General Anatomy and Embryology 2021 PDF

GENERAL ANATOMY Domestic Animal Species Herbivores ‫آكالت األعشاب‬ - Equidae (Horse, donkey, mule) - Ruminatia (Large: ox) (Small: sheep and goat) - Tylopoda (camels, Llama ….etc.)  The existing camel species now : Camelus dromedarius & Camelus bactrianus ...

GENERAL ANATOMY Domestic Animal Species Herbivores ‫آكالت األعشاب‬ - Equidae (Horse, donkey, mule) - Ruminatia (Large: ox) (Small: sheep and goat) - Tylopoda (camels, Llama ….etc.)  The existing camel species now : Camelus dromedarius & Camelus bactrianus  The lama guanaco existing Lama species now : alpaca vicuna Endocrine Regulates body functions such as thyroid gland, growth and reproduction by means pituitary of hormones gland, pancreas Regulates day-to-day metabolism by means of hormones Circulatory Transports oxygen and nutrients heart, blood, to tissues and removes waste arteries products Lymphatic Returns tissue fluid to the blood spleen, lymph Destroys pathogens that enter the nodes body and provides immunity Respiratory Exchanges oxygen and carbon lungs, trachea, dioxide between the air and blood larynx, diaphragm  Carnivores ‫آكالت اللحوم‬ - Canines (dogs) - Felines (cats) ‫آكالت األعشاب واللحوم ‪ Omnivores‬‬ ‫‪- Swines‬‬ ‫)‪(pigs‬‬ ‫(اإلنسان) ‪- Human‬‬  General Introduction Gross or Macroscopic Anatomy. Comparative Anatomy. Special Anatomy. Veterinary Anatomy. Microscopic Anatomy (Histology). Embryology (developmental anatomy). Ontogeny (Individual development). Phylogeny (Historical development). Refer to Dorland’s dictionary for more anatomy definitions ▪ Anatomy is the study of the form and structure of the organism. ▪ The word anatomy means to cut apart. ▪ Anatomical studies can be done on preserved carcass and living animals both macro and microscopically. 1- Gross or Macroscopic Anatomy It describes simple dissection using scalpels, forceps and scissors either by the naked eye or by help of hand lenses or low power magnification systems. 2- Microscopic Anatomy It describes the study of the minute structure of cells (Cytology) and the tissues (Histology) with the aid of different types of microscopes. 3- Applied (or clinical) anatomy: Application of all anatomical facts on living subject to be able to examine different organs. 4- Surgical anatomy: A Subdivision of clinical anatomy which deals with sur- gical procedures based on anatomical facts such as planning of incisions, app- roach to deep structures, punctures… etc. 3- Applied (or clinical) anatomy: Application of all anatomical facts on living subject to be able to examine different organs. 4- Surgical anatomy: A Subdivision of clinical anatomy which deals with sur- gical procedures based on anatomical facts such as planning of incisions, app- roach to deep structures, punctures… etc. 5- Surface anatomy: Deals with projections (land marks) of internal structures on the surface for the purpose of clinical examination. 6- Comparative anatomy: Comparison of the structure of animal and forms the basis for their classification (structural evolution). 7- Special anatomy: Description of the structure of a single type or species e.g. birds anatomy and fish anatomy 8- Instrumental anatomy The use of helpful instruments to visualize deeply seated living structures in action, or hollowed organs prior to any treatment. Many methods are included:- a) Endoscope: by using special optical instruments such as gastroscope. bronchoscope, laparoscope, cystoscope… etc. b) Radiography: By x-ray for recording anatomical structures in the region. Radiography can be done simply (plan radiography) or by the use of contrast media (salt or heavy metals) either by injection or swallowing. c) Computerized Aid Tomography (CAT): It is scanning which show the interior of transverse section of a living part. d) Ultrasonography (Sonar): It is the study of different anatomical facts and relations especially in moving organs like the heart or the fetus during pregnancy. e) Nuclear Magnetic resonance (NMR): This technique records the resonance of the protons against the nuclei of the cells when they are exposed to a strong magnetic field. It is the latest method of recording not only anatomical information, but even the physiological and biochemical changes occurring in the tissue before they become evident pathologically. NMR gives minute details more than those recorded by x-rays or CAT scanning. Transverse scan of abdomen showing liver metastasis with area of low signal from calsification (arrow). Note low signal from vessels. e) Nuclear Magnetic resonance (NMR): This technique records the resonance of the protons against the nuclei of the cells when they are exposed to a strong magnetic field. It is the latest method of recording not only anatomical information, but even the physiological and biochemical changes occurring in the tissue before they become evident pathologically. NMR gives minute details more than those recorded by x-rays or CAT scanning. 9- Developmental anatomy (Embryology). It is the study of the developmental changes which occur to the individual from the time of fertilization of the ovum until birth.  There are three methods to study veterinary anatomy  1) Systemic Anatomy Osteology: Description of the skeleton (bones, cartilages). Syndesmology: Description of the joints. Myology: Description of the muscles and the accessory structures. Splanchology: Description of the viscera digestive system & respiratory system & uro-genital system & peritoneum & endocrine glands (endocrinology) Angiology: Description of the organ of circulation (heart, arteries, veins, lymphatics and spleen). Neurology: Description of the nervous system. Esthesiology (sensory organs): Description of the eye and ear. Dermatology (common integument): The study of the skin and its structures. 2) Topographic Anatomy Is the study of parts of the body in relation to their surrounding structures.  3) Applied Anatomy Means the consideration of the anatomical facts in their relation to surgery ,physical diagnosis, and other practical branches.  DIRECTIONAL TERMS Directional terms are used to precisely locate one part of the body relative to another and to reduce length of explanations.  DIRECTIONAL TERMS Directional terms are used to describe relative position. Directional terms come in opposing pairs (like East/West & North/South). It is important to familiarize your self with these terms because they make up the basic language used in anatomy. N.B.: ---al = adjective, e.g., dorsal; ---ad = adverb (toward), e.g., dorsad  DIRECTIONAL TERMS Medial: toward midline Lateral: away from midline Intermediate: between 2 points Ipsilateral: same side Contralateral: opposite side Proximal: near origin Distal: away from origin  DIRECTIONAL TERMS External (Outer) Internal (Inner) Central Peripheral Parietal Visceral  AREAS Head & Neck Trunk –Thorax –Abdomen –Pelvis & Perineum Extremities (or limbs) –Forelimb –Hind limb Caudal Cranial  Describing the location on a bone Cranial - toward the head.‫أمامى‬ Caudal - toward the tail. ‫خلفى‬ Medial - toward the center (midline) in normal anatomic position.‫داخلى‬ Lateral - away from the center (midline) in normal anatomic position.‫خارجى‬ Superior - upper (opposite of inferior) ‫علوى‬ Inferior - lower (opposite of superior( ‫سفلى‬ Dorsal - towards the top (opposite of ventral) ‫ظهرى‬ Ventral - towards the bottom ‫*( بطنى‬dorsal) Distal - away from an attachment. ‫سفلى‬ Proximal - towards an attachment ‫علوى‬ Rostral: Toward the front Caudal: Toward the back, toward the tail Cranial  Other commonly used terms (supra) Above Inferior (infra) Below Superficial Close to surface Deep (profundus) Beneath surface Peripheral (Peri) The part nearest to the surface Parietal The body wall or the wall of the cavity Visceral The viscera or organ in the cavity Somatic The part of the body other than viscera Axial Towards the central line of body or any body part Abaxial Away from the central line of body or any body part Dexter Means right Sinister Means left Intra Inside Inter Between Sub Under  In the leg region Knee (proximal) palmer plantar dorsal Hoof (distal)  In the head region: caudal rostral  Body Planes Median Plane: The Median Plane is a plane that divides the body into right/left halves [head, trunk, tail].(equal parts) Sagittal Plane: A Sagittal Plane divides the body into right/left parts [head, trunk, tail]. (unequal parts). It is parallel to the median plane.  Body Planes Transverse Plane: A Transverse Plane is perpendicular to the long axis of the body [head, trunk, tail]. Caudal part Cranial part  Body Planes Frontal Plane (dorsal): A Frontal (Dorsal) Plane is parallel to the back [head, trunk, tail]. Divides the body into dorsal/ventral parts. Limb Plans: Limbs are generally cut by planes: 1) Median & Paramedian planes 2) Transverse Plane 3) Frontal (Dorsal) Plane Median & Paramedian planes Transverse plane Dorsal plane  PLANES Planes are imaginary flat surfaces that are used to divide the body or organs into definite areas & include: – Midsagittal (medial) and parasagittal, frontal (coronal), transverse (cross-sectional or horizontal) and oblique. Body Planes & Directional Terms  Summary Checklist Tick off when you can describe and discuss the following items: ❑ Definition of Anatomy. ❑ Methods of studying Anatomy. ❑ Animal species for comparative anatomy ❑ Topographic Terms. ❑ Directional terms. ❑ Plans of the body.  Systemic Anatomy Osteology Syndesmology; Arthorology Myology Splanchinology description of the viscera e.g. digestive system, respiratory system, urogenital system, Angiology organs of circulation (heart, arteries, veins, lymphatic and spleen) Neurology Ethesiology (Sensory organs) Dermatology (Common integument)  A- Osteology Bone Classification Schemes Development: Endochondral bones — develop from cartilage precursors [most bones] Intramembranous bones — directly from mesenchyme (fascia) [bones of calvaria & face]  Location: Axial skeleton — head, vertebral column ( including tail), ribs & sternum Appendicular skeleton — bones of limbs, including scapula & os coxae(hip bone) Heterotopic bones os penis...... [ carnivore; rodent ], os cardis ….. cattle  Shape: Long bones — length greater than diameter Short bones — approximately equivalent dimensions Flat bones — e.g., scapula, os coxae, many bones of skull Irregular bones — short & multiple processes (vertebrae) Sesamoid bones — small “seed-like” within tendons,e.g., patella (knee cap) ossification Syndesmology (Arthrology) Joints may be classified— (a)anatomically, according to their ----mode of development, -----the nature of the uniting medium, ------and the form of the joint surfaces; (b) physiologically, with regard to the amount and kind of movement or the absence of mobility in them (c) by a combination of the foregoing consideration. Three chief subdivisions of joints are usually recognized, synarthroses, diarthroses, and amphiarthroses.  Synovial joints can be classified according to the following criteria: 1.Number of bones forming the joint Simple joints (articulatio simplex) with one pair of articular surfaces, e.g. the shoulder joint, Composite joints (articulatio composita) in which more than two surfaces are involved, e.g. the carpus. 2.Degree and kind of mobility Uniaxial joints: Hinge joint (ginglymus): the joint axis is perpendicular to the long axis of the bone, e.g. elbow joint, Pivot joint (articulatio trochoidea): the joint axis is parallel to the long axis of the bones, e.g. atlantoaxial joint,  Biaxial joints: Saddle joint (articulatio sellaris), e.g. interphalangeal joints, Ellipsoidal joint (articulatio ellipsoidea), e.g. atlantooccipital joint. Multiaxial joints, e.g. shoulder and hip joint,  3. Form of the articular surfaces Spheroidal or ball-and-socket joint Cotyloid joint e.g. shoulder Spheroidal joint, where the convex articular surface is enclosed in the articular cavity, e.g. hip joint, Ellipsoidal joint e.g. atlantooccipital joint, Saddle joint e.g. interphalangeal joints, Condylar joint. The last group comprises the following subdivisions, characterised by special functional features: Hinge joint (ginglymus), e.g. elbow joint, Cochlear joint ,e.g. tarsocrural joint of the horse, Sledge joint e.g. the femoropatellar joint, Plane joints (articulatio plana), e.g. the joints between the articular processes of the vertebrae,  C- Myology Science deals with muscles Three types of muscle tissue: 1] smooth muscle = not striated; associated with viscera (gut, vessels, glands) 2] cardiac muscle = striated; musculature of the heart 3] skeletal muscle = striated; generally attached to bone; usually under voluntary control  The description of the muscles under the following heads (1)Name (2) shape and position (3) attachments (4) action (5) structure (6)relations (7) blood and nerve supply  Muscle names Muscle names may be latinized (flexor digitorum profundus) or anglicized (deep digital flexor). Muscle are named for their:  shape …………deltoideus, serratus ventralis Direction of fibers …………rectus abdominus  position and relationship of muscles to another ………………… internal and external oblique abd. M.  Action …………………… adductor Location………………….(brachialis)  attachments …………..(sternohyoideus)  structure ……………………(biceps)  function or combinations of these (superficial digital flexor; serratus ventralis; flexor carpi radialis;etc.) Muscles may be called long, short or broad Long muscles: in the extremities Broad muscles: in the trunk Short muscles: between the ribs and vertbrae  Muscle associated fascia 1. epimysium = loose or dense connective tissue surrounding an entire muscle 2. perimysium = loose connective tissue defining muscle fascicles 3. endomysium = small amounts of loose c.t. surrounding individual muscle fibers Muscle associated fascia: 1. epimysium = loose or dense connective tissue surrounding an entire muscle 2. perimysium = loose connective tissue defining muscle fascicles 3. endomysium = small amounts of loose c.t. surrounding individual muscle fibers  Fascicle & fiber arrangement: Parallel arrangement, e.g., strap or spindle arrangement, fibers/fascicles arranged parallel to the tendon of insertion. This results in a greater range of shortening and thus yields greater movement velocity (distance per time). Pennate arrangement = fibers/fascicles arranged at an angle to the direction in which the tendon moves. This results in a greater area of muscle fibers along axes of contraction and produces more strength (at the expense of a reduced range of contraction).  Muscle-related connective tissue Muscle fibers are within a connective tissue framework that is continuous with tendons As a result, passive muscles are able to serve as ties that reinforce joints & oppose forces on bones  The main charcters of muscles Irritability or excitability Contractility Extensibility Elastisity  Skeletal Muscle Different types of muscle fibers are found among the skeletal muscles of the body, e.g., — slow contracting, fatigue resistant, aerobic metabolism (Type I) — fast contracting, fatigue resistance, aerobic metabolism (Type 2A) — fast contracting, fatigue susceptible, anaerobic metabolism (Type 2B) Skeletal muscle will not contract in the absence of a functional nerve supply (denervation atropy occurs). Innervations of the muscle  Tendon protection Synovial tendon fluid tendon Deep fascia Synovial fluid bone Bursa tendon synovial sheath A. bursa = synovial pocket inserted between a tendon and a bony prominence B. tendon synovial sheath = lubrication where tendons are bound, e.g., by retinaculum  Muscle Attachments Fleshy attachment Tendon proper Flat sheet tendon; aponeuroses Most muscles attach to two different bones; the least movable called Origin and the more movable called Insertion  Functional grouping of muscles Flexor Extensors Adductors Abductors Sphinctors Cutaneous muscles Articular muscles Types of pennate arrangement : — unipennate, e.g., ulnar & radial heads of the deep digital flexor muscle; — bipennate, e.g., infraspinatus muscle; — multipennate, e.g., humeral head of the deep digital flexor muscle  骨格筋の形態と機能 長筋 多腹筋 二腹筋 三頭筋 三角筋 方形筋 紡錘状筋 十字状筋 放射状筋 半羽状筋 羽状筋 多羽状筋 螺旋状筋 他に広筋、円筋、鋸筋、二頭筋、四頭筋、多裂筋などがある。 Muscle roles within a given movement agonist = prime mover or principal muscle(s) executing the particular joint movement antagonist = muscle(s) that oppose the action of the agonist on the joint(s) synergist = muscle(s) that assist the agonist; e.g., fixators stabilize distant joints  Some clinical expressions , which are related to anatomical terms: Osteopathy, ostitis, osteomyelitis, periostitis, osteosynthesis, osteolysis, osteomyelofibrosis, osteonecrosis, osteoperiostitis, ossificans, osteopetrosis, osteoporosis, osteochondrosis, osteosarcoma, arthropathia, arthritis, arthrosis, arthroscopy, arthrolysis, hip dysplasia, myopathy, myodistrophy, myofibrosis, myometritis, myocarditis, myospasm, tendopathy, tendinitis, bursitis, synovitis.  Syndesmology (arthrologia) The degree of mobility permitted between two adjacent bones largely depends on the type of articulation Articulations without a joint Articulations with a space are termed joint space are termed synarthroses. (diarthrosis). These joints can Synovial joints are either be filled with characterised by a joint cavity), filled. soft tissue, forming or with cartilage, fibrous junctions or with joint fluid forming (synovia). joints). cartilaginous joints  Fibrous joints can be subdivided into: 1-Syndesmoses, the attachment of the radius 3- Gomphosis: e.g. the and ulna, dewclaws implanation of the teeth in 2- Sutures unite the bones of the the dental alveoli by the skull, There are different periodontal membrane types of sutures: Serrate suture (sutura serrata), Plane suture (sutura plana), Squamous suture (sutura squamosa) Foliate suture (sutura foliata)  Cartilaginous joints can be subdivided in: Hyaline cartilage joints (synchondroses): – e.g. between the base of the skull and the hyoid bone, – Fibrocartilaginous joints (symphyses): e.g. between the two halves of the pelvis or the mandible and  Synovial joints Synovial, or true joints, vary with regards to The number of bones composing the joint, The amount and kind of mobility in them The form of the joint surfaces. However, all joints have certain common structural and functional features They share the following characteristics: – 1- Articular cartilage usually hyaline, covers the articular surfaces of the bones, – 2- Joint cavity and – 3- Joint capsule. consists of two layers, the external layer is composed of fibrous tissue (stratum fibrosum), the internal layer is richly supplied by a network of blood vessels and nerves and is termed the synovial layer or membrane  Joints are held in place by intracapsular, capsular or extracapsular joint ligaments. A few joints also contain fibrocartilagous meniscus in the knee joint or discs in the mandibular joint, in order to compensate the incongruency of the joint surfaces and stabilize the joint Synovial joints can be classified according to the following criteria: 1.Number of bones forming the joint Simple joints (articulatio simplex) with one pair of articular surfaces, e.g. the shoulder joint, Composite joints (articulatio composita) in which more than two surfaces are involved, e.g. the carpus. 2.Degree and kind of mobility Uniaxial joints: Hinge joint (ginglymus): the joint axis is perpendicular to the long axis of the bone, e.g. elbow joint, Pivot joint (articulatio trochoidea): the joint axis is parallel to the long axis of the bones, e.g. atlantoaxial joint, Biaxial joints: Saddle joint (articulatio sellaris), e.g. interphalangeal joints, Ellipsoidal joint (articulatio ellipsoidea), e.g. atlantooccipital joint. Multiaxial joints, e.g. shoulder and hip joint,. Tight joints (amphiarthrosis), e.g. sacroiliac joint.  3. Form of the articular surfaces Spheroidal or ball-and-socket joint Cotyloid joint e.g. shoulder Spheroidal joint, where the convex articular surface is enclosed in the articular cavity, e.g. hip joint, Ellipsoidal joint e.g. atlantooccipital joint, Saddle joint e.g. interphalangeal joints, Condylar joint. The last group comprises the following subdivisions, characterised by special functional features: Hinge joint (ginglymus), e.g. elbow joint, Cochlear joint ,e.g. tarsocrural joint of the horse, Sledge joint e.g. the femoropatellar joint, Plane joints (articulatio plana), e.g. the joints between the articular processes of the vertebrae,  4- degree of congruency Congruent incongruent Incongruent joints: the form of the articular surfaces do not correspond, e.g. the femorotibial joint or the temporomandibular joint. In these joints fibroarticular menisci or discs render the surfaces congruent.  A- plantigrade locomotion B- digitigrade C- unguligrade  Nervous System 1. The central nervous system (CNS) which consists of the brain and spinal cord. 2. The peripheral nervous system (PNS) which consists of the nerves that connect to the brain and spinal cord (cranial and spinal nerves) as well as the autonomic (or involuntary) nervous system.  Common embryologic terms Embryology: It is the science which studies the prenatal (before birth) development of embryo and foetus; however, the term can also refer to postnatal development. Embryogenesis Morphogenesis (general embryology) (special embryology) Histogenesis Organogenesis Oocyte (= ovum or egg) It is the female gamete which produced in the ovaries. It carries the half of the chromosomes (haploid, 1N). Sperm (= spermatozoon) It is the male gamete which produced in the testes. It also carries the half of the chromosomes (haploid, 1N). Zygote: Cell results from the union of an oocyte and a sperm after fertilization. It carries the full chromosomes (diploid, 2N). Embryo (embryonic period): It is the time from fertilization to the earliest (primordial) stages of organ development (about 30 days in dog, cat, sheep, pig; almost 60 days in horse, cattle, and human). Foetus (foetal period): It is the time after the embryonic period and until birth in which differentiation and growth of the tissues and organs formed during the foetal period occur. Teratology It is a division of embryology which deals with abnormal development (birth defects or anomalies). Cleavage Series of mitotic cell divisions of the zygote that results in the formation of early embryonic cells which called blastomeres. Start with 2 cell stage, then 4 cells stages and so on. Morula: It is a solid mass of approximately 8 (early morula) – 16 (compact morula) blastomeres (according to species) which is formed by cleavage of the zygote. Blastocyst It is the morula when a fluid-filled cavity, develops inside it. It consists of inner cell mass which will give rise to the embryo and outer trophoblast (trophectoderm) which participates in formation of fetal membrane and placenta. NB: In birds, the blastosyst is called blastoderm which is disc shape. Blastula It is the embryo during blastocyst formation (has only two layers, bilaminar embryo). Implantation The process during which the blastocyst attaches to the endometrium and subsequently embeds in it (in mammals only). NB: Before implantation, the blastocyst becomes slightly expanded, then hatches from the surround zona pellucid and finally becomes highly expanded. Gastrulation Formation of the three germ layers (ectoderm, mesoderm, and endoderm) from the bilaminar (two layers) embryo. Gastrula It is the embryo during and after gastrulation (has three layers, trilaminar embryo). Neurula The early embryo when the neural tube is developing. It is the first appearance of the nervous system and the next stage after the gastrula. Fetal membranes (chorion, amnion, yolk sac and allantois) They are structures which develop from the zygote but do not form part of the embryo itself. These membranes are only required during embryological development and so they are either shed or absorbed at hatching or birth. NB: Conceptus = embryo/fetus + fetal membranes. Placenta It is a specialized structure of mammals which separates the blood of mother from that of fetus. Umbilical cord It is the connecting stalk (life line) between fetus and fetal side of placenta (chorion). Sperm (1N) Oocyte (1N) Fertilization Zygote (2N) Cleavage Morula Cleavage Blastula (bastocyst / blastoderm) Gastrulation Gastrula Neurulation Neurula Differentiation Fetus  Initial development of the mammalian embryo: A: Zygote; B: 2-cell embryo; C: 4-cell embryo; D: Early morula; E: Compact morula; F: Blastocyst; G: Expanded blastocyst; H: Blastocyst in the process of hatching from the zona pellucida; I: Ovoid blastocyst with embryonic disc; J: Elongated blastocyst; K: Embryonic disc in the process of gastrulation. 1: Inner cell mass; 2: Trophectoderm; 3: Epiblast; 4: Hypoblast; 5: Embryonic disc; 6: Amniotic  Cell division (proliferation) A) Mitosis - This type of cell division occurs in all somatic cells and results in two new (daughter) diploid (2N) cells similar to each other and to their parent. - Mitosis is preceded by a preparatory phase called interphase in which the DNA (chromosomes) are replicated (formation of sister chromatids, dyads) and the centrioles duplicated to form centrosomes. B) Meiosis - It takes place in germ (sex) cells only. - It results in formation of haploid (1N) gametes (sperms and oocytes). - It leads to formation of daughter cells not genetically identical due to exchange of genetic material between non-sister chromatids (crossover). -Before meiosis: Germ cell enters meiosis with diploid chromosomes (2N). - There are two successive meiotic divisions (meiosis I and meiosis II)  Diagrammatic representation of meiosis. A to D, Stages of prophase of the first meiotic division. The homologous chromosomes approach each other and pair; each member of the pair consists of two chromatids. Observe the single crossover in one pair of chromosomes, resulting in the interchange of chromatid segments. E, Metaphase. The two members of each pair become oriented on the meiotic spindle. F, Anaphase. G, Telophase. The chromosomes migrate to opposite poles. H, Distribution of parental chromosome pairs at the end of the first meiotic division. I to K, Second meiotic division. It is similar to mitosis except that the cells are haploid.  Species Number of chromosomes in pairs Chicken, dog 39 Horse 32 Cattle, goat 30 Sheep 27 Human 23 Pig, cat 19  Gametogenesis Gametogenesis is the process of formation and development of male gamete (sperm = spermatozon) and female gamete (oocyte = ovum = egg). Spermatogenesis Oogenesis The gametes are originated from the primordial germ cells (PGCs) which are diploid (2N) cells that originate from the primary ectoderm (epiblast) and terminate in the gonadal ridge.  Emigration (migration) of PCGs In mammals; - PGCs move (by pseudopodia, in an amoeboid manner) from the primary ectoderm into endoderm of yolk sac and then migrate up through the intestinal wall and the dorsal mesentery to finally colonize the gonadal ridge. - The gonadal ridge together with PCGs form the gonad which is the primordium of either the testis or the ovary. In birds - The PGC have no pseudopodia and are unable to move. - Therefore, they re-enter the body via blood circulation of the yolk sac. - They then reach the gonadal ridge passively through blood.  Spermatogenesis - Spermatogenesis is the process of formation and maturation of the spermatozoa (male sex cells). - It occurs in seminiferous tubules of the testis from prenatal life to death. - During spermatogenesis the germ cells move towards the lumen (centripetal movement).  Spermatogenesis B. Spermiogenesis A. Spermatocytogenesis (spermiohistogenesis) It starts from primordial From spermatid to germ cell (PGC) to spermatozon spermatid 2) Period of maturation 1) Period of proliferation (From primary (from PGC to spermatocyte to spermatogonia type B) spermatid) A1) Proliferation period of spermatocytogenesis (clonal expansion) - During fetal life: The primordial germ cells (2N) give rise to spermatogonia type A (2N) which remain dormant in the basal layer of seminiferous tubules of the testis. - At puberty: The spermatogonia type A proliferate by mitosis with homonymous division (complete separation of cells) to give rise to two types of cells: 1) One dormant cell 2) The other cell becomes which acts as stem type B spermatogonia (2N) cell to renew the stock which also multiplicate by of type A cells. This mitosis but with dormant heteronymous division spermatogonia A stay (incomplete separation of to ensure a cells due to presence of continuous supply of cytoplasmic bridges. This spermatogonia when ensures that all cells depleted. This can undergo the same explain why the male morphological changes at can produce sperm the same time and so for ever (until death). produce identical sperms). A2) Maturation period of spermatocytogenesis - Spermatogonia type B divide by mitosis giving rise to primary spermatocyte (2N) which characterized by larger nucleus size. - Primary spermatocyte divides by first meiotic division (meiosis I) into two equal size secondary spermatocytes each of them contain haploid (1N). - In mammals, one secondary spermatocyte contains X chromosome and the other contains Y chromosome. - Each secondary spermatocyte (1N) divides by second meiotic division (meiosis II) to produce two spermatids (1N). B) Spermiogenesis (spermiohistogenesis) - It is the differentiation of the spermatids into sperm cells. 1- Nuclear condensation: The nucleus becomes smaller, denser, oval and eccentric. 2- Acrosome formation: - The Golgi complex produces small vesicles, which then fuse into a single large acrosomal vesicle close to the nucleus. - When the vesicle covers the cranial aspect of the condensed nucleus, it is called acrosome. 3- Flagellum formation: The centrioles which migrate to the pole of nucleus opposite to acrosome, form the axial filament (axoneme) from which mid piece and tail of the sperm develop. 4- Spiral filament formation: The mitochondria arrange spirally around the flagellum in the mid piece. 5- Cytoplasmic reduction: - Elimination of all unnecessary cytoplasm (collectively called residual body) by Sertoli phagocytosis. - Some cytoplasmic remnants may be left and hang to the mid piece of the immature sperm as protoplasmic droplet. - Once the sperm becomes mature, it free itself from the Sertoli and go to the lumen of seminiferous tubule, this process is called spermiation.  6 5 4 11 Diagram showing Spermiogenesis 3 2 1 stages Three differing stages of spermiogenesis: on the left a fresh spermatid, on the right an immature sperm cell, and in the middle an in-between stage. 1. Axonemal structure, first flagellar primordium, 2. Golgi complex, 3. Acrosomal vesicle, 4.Pair of centrioles (distal and proximal), 5. Mitochondrion, 6. Nucleus, 7. Flagellar primordium, 8. Microtubules, 9. Sperm cells tail, 10. Acrosomal cap, 11. protoplasmic droblet  Sperm structure Mature sperms are free-swimming, actively motile cells consisting of a head, neck, mid piece and tail. (1) The head - forms most of the bulk of the sperm and contains the haploid nucleus. - The anterior two thirds of the nucleus is covered by the acrosome, a cap like saccular organelle containing several enzymes (hyaloronidase and others) which help the sperm to penetrate of the ova during fertilization. - The posterior one third covered only by post acrosomal plasma membrane. (2) The neck - contains the two centrioles (proximal and distal, the distal one forms the flagellum). - The weakest and fragile part of the sperm  (3) The mid (middle) piece - consists of ring-shaped mitochondria forming spiral filament around the sheath of the thick outer fibers and the axoneme. - provides the energy for the flagellar movement. - The terminal ring; presents at the junction with the tail to prevent caudal displacement of the mitochondria during motility. (4) The tail - The tail provides the motility of the sperm that assists its transport to the site of fertilization. It composes of two pieces: A- Principal piece - Has a sheath of ring fibers around the axoneme. -The axoneme or axial filament is composed of 9 microtubules doublets plus 2 central microtubules. B- End piece. - Consists of only the 9+2 structure of the axoneme.  Oogenesis - Oogenesis is the sequence of events by which oogonia are transformed into mature oocytes. - It begins before birth and is completed after puberty. - It continues to menopause, which is permanent cessation of the menstrual cycle (in woman only). - It is divided into two periods; period of prenatal maturation and period of postnatal maturation of oocytes. A) Period of Prenatal Maturation - Occurs only in the prenatal life (before birth). - Starts with primordial germ cells (2N) which divide mitotically to give rise to oogonia (2N). - Oogonia proliferate by mitosis and when enlarge; they form primary oocytes (2N). - Primary oocytes begin the first meiotic division before birth but arrested in prophase I. - The primary oocyte becomes surrounded by flat follicular cells and together they form the primordial follicle.  B) Period of Postnatal Maturation of Oocytes Time Changes After primary oocytes (arrested in prophase I by oocyte birth maturation inhibitor factor secreted from the follicular cells surrounding the primary oocyte) remain dormant within primordial follicles. NB: In contrast to continuous production of primary spermatocytes in males, no primary oocytes form after birth in females and so each female born with limited numbers of primary oocytes. This can explain why women have menopause when they become older (between 45 and 50 years). At primary oocyte enlarges and primordial follicle puberty becomes primary follicle.  Hours Primary oocyte completes the first before meiotic division to give rise to a ovulation secondary oocyte (1N, large) and the first polar body (1N, small, soon degenerates). At ovulation Secondary oocyte begins Meiosis II, but arrested in metaphase II. After Secondary oocyte completes Meiosis II fertilization and the fertilized oocyte and the second polar body formed.  Folliculogenesis - The growth and development of ovarian follicles from the primordial to the ovulatory stages. - The female mammal is born with a fixed number of follicles i.e. at birth the ovary contains all the follicles it is ever going to have. 1. Primordial follicle - consists of one primary oocyte surrounded by a single layer of squamous (flat) follicular cells. 2. Primary follicle - formed when the surrounding of primary oocyte becomes a single layer of cuboidal follicular cells 3. Secondary follicle - It is characterized by formation of; a. Granulosa cells: - Formed when the follicular cells proliferate into multiple layers (stratified granulosum). b. Zona pellucida: - A fenestrated translucent glycoprotein layer between the oocyte and follicular epithelium. NB: Cytoplasmic processes of the granulosa cells pass through the fenestration of the zona pellucida to connect with the oocyte, thereby assure their communication and supply the primary oocyte by yolk. c. Theca follicular cells: - Formed by the ovarian stroma surrounding the basal lamina of granulosa cells. 4. Tertiary (antral) follicle - Formed when granulosa cells secrete fluid which leads to formation of fluid-filled spaces, which fuse to form a single large cavity, the antrum, which contains follicular fluid. - The primary oocyte reaches full size and is pushed to one side of the follicle, - The inner follicular cells surrounding the oocyte are called the cumulus oophorus. - The cumulus oophorus cells radially arranged on the cell membrane of the oocyte is called the corona radiata. - The theca layer becomes organized into the theca interna (vascular and glandular layer) and theca externa (fibrous). 5. Graafian (mature) follicle - It is a selected tertiary follicle that continues to enlarge until reaches maturity and produces a swelling on the surface of the ovary. Atresia occurs for the other non selected tertiary follicles. Changes occur in Graafian follicle before ovulation: - The processes of the granulosa cells retracted from the oocyte surface into the zona pellucida. This leads to the formation of the perivitelline space. - In this space the ejection of the first polar body takes place as a sign that the first meiosis ended and the secondary oocyte formed. Diagram showing changes occur in Graafian follicle before ovulation 1. Theca interna and externa, 2.basal membrane, 3.Granulosa cells, 4. Graafian follicle, 5. Primary oocyte, 6. Cumulus oophorus, 7. Ovarian tissue, 8.Tunica albuginea, 9. Abdominal space, 10. oocyte plasma membrane (oolemma), 11. diakinesis stage of prophase I, 12 corona radiata, 13. processes of 1. zona pellucida, 2. perivetilline space, 3. mitotic spindle, 4. retracted processes of granulosa cells, 5. oolemma, 6. granulosa cells, 7. first polar body.  Types of ova A) Amount of yolk 1. Oligolecithal 2. Mesolecithal 3. Polylecithal little amount of moderate large amount of yolk yolk and smaller amount of yolk and larger egg ova because because embryo embryo develops develops outside inside mother and mother body and so feeds by placenta needs high yolk to feed on Example; Fishes and frogs Birds and reptiles Mammals  B) Distribution of yolk in the cytoplasm 1. Isolecithal ova: They contain few amount of yolk which is equally (uniformly) distributed in the cytoplasm. Example: oligolecithal ova (Mammals) 2. Anisolecithal ova: They contain moderate or larger amount of yolk which displace the small amount of cytoplasm and nucleus to the animal pole and so the yolk is unequally distributed. Example: meso-and polylecithal ova.  Telolecithal ova Centrolecithal ova The yolk accumulates in the vegetable The yolk accumulates in pole. the center of ova and so They are subdivided into: the nucleus and Micro- Macro- telolecithal cytoplasm are pushed to telolecithal the periphery. Incomplete complete isolation Example. Insects. isolation of yolk (there is a from cytoplasm demarcation line) (no demarcation line between yolk and cytoplasm) Examples: Polylecithal (birds (mesolecithal and reptiles) lower fishes and frogs)  animal pole Nucleus Vegetable Yolk uniformly pole distributed with cytoplasm Chalaza Corona radiata Bird ovum (polylecithal, macro-telo-lecithal) Mammalian ovum (oligo-isolecithal) Jelly coat animal pole Yolk Nucleus Cytoplasm Vitelline membrane Vegetable pole Yolk mixed with few cytoplasm Insect ovum (centrolecithal) Frog ovum (mesolecithal, micro- telolecithal  Egg (ovum) coverings Animal pole Vegetable pole The part containing Contains yolk cytoplasm and nucleus Characterised by active, Slow rate of growth rapid growth rate Cleavage divisions occur Slowly or lack divisions rapidly Form the embryo Form yolk sac, placenta and allantois  Spermatogenesis Oogenesis Occurrence In seminiferous tubules of testis In the ovary cortex Start time before birth before birth When the cells mature, they When the cells mature, they Direction of move toward the center (lumen), move toward the periphery cell movement centripetal movement (surface), centrifugal movement Start late (at puberty) but Start and end before birth so continue for ever (until death) so the number of oocyte is unlimited cell number limited Produce spermatogonia type A & B Only one type of oogonia Proliferation (mitosis) Spermatogonia can give rise to All primary oocytes are primary spermatocyte after birth formed before birth Start with homonymous then Only homonymous division continue as heteronymous division (complete separation of cells)  Occurs at any time from Start before birth and puberty to death completed few hours before ovulation Meiosis I Lead to formation of two equal One big cell (secondary size cells (secondary oogonium,1N) and one small spermatocyte, 1N) polar body Occurs at any time from Begin at ovulation and Meiosis II puberty to death completed in uterine tube after fertilization Transformation Present (spermiogenesis) No From one spermatogonia B, 8 From one oogonia, one ova sperms are formed with the and three smaller polar End result same size and each has 1N and bodies are formed each has either X or Y chromosome 1N and only X chromosome  Sperm Ovum Size Small Large Shape Elongated Spherical Quantity Large number (million) 1- 25 (acc. to the animal) Sex X and Y chromosome Only in birds determination Motility Vigorous (by the tail) Lack (no tail) Protection Only plasma membrane Plasma membrane + other membranes + follicles Nucleus Condensed and form Not condensed sperm head Yolk Non Little to much Golgi complex Acrosomal cap Diffused Mitochondria Spiral filament in mid Diffused piece Centrioles Retained and form the Disappeared axial filament  Fertilization It is the union of a haploid oocyte with a haploid sperm to produce a diploid zygote. Types of fertilization 1. External fertilization; - occurs outside the body as in fish and frog. 2. Internal fertilization; - occurs inside the body as in mammals, birds and reptiles. These animals can be classified into: a. oviparous; The zygote developed outside the body in the eggs as in birds and reptiles. b. viviparous; The zygote developed inside the body in the uterus as in mammals.  Site of Fertilization At the ampula of the uterine tube (uterine tube in animal = fallopian tube in human or oviduct in birds and reptiles). So sperms must reach this site (either by coitus or artificial insemination) (journey of the sperm) and oocyte should also goes there (journey of the ovum).  Ovulation Definition: It is the process by which the secondary oocyte releases from the mature Graafian follicle and ovarian surface. Types A. Spontaneous: (not need male) as in all animals except those mentioned below. B. Induced: (need male, not ovulated except after coitus), as in ferret, rabbit, cat and camel.  Time of ovulation Animal Duration of Number of Time at which ovulation oestrus released ova occurs (heat) Bitch 9 days 2 to 10 2 to 3 days after onset of oestrus Cow 18 hours 1 14 hours after end of oestrus Ewe , 36 hours 1 to 3 24 to 30 hours after onset Goat of oestrus Mare 4 to 8 days 1 1 to 2 days before end of oestrus Queen 3 to 6 days 2 to 8 24 hours after coitus Sow 48 hours 10 to 15 36 to 48 after onset of oestrus Mechanism of ovulation - The exact mechanism is poorly understood. - After the FSH/LH peak, the dominant follicle's displaced towards the ovarian surface where it finally bulges out. - The cumulus cells loosen and the oocyte and the surrounding corona radiata free themselves and now swim in the follicle fluid. - On the ovarian surface above the follicle that is about to burst, a white point forms shortly before the rupture (due to compression of the blood vessels), the so-called stigma. - When stigma ruptures, the secondary oocyte with corona radiata and the follicular fluid expelled. Three factors share in ovulation 1. The hyaluronic acid: - It is secreted by the granulosa cells, has the property that it binds water molecules: The more hyaluronic acid is made, the more water can be absorbed. In this way, a rapid increase of the amount of follicle fluid occurs and this leads to a dramatic increase of the tension in the follicle wall. 2. Contraction of theca externa muscles; 1+2 increase the intrafollicular prssure 3. The lytic enzyme: causes retardation of blood supply and stigma formation Capture (up-take) and transport of oocyte by/in uterine tube - Shortly before ovulation, the fimbriae of the infundibulum have placed themselves around the ovarian stigma. - The sweeping ) ‫( كنس‬action of the fimbriae and fluid currents produced by the cilia of the mucosal cells of the fimbriae move the secondary oocyte and corona radiata into the funnel-shaped infundibulum. -The secondary oocyte then passes into the ampulla of the tube by aid of peristalsis movements (toward the uterus) of the wall of the tube. 1. Ovary, 2. Follicle that is about to rupture, 3. Infundibulum,4. Fimbriae, 5. ampula of uterine tube tube, 6. proper ovarian Ligament, 7. Suspensory ligament 1. Uterine tube cut open, 2. Closely apposed fimbriae, 3. Follicle fluid that has flowed out, 4. Secondary oocyte with corona radiata, 5. Ovary with follicles in various stages of development and atresia, 6. zona pellucida, 7. First polar body, 8. Secondary oocyte, 9. corona radiata, 10. Arrested spindle apparatus  The fate of the ovulated follicle - Shortly after ovulation, the walls of the ovarian follicle and theca folliculi collapse and form corpus haemorrhagic (CH). - Under LH influence, the CH develops into the corpus luteum (CL) which secretes progesterone to prepare the endometrium for implantation. - If the oocyte is fertilized, the CL enlarges to form CL of pregnancy which remains functionally active until the placenta starts the production of progesterone that is necessary for the maintenance of pregnancy. - If the oocyte is not fertilized, the CL degenerates. It is then called CL of estrus which is subsequently transformed into a corpus albicans.  2. The journey of the sperm - The newly formed sperms in the testis are not able to fertilize the ovum because the sperms have to go through several maturation and activation steps in a series of different locations to penetrate the oocyte. Maturation steps of sperms in epididymis - After spermiation, the sperms move to lumen of seminiferous tubules then to rete testis and then pass to the epidydmial duct through the effernt ductules to finally store in the tail of the epididymis where they undergo the following changes: a. The sperm head becomes smaller and more compact because DNA becomes more condensed. b. Molecular and chemical changes in the structure of the plasma membrane. c. The ability for motility is achieved but at the same time inhibited by the environment.  7 1. Testis / testicles, 2. Ductus 1.Testis, epididymidis, 3. Ductus deferens 2. Seminiferous tubules (tubuli seminiferi contorti), 4. Ampulla ductus deferens 3. testicular Lobules, 5. Seminal vesicle gland 4. efferentes Ductuli, 6. Ejaculatory duct , 7. Prostate 5. Ductus epididymidis, 5a. head of epididymidis, 8. excretory duct of the prostate 5.b. body of epididymidis, 5.c. tail of epididymidis, 9, Bulbourethral gland 6. ductus deferens, 10. Urethral gland (Littre's gland) 7. rete testis 11. Urethra, 12. Urinary bladder  Activation steps of the sperms during ejaculation - Ejaculation is a process by which the semen deposited in female genital system (mainly vagina or uterus, see table). NB: The semen consists of sperms (10%) which produced by testis and seminal plasma (90%) which produced by seminal glands (seminal vesicle, prostate, bulbo urethral). - During ejaculation, sperms become more motile due to: 1. The mechanical stimulation. 2. The seminal plasma - which nourishes sperms - forms a favourable alkaline media in vagina. NB: Freshly ejaculated sperms are unable to fertilize oocytes. Sperms must undergo two modifications in the female genital tract to fertilize the ova. These modifications are A) Sperm capacitation and B) acrosome reaction. Sperm capacitation - Capacitated sperms show no morphologic changes, but they are more active (hyperactivity) due to removal of inhibiting factors from acrosome and cell membrane of sperm head. - Capacitation starts in the uterus and ends in isthmus of uterine tube. - Completion of capacitation permits acrosome reaction to occur (i.e. sperms which are not capacitated, can’t fertilize ova).  Transport of the sperm in the female genital tract - Passage of sperms through female genital tract aided by: a. muscular contractions of the walls of these organs. b. prostaglandins in the semen stimulate uterine motility. - Fructose, secreted by seminal glands, is an energy source for sperms in the semen. - The sperm takes from 5 to 45 minutes to complete the journey. - Sperms probably do not survive for more than 48 hours in the female genital tract. However in birds, sperms can be survive in the spermatic nest in the female vagina for 21 days.  Sperm filtration - Although millions of sperms deposited in female genital tract after ejaculation, only approximately 200 sperms reach the fertilization site. Most sperms are filtered and then degenerated by the female genital tract. - The sites of sperm filtration are: a. Portio vaginalis because it is so narrow. b. Cervix - Before ovulation, the cervical mucus is viscus and forms network that hinder the sperm movement. - After ovulation occurs, the cervix becomes wider; the cervical mucus becomes less viscid, making it more favourable for transport of typical sperms. - The cervical mucus functions as a filter in which atypical sperm cells remain hanging. c. Isthmus and ampulla of uterine tube because they are so narrow.  1. vagina, 2.Portio vaginalis of cervix, 3 Cervix canal,4. Isthmus, 5.Ampulla, 6. Fimbriae, 7.Endo, 8.Myo-metrium, 9. Uterine cavity, 10. Before ovulation After ovulation fertilization site the cervical mucus (2) the cervix becomes The sperms travel is marked in yellow. The (which secreted from wider, the cervical triangles indicate the places where sperms can cervical glands, 3) is mucus (4) becomes wait for longer periods of time. They are the viscus and form network less viscid, making it crypts in the cervix, the region of the isthmus that hinder the sperm at more favourable for and the ampullary part of the uterine tube. portio (1). sperms transport (5). Fertilization sequences (mechanism of fertilization) 1- Penetration of corona radiata Capacitated sperms can penetrate corona radiata by help of: a. Hyaluronidase enzymes; which release from the acrosome to dissolve the cement material (hyaluronic acid) that connects between the corona cells. b. Whipping ) ‫ (ضرب بالسور‬motions of sperms tails; which push sperms through the corona radiata. 2- Binding to zona pellucida - A sperm binds to a specific receptor on the zona pellucida [this binding is species specific and so prevents union with foreign sperm]. - The most important sperm receptor, which plays a crucial role in fertilization, is the ZP3. 3- Penetration of zona pellucida (acrosomal reaction) 1. multiple fusions reaction between the outer acrosomal membrane and the overlying plasma membrane. 2. extensive formation of vesicles at fusion sites. 3. breakdown of these vesicles produces multiple openings. 4. release of acrosomal contents (including; hyaluronidase and acrosin) to participate in corna radiata and zona penetration, rspectively. 5. exposure of inner acrosomal membrane 4. Fusion with plasma membrane a. After penetration of zona pellucida, the sperm head enters the perivitelline space. b. Fusion occurs between the post-acrosomal cell membrane of the sperm head and the cell membrane of the oocyte (species- specific fusion). c. The processes of the oocyte cell membrane form a fertilization cone around the sperm head and mid piece. When this cone retracted, the sperm contents engulfed inside the oocyte and the tail membrane remain as appendage. NB; Sperm mitochondria degenerate once they enter the cytoplasm of the oocyte, and thus all mitochondria in the cells of an individual derive from the mother alone. 5. Oocyte activation and reaction (a) Cell membrane reaction (vitelline block) - Once cell membrane of sperm and ovum fused, the structure of their surface receptors become changed and so other sperms can’t fuse with cell membrane of oocyte. This prevents polyspermy (penetration of an ovum by more than one sperm). (b) Cortical reaction - Before fusion between the two cell membranes, cortical granules contact with the inner surface of oocyte cell membrane. - After membrane fusion, these granules rupture and release their contents outside making the cell membrane impermeable to other sperms and harden zona pellucida. (c) Zona pellucida reaction - After membrane fusion, the zona pellucida reacts to prevent polyspermy through; a. Blocking (or changing) the specific binding receptors (ZP3) for other sperms. b. Hardening of their structure (by cortical granules contents). (d) Resumption of meiosis II - The secondary oocyte (which was previously hindered in metaphase II) completes meiosis II. This produces mature oocyte (with pronucleus) and second polar body. 6. Formation of zygote - When the two pronuclie approach to each other, their nucleic membranes dissolve and their chromosomes align themselves on a common spindle at the equator and the cell now called zygote (2N).  Cell mem. of ovum X Cell mem. of sperm tail Centeriol Nucleus Monospermy and polyspermy Monospermy means fertilization of ovum by only one sperm Polyspermy means fertilization of ovum by more than one sperm. It may be; Physiological polyspermy Pathological polyspermy - Occurs normally in birds, Abnormal case which leads to insects and reptiles so no death of zygote death for zygote - One sperm forms paternal pronucleus while the others degenerate inside the ovum Parthenogenesis - The developing of an embryo from an ovum that has been activated by means other than sperms. It may be; Natural Artificial parthenogenesis parthenogenesis Such as in turkey, Induced by X-rays, chicken (rare), frog, ultra violet irradiation, star fish and bony fish needle, heat shock some salts and acids  Cleavage Cleavage (segmentation) can be defined as successive mitotic division of zygote occurs directly after fertilization inside the uterine tube. It is characterized by: 1. Increase number of cells (which now called blastomeres) without increase in size (no growth). Therefore daughter cells become smaller with each division (so it called segmentation). 2. The increased numbers of blastomeres follows double sequences (2, 4, 8, 16……etc) in majority of species. 3. No changes in general shape except formation of blastocoele. A) Holoblastic (total) cleavage B) Meroblastic (partial) cleavage - The entire ovum (cytoplasm + nucleus - Only cytoplasm and nucleus are + yolk) are divided during cleavage. divided - Subdivided according to size of - Subdivided according to site of mitosis blastomeres into; into; 1. Equal 2. Unequal 1.Discoidal 2. Superficial all blastomeres are blastomeres are divisions occur only divisions occur equal in size smaller at animal in animal pole only at the pole (micromeres) (where cytoplasm periphery (where but larger at and nucleus found) nucleus and vegetable pole and form blastoderm cytoplasm found) (macromeres) due to over yolk around the yolk yolk retards cytokinesis at vegetable pole Occur in Micro-telolecithal, Macro-telolecithal, Centrolecithal ova Isolecithal, Mesolecithal ova polylecithal ova (insects) Oligolecithal ova (lower fishes and (higher  Cleavage in birds - Type of ova is Polylecithal, anisolecithal (macro- telolecithal). - Type of cleavage is Discoidal meroblastic. NB: - Nucleus and cytoplasm of the ovum (egg) are located in animal pole and together they called blastodisc which forms a cap on the yolk. - The cleavage is completed during the journey of ova in the oviduct (24hr). Mechanism of cleavage in birds - The 1st cleavage division (furrow) is vertical (= meridional, but only at animal pole) and divides the zygote into two equal blastomeres. - The 2nd furrow is also meridional and right angle to the 1st one. This forms 4 blastomeres. - The 3rdfurrow is radial on the 2nd one and form 8 blastomeres. - The 4th furrow is circumferential which divides the blastomeres into central (8 micromeres) and peripheral (8 macromeres) rows. NB; - Because planes of first four divisions are vertical, blastomeres are formed in one plane (row). --- Because their furrows do not extend to their deepest portions, the first 16 blastomeres are not completely enclosed in their own cell membranes but are open ventrally  Vitelline membrane Vitelline membrane Yolk Cleavage furrow 1 2 3 4 1 2 1 2 4 3 Yolk open blastomere cell G - A-F top (dorsal) view 2 cells Blastodisc 4 cells - G, H sagittal sections at level indicated in C and H G - Roman numbers (I, II, III……etc) indicates the cleavage furrows. - Arabic numbers (1,2,3……etc) indicates the blastomeres. 8 cells 16 cells H Subgerminal cavity 32 cells H 64 cells - All further furrows are horizontal (equatorial) which produce multilayers central cells and single layer peripheral cells. -By 64-128 cell stage, the most centrally situated cells become completely surrounded by a cell membrane, and so loss contact with the underlying yolk. This leads to formation of the subgerminal cavity between the embryo and yolk. There is still a region of mixed yolk and cytoplasm at the periphery. -- By the time of laying, the blastodisc becomes a flat and now called the blastoderm which consists of several regions which are; a- The area pellucida - It is a translucent central region of the blastoderm which consists of a thin layer of cells that overly the subgerminal cavity. - It forms embryonic tissues. b- The area opaca - The more opaque peripheral ring that surrounds area pellucida. - It is darker due to presence of large numbers of intracellular yolk droplets. - It composes of large cells which digest the yolk which nourishes the embryo. - Forms extra-embryonic tissues. c- The marginal zone - It lies between the area opaca and area pellucida. - It is more thicker posteriorly forming the posterior marginal zone (PMZ). d- Koller’s sickle - It is a crescent-shaped area anterior to PMZ.  2 J Anterior Posterior 1 Anterior upper layer 2 2 1 3 4 5 3 I J lower layer (hypoblast) 1. area pellucida, 2. area opaca, 3. posterior marginal zone, 4. subgerminal cavity, 5. delaminated upper cells to form the hypoblast, 6. Koller’s sickle cells Epiblast K Blastocoele Area opaca L L P A Hypoblast Subgerminal space The end result of cleavage - Formation of modified blastula known as balstoderm (in area pellucida) which consists of epiblast (dorsal), hypoblast (ventral) and blastocoele in between. This bilaminar layer formed as following: A) Formation of hypoblast layer - The hypoblast formed as a one layer of cells within the subgerminal cavity and over the yolk. - It has tow origins; a. From the upper multilayer cells by dorsoventral delamination (the epiblast cells separated from each other and migrate ventrally toward the yolk). b. From PMZ and koller’s sickle cells by posterior to anterior migration. - Later, it will form the extraembryonic endoderm of the yolk sac. B) Formation of epiblast layer - It is the one layer cells of the upper layer which does not delaminate. C) Formation of blastocoele - Due to formation of hypoblast, the subgerminal cavity becomes divided into upper portion between epiblast and hypoblast, known as blastocoele and a smaller lower subgerminal space (between hypoblast and yolk). Cleavage in mammals - Type of ova is Oligo-isolecithal and type of cleavage is Equal holoblastic. Mechanism of cleavage 1. The 1st cleavage division is meridional and produces 2-cells stage. This division takes 24 hr. NB: Each further cleavage division takes only 12hr. 2. In the 2nd cleavage; one of the two blastomeres divides meridionally and the other divides equatorially. The daughter cells rotate 90° to make the two planes of division right angle on each other. This type of cleavage is called rotational cleavage. 3. The 3rd, 4th and 5th cleavages are equatorial (horizontal) and forms 8-cells, 16-cells and 32-cells stage, respectively, which undergo compaction which is characterized by; a. The blastomeres maximize their contact with one another (by tight junction between the outer cells and gap junction between the inner cells). b. Formation of a compact ball of cells which called Morula. - The morula consists of few inner blastomeres surrounded by more outer blastomeres. - The actual numbers of blastomeres forming the morula is variable according to species; a. 8-cells in human and mice b. 16-cells in sheep c. 32-cells in cattle, this is ideal for embryo transfer - Most embryos are in morula stage when enter the uterus from the uterine tube and this stage is reached in 4 to 5 days in most of domestic animals.  Rotational cleavage Other animals Mammals EM 4. Further cleavage divisions lead to formation of the blastocyst as following; - The outer cells of the morula secrete fluid into the inner cells to create a fluid-filled space called the blastocystic cavity or blastocoele. As fluid increases, it separates the blastomeres into two parts: a. A smaller outer cell layer called trophoblast or trophectoderm (trophe = food), which facilitates absorption of nutrients early in development and later share in formation of chorion and amnion. b. A larger centrally located blastomeres called inner cell mass (embryoblast), which gives rise to the embryo and its associated fetal membranes.  - Species variation was observed in the fate of the layer of trophoblast cells located over the inner cell mass (these cells called Rauber’s layer); 1) In primates (human and monkeys) and rats; Rauber’s layer persists. 2) In domestic animals; Rauber’s layer degenerates prior to implantation and thus allows the inner cell mass (which now called embryonic disc) to become part of cellular wall of the blastula and to expose to the same environment as are the remaining trophoblast. NB; - Because blastomeres arise only through the cleavage of the zygote and all are found inside the zona pellucida, which cannot expand, no growth is seen. Every new cell is thus only half as large as the cell from which it derives. Thus, cleavage leads to increase in number of cells but without increasing the volume of the total cell mass. This is very important to decrease the size of zygote (which is 150 micron) to the normal adult cell size (7 micron). - All mammals blastocyst have inner cell mass except in Marsupials (such as Kangaroo) which has only trophoblast and blastocoele. Blastocyst Species variation Rauber’s layer A. In Marsupials such as Kangaroo (No inner cell mass) B. In primates and rats, the Rauber’s layer remains above the inner cell mass. C and D. In domestic animals Rauber’s layer degenerates and the inner cell mass now called embryonic disc. Emergence of blastocyst from Zona Pellucida - During cleavage, the zona pellucida prevents the blastocyst from adhering to the oviduct walls. When such adherence does take place, it is called an ectopic or tubal pregnancy. This is a dangerous condition because the implantation of the embryo into the oviduct can cause a life-threatening hemorrhage. - When the embryo reaches the uterus, however, it must emerge from the zona so that it can adhere to the uterine wall. - The blastocyst emerges by two methods; a. In rat and horse, the zona pellucida completely degenerates. b. In human, cattle, pig, sheep and dog the blastocyst hatches from a cracked (holed) zona pellucida. A trypsin-like protease from trophoblast lyses a small hole in the zona and blastocyst squeezes through that hole. - The sodium pump (a Na+/K+-ATPase) in the plasma membranes of trophoblast facing the blastocoel also aids in blastocyst emergence. It pumps sodium ions into blastocoele which then draws in water osmotically, thus enlarging the blastocoel within the zona pellucida. Blastocyst hatching from zona pellucida Exp 1. Zona pellucida Em 2. Trpphoblast 3. Hypoblast of inner cell mass 4. Blastocoele 5. Epiblast of inner cell mass Note only the in 6. Expansion of blastocyst. - Shedding of the zona pellucida permits the hatched blastocyst to increase rapidly in size (mainly due to enlargement of blastocoele and trophoblast) and to change slightly in shape. - Species variation is evident in the expansion of the blastocyst prior to implantation: 1- In primates (human and monkeys) and rats, because the blastocyst invades the endometrium, very little expansion occurs. 2- In dogs, cats and rabbits, associated with central attachment, there is moderate oval expansion of blastocyst. 3- In Equine, the blastocyst shows spherical expansion but becomes surrounded by a capsule which allows free movement of blastocyst throughout the uterus, thus exposing a greater amount of uterine surface for placentation. The capsule remains until 21 days of pregnancy and is a factor for delayed implantation in equines. 4- Ruminant and pig blastocysts undergo huge thread- like expansion at a rate of 1cm per 1 hour and can reach a length of 1.57 m on the 13rd post coitus day and so no need for movement of blastocyst (as in equine) as the largely expanded blastocyst contact with a larger area of uterus. For this huge size we can find the trophoblast and blastocoele of blastocyst in both gravid and non gravid uterine horn. da Expanded blastocyst in pig and ruminant Embryo (post coitus) Note that the expanded part of blastocyst involves only the trophoblast and the blastocoele, while the inner cell mass grows only slightly (arrow). End result of cleavage in mammals (Formation of bilaminar embryonic disc) - From the inner cell mass, cells delaminate and migrate ventrally to form a new cell layer inside the trophoblast layer. The new layer of cells is called the hypoblast. The remaining inner cell mass is called epiblast. The embryo now takes a bilaminar disc shape formed of Epiblast and Hypoblast. - As in birds, the epiblast gives rise to all three germ layers, however Hypoblast makes no contribution to germs layers and only gives rise to the extra embryonic endoderm of umbilical vesicle (yolk sac) and allantois. - The separation of the hypoblast layer from the epiblast establishes a space (coelom) which later gives rise to body cavities.  Cleavage in Fallopian tube Fertilization Implantation Zona pellucida Compaction Zygote 2-cells stage 4-cells 8-cells 16-cells Blastocyst Morulla 1- ovary 2- Fallopian tube 3- Endoometrium 4- Myometrium 5- Uterine cavity  Differences between cleavage in mammals and other animals 1. Cleavages in mammalian ova are among the slowest in the animal kingdom. 2. Presence of rotational cleavage (2nd cleavage). 3. Asynchrony of early cell division. Mammalian blastomeres do not divide at the same time. Thus, mammalian embryos do not increase from 2- to 4- to 8-cell stages, but rather any number of cells can be present in the embryo. 4. Unlike almost all other animal genomes, the mammalian genome is activated during early cleavage. 5. Presence of compaction. 6. Formation of specialized type of balstula which called blastocyst.  Cleavage timing in cattle (ideal for embryo transfer) Blastocyst Species variation Rauber’s layer A. In Marsupials such as Kangaroo (No inner cell mass) B. In primates and rats, the Rauber’s layer remains above the inner cell mass. C and D. In domestic animals Rauber’s layer degenerates and the inner cell mass now called embryonic disc.  2 J Anterior Posterior 1 Anterior upper layer 2 2 1 3 4 5 3 I J lower layer (hypoblast) 1. area pellucida, 2. area opaca, 3. posterior marginal zone, 4. subgerminal cavity, 5. delaminated upper cells to form the hypoblast, 6. Koller’s sickle cells Epiblast K Blastocoele Area opaca L L P A Hypoblast Subgerminal space  Gastrulation Gastrulation is a stage in embryonic development during which the bilaminar (two layers) blastula is converted into a trilaminar (three layers) structure consisting of an outer ectodermal, a middle mesodermal and an inner endodermal layer from which the embryo organs will be developed. The events of gastrulation are similar in both birds and mammals. These events start with formation of the primitive streak and primitive node through them the epiblast cells ingress forming (endoderm and mesoderm) and the remaining non-ingressed epiblast cells form the ectoderm. 1- Formation of the primitive streak (PS) - The PS initially appears as a dark triangular region in the caudal part of epiblast in midline of blastoderm within area pellucida. The darkness is due to thickening of the blastoderm as a result of convergence of the epiblast cells toward the midline. - It then elongates from posterior to anterior direction until reaching the full length (about two-thirds of area pellucida). - The PS consists of a central furrow called primitive groove which surrounded by two primitive ridges or folds. - The Hensen’s (primitive) node appears as a condensation of epiblast in the cranial end of the PS - The Hensen’s (primitive) pit is a small pit within the node and continues caudally with the primitive groove. Accumulation of epiblast cells in the posterior portion of the embryonic disc results in the formation of the primitive streak Anterior Posterior Formation of primitive streak and primitive node  2- Formation of trilaminar disc (cell ingress and migration through PS) - Epiblast cells at the edge of the blastoderm continually converge toward the midline and then ingress through the primitive streak into the blastocoele. - The ingressed cells lose their epithelial character and become mesenchymal cells to migrate and form the endoderm and mesoderm. 2. a Formation of endoderm - The first population of epiblast cells which pass through PS form endoderm. - After ingression, they displace the hypoblast outwardly toward the edge of area opaca. Formation of the endoderm the first ingressed epiblast cells form endoderm which will displace the hypoblast cells 2.b Formation of mesoderm 1. Axial mesoderm 2. Paraxial mesoderm 3. Lateral mesoderm Come from anterior come from middle come from the epiblasts epiblasts caudal epiblasts ingress through ingress though the ingress through Hensen’s node cranial third of PS caudal two third of and the caudal half PS of the Hensen’s node then migrate cranially Then migrate then migrate cranio- but stay in the midline cranially and on laterally each side of axial mesoderm. Includes the The presomitic Intermediate and notochord mesoderm and lateral plate - The mesodermal cells can be found at any portion of the space between epiblast and endoderm except at the following sites; a. Buccopharyngeal or oropharyngeal membrane b. Cloacal membrane. At these two membranes the ectoderm directly fuses with the endoderm without any mesoderm in between.  Anterior A Formation of mesoderm and ectoderm Paraxial mesoderm Intermediate, Lateral plate mesoderm Posterior B Anterior Posterior Primitive node Epiblast (ectoderm) Primitive streak A hypoblast Notochord  3. PS regression After the end of ingression (when the division of the cells at the periphery decrease and compensation for the depletion of cells lost from epiblast cease), the PS becomes regressed and the Hensen’s node moves more caudally especially with large development of neural tube. Regression of primitive streak and node  Derivative of the three germinal layers 1- Derivatives of the endoderm - In general the endoderm gives rise to the lining epithelia of the gut which will form the lining epithelia of; a. The digestive system (from pharynx to rectum) except the oral cavity and the caudal portion of anal canal which are ectodermal in origin. b. The lower respiratory tract (from larynx to lung). 2- Derivatives of the ectoderm a. Surface ectoderm; gives rise to the epidermis of skin. b. Neural ectoderm; gives rise to; 1. Neural tube; which will form the central nervous system (brain and spinal cord) 2. Neural crest; which will form,  i) Peripheral nervous system (which includes peripheral nerves (cranial, spinal nerves) + autonomic nerves (sympathetic and parasympathetic) + head and spinal ganglia). ii) Others (melanocytes, chromaffin cells of adrenal gland, some bones of skull). 3. Neural placodes, which will form cranial ganglia. 3- Derivatives of the mesoderm 3. a The notochord (axial mesoderm) - Formation of notochord passes into 4 successive stages which are; 1. The notochordal (chordal) process: - It is a median cellular cord of mesenchymal cells that ingressed from Hensen’s node and extends cranially between the ectoderm and endoderm until the prechordal plate (which is the primordium of the oropharyngeal membrane). 2. The notochordal canal: - The primitive pit of the Hensen’s node extends anteriorly inside the notochordal process which becomes canalized and form a notochordal canal which communicates with the amniotic cavity. Formation of Notochord Formation of Notochord 3. The notochordal plate: - The floor of the notochordal process fuses with the underlying endoderm and these fused layers gradually undergo degeneration resulting in losing of notochordal canal floor. This leads to a temporary communication between amniotic cavity and yolk sac by means of neurenteric canal. - The roof of the notochordal canal remains and forms of a notochordal plate which attached to the endoderm and form part of yolk sac roof. 4. The definitive notochord - The notochordal plate infolds (from cranial to caudal) to form a rod solid structure called the definitive notochord. - The notochord becomes finally detached from the endoderm of the yolk sac.  Formation of Notochord E F G G  Formation of Notochord I H J I J Notochord functions a) Defines the longitudinal axis of the embryo. b) Gives the embryo some rigidity (so act as a primitive skeleton). c) Induces the neuroectoderm to form the neural plate, the primordium of the central nervous system. d) Induces somites to form the musculoskeletal structures. Fate of notochord The notochord degenerates as the bodies of the vertebrae form, but small portions of it persist as the nucleus pulposus of each intervertebral disc. 3.b The paraxial mesoderm Definition It is thick bands of mesodermal cells lie on either side of the neural tube and notochord and medial to the intermediate and lateral plate mesoderm. Portions a. Unsegmented caudal portion (the presomitic mesoderm) from which (b) arise. b. Segmented middle portion (the somites) c. Unsegmented cranial portion (the head mesenchyme) 3.b1 Somite formation (somitogenesis) - The somites are paired blocks of mesoderm that formed by budding off from the rostral end of the presomitic mesoderm. - The number of somites in an embryo is indicative of age because somites develop chronologically (at regular periodic intervals, each 90 minutes one pair of somites is formed), in craniocaudal order. - Each somite composed of outer epithelial wall and inner mesenchymal core called somitocoele. - The successive somites separated from each other by intersomitic cleft. Each somite divides into anterior (cranial) and posterior (caudal) half by Von Ebner’s fissure. Formation of somites Somites Epaxial myotome Presomitic mesoderm Hypaxial myotome Hensen’s node Primitive Streak - Derivatives of somites are; (1) Vertebrae and ribs. (2) Muscles of the rib cage, body wall, limbs and back. (3) Dermis of the skin of the back region. A= Paraxial mesoderm, B= Intermediate mesoderm, C= Lateral plate mesoderm, 1= Ectoderm, 2= Somite, 3= Somitocoel, 4= Neural tube 3.b2 Somites subdivision (fate of somites) 1. The sclerotome - It is the precursor for the vertebrae and proximal part of ribs. - It arises from the ventromedial cells of the epithelial somite. These cells undergo epithelio-mesenchymal transition (transformed from epithelial to mesenchymal cells). 2. The dermomyotoe - It is the precursor for muscles and dermis. - It arises from the dorsolateral portion of the epithelial somites which remains epithelial. The dermomyotom is further subdivided into: a. The dermatome which forms the dermis of skin in back region. b. The myotome which forms the muscles of back, abdomen and limbs. - The myotome is also subdivided into; i) The epaxial myotome (the dorsomedial lip) from region closest to the neural tube. It will form the epaxial muscles (back muscles). ii) The hypaxial myotome (the ventrolateral lip) from region furthest from the neural tube. It will form hypaxial muscles (intercostal, abdominal and limb muscles). Cross sections of embryos showing the gradual (A, B, C through D) differentiation of the somites. 1: Neural groove; 2: Somite; 3: Notochord; 4: Sclerotome; 5: Dermamyotome; 6: Neural tube; 7: Dorsal aorta; 8: Dermatome; 9: Myotome. 3.c The intermediate mesoderm - It lies between the paraxial mesoderm and the lateral plate mesoderm. - It forms the urogenital ridge which will give rise to the kidney and gonads. 3.d The lateral plate mesoderm - It is composed of the following two layers: a- Somatic mesoderm - It connects with the ectoderm and form the somatopleure, which gives rise to the lateral and ventral body wall. - Somatic mesoderm forms the: - Dermis of the skin in the ventral region - Parietal serosa of the ventral body cavity - Bones, ligaments, and dermis of the limbs b) Visceral (splanchnic) mesoderm - It connects with the endoderm and form the splanchnopleure, which gives rise to the wall of the digestive tube. - Splanchnic mesoderm forms: the heart and blood vessels, visceral serosa, smooth muscles of gut and most connective tissues of the body. - The bones of the body are derived from the following three sources: 1. Sclerotome: Forms the endochondral vertebrae and proximal part of ribs. 2. Somatic mesoderm: Forms the endochondral appendicular skeleton and intramembranous sternum. 3. Neural crest: Forms the intramembranous bones of skull (bones of skull roof). - All muscles develop from mesoderm: skeletal muscle originate from somites, while cardiac and smooth muscles originate from splanchnic mesoderm. Derivatives of mesoderm Notochord  Placentation Definition of placenta Placenta is a specialized structure of mammals which separates the blood of mother from that of fetus. Placentation means placenta formation. Steps of placentation: A) Implantation, B) Decidua and C) Formation of placenta A) Implantation Definition of implantation It is the interaction process between blastocysts and the uterine mucosa (endometrium) which normally takes place in uterine body (human) and/or uterine horn (domestic animals). Implantation is very important for placentation to occur and so it precedes placentation. Types of implantation Central Eccentric Interstitial (superficial) (invasive) Characters The The blastocyst Blastocyst is blastocyst presents in a crypt completely remains in (recess) so it has two embedded inside the uterine portions, one inside the endometrium cavity the endometrium and the other outside (in the uterine lumen) Examples Ungulates Carnivores and Human (equine, rodents ruminants and pigs) Types of implantations Mechanism (stages) of implantation 1. Adplantation (apposition) of the blastocyst to the uterine mucosa After blastocyst emergence from the zona pellucida, it starts to contact with the maternal endometrium with its embryonic pole. NB: During the whole time from ovulation up to implantation, the oocyte is enveloped by the zona pellucida, the role of which changes. a) Before fertilization, it is united with the cells of the corona radiata during the transport of the oocyte within the fallopian tube and so facilitates transport of nutrients to the oocyte. b) At fertilization, it facilitates the acrosomal reaction of the sperm cells. c) After the cortical reaction has taken place, it undergoes physical and chemical changes prevent polyspermy. d) It prevents premature implantation of the embryo in the uterine tube. e) It possesses no surface antigen and thus acts as an immunological barrier in relation to the mother. 2. Adhesion to endometrium Trophoblast cells at embryonic pole adhere to endometrium by aid of cell surface glycoproteins. 3. Invasion of the trophoblast - Trophoblast differentiates into: 1- Inner cytotrophoblast (CT) which consists of an inner layer of ovoid, single- nucleus cells. 2- Outer syncytiotrophoblast (ST) which forms a syncytium, i.e., a multi-nucleic layer without cell boundaries that arises from fusion of cytotrophoblast cells at periphery of embryonic pole. Mechanism of invasion - The ST produces lytic enzymes that cause apoptosis of endometrial cells and then penetrates into stroma. This results in formation of penetration defect (opening) in endometrium and erosion of maternal capillaries wall making blastocyst floats in a lake of maternal blood. - The invasion becomes complete when ST entirely surrounds the embryo and the penetration defect is closed by fibrin plug. - Lacunae (spaces) appear in the ST (this stage called lacunar stage). - The lacunae communicate with each other and with the eroded maternal capillaries forming a single intervillous space which gives the ST a trabeculated appearance (this stage called trabecular stage) and these trabeculae called chorionic villi which distribute over the chorionic sac. NB:→ The first two stages of implantation Stages of (apposition and Implantation adhesion) occur in all mammals A) Adplantation however, the last B) Adhesion stage (the invasion) occurs only in (A) human (completely) and carnivores and rodents (partially). → The invasion only occurs in the compact layer of (B) the endometrium and does not extend to the other layers (spongy and basal layer).  1 4 5 2 3 2 5 4 6 Stages of Implantation (C – F) Invasion 1. Endometrium 2. Cyto- - trophoblast 3. Syncytio- 3 - trophoblast 4. Epiblast (C) (D) 5. Hypoblast 6. Blastocoele 5 9 7. Eroded maternal 8 capillary 8. Lacuna 8 9. Fibrin plug 10. Primary villus 10 7 (E) 7 (F) Abnormal sites of implantation (ectopic implantation) A) Outside the uterus 1- Ovarian (in the ovary) 2- Infundibular (in the infundibulum of the tube) 3- Tubal (in the tube, most common one, can cause sever internal haemorrhage) 4- Abdominal (in the peritoneum, the only one can continue until birth) 5- Pelvic (in the pelvis) B) Inside the uterus In any place rather than the normal implantation site. 1- Interstitial (between the tube and the uterus) 2- Cervical (in cervix)  Abnormal sites of implantation (ectopic implantation) 1. Ovarian 2. Infundibular 3. Tubal 4. Interstitial 5. Cervical 6. Abdominal 7. Pelvic  B) Decidua Decidua (fall off) is the compact layer of the endometrium of pregnant uterus which react to the implantation. Decidua basalis Decidua Decidua capsularis parietalis Definition The part of decidua The part of The part of where the decidua covers decidua lines the implantation takes the embryo uterus away place. from the embryo Location lies between the lies between the lies on the embryo and the embryo and the opposite (non muscular wall of uterine cavity implanted) the uterus uterus wall Occurren All mammals Human All mammals ce Later on, when the human fetus becomes so large, the decidua capsularis comes into contact with the decidua parietalis causing  Fate of Decidua (classification of placenta acc. to Decidua fate) 1- Decidua 2- Non- Decidua 3- Contra placenta Decidua Character Placenta comes Placenta comes Placenta does s out after birth out after birth not come out followed by loss but does not after birth and of maternal followed by loss so no loss for tissues which of maternal either maternal cause bleeding. tissues and so no or fetal bleeding. membranes and so no bleeding. Example animals with animals with Kangeroo. (fetal interstitial and central membranes  (A) Early pregnancy (B) Late pregnancy 1. Decidua parietalis, 2. Decidua capsularis, 3. Decidua basalis, 4. Uterine cavity , 5. Smooth chorion (laeve), 6. Bushy chorion (chorionic frondosum), 7. Amniotic cavity, 8. Decidua capsularis and parietalis fuse together.  C) Formation of the placenta Placenta = chorion frondosum which carries the chorionic villi (fetal part) + Decidua basalis (maternal part). Development of chorionic villi 1- Primary villus 2- Secondary 3- Tertiary villus villus Formed when a Formed when the Formed w