General Embryology Book 3 PDF

Anatomy and embryology department Faculty of medicine Menoufia University foundation i b general embryology by Staff members of anatomy and embryology department 2024/2025  ‫‪Foundation I B: G...

Anatomy and embryology department Faculty of medicine Menoufia University foundation i b general embryology by Staff members of anatomy and embryology department 2024/2025  ‫‪Foundation I B: General embryology‬‬ ‫رؤية كلية الطب‬ ‫أن تكون كلية معتمدة أكاديميا ذات سمعة محلية وإق ليمية ودولية تتميز‬ ‫بالريادة في جودة التعليم الطبي والرعاية الصحية‬ ‫رسالة كلية الطب‬ ‫أن تلتزم كلية الطب جامعة المنوفية بتخريج طبيب بشري طبق ا للمعايير‬ ‫القومية األكاديمية المرجعية‪ ،‬ق ادرا على تلبية احتياجات سوق العمل المحلي‬ ‫واإلق ليمي‪ ،‬ذو مهارة في إجراء أبحاث علمية لتطوير المهنة والخدمة الطبية‬ ‫المقدمة‪ ،‬حريصا على التدريب والتعليم المستمر بما يدعم خدمة المجتمع‬ ‫والبيئة المحيطة في إطار االلتزام بأخالقيات المهنة‪.‬‬ ‫‪Contents‬‬ ‫‪Page 2‬‬  Foundation I B: General embryology Foundation I University: Menoufia Faculty: Medicine A - Administrative Information Module Title: Foundation I Code :MED 101 Department offering the Module: Anatomy, histology, physiology and biochemistry departments Program on which the Module is given: Menoufia M.B.B.Ch Credit-points Program (5+2) Academic year: 1st Year Semester: I Date of specification: 2023 Date of approval by faculty council: 2023 Credit/taught hours: Credit points: 12 points Teaching hours Lectures Practical Activities A- Anatomy department 30 hours 30 hours 12 hours B- Histology department 15 hours 15 hours 6 hours C- Physiology department 15 hours 15 hours 6 hours D- Biochemistry department 30 hours 30 hours 12 hours Total 90 hours 90 hours 36 hours Contents Page 3  Foundation I B: General embryology B- Professional Information I. Overall aims of the course : a) Upon completion of this course the student will have reliably demonstrated the ability to provide a basic knowledge about anatomical structure and embryological development of the human body and the basic histological structure and ultrastructure and identify them under the microscope, with functional and clinical correlation whenever possible. b) Acquire an appropriate functional background of cells, tissues, organs& systems. c) Integrate physiological data and mechanisms with the ongoing basicsciences and clinical applications. d) Be familiar with the physiological frameworks within which healthand diseases states could be distinguished. e) Follow the rapidly changing & inflating details about molecular biology & genetics f) Help students to become familiar with the biochemical knowledgethat will assist students in understanding biochemical alteration in health and disease. g) Enable students to be oriented with structure and functions of important structures as carbohydrates, proteins and lipids. II- Intended Learning Outcomes: Competency Area 4.1 4.1.1. Identify the normal structure of the skeletal, joint and body cavities. 4.1.2. Describe the basic anatomical structure of body bones 4.1.3. Demonstrate the surface landmarks of the underlying bones, muscles, joints and tendons. Contents Page 4  Foundation I B: General embryology Contents Structure of male genital system............................................................... 6 Structure of female genital system............................................................ 8 Gametogenesis........................................................................................... 10 Female reproductive Cycles.................................................................... 18 First week of pregnancy........................................................................... 27 Second week of pregnancy....................................................................... 36 Third week of pregnancy.......................................................................... 40 Fourth week of pregnancy........................................................................ 51 Fetal membranes....................................................................................... 55 References.................................................................................................. 71 Contents Page 5  Foundation I B: General embryology Structure of male genital system Contents Page 6  Foundation I B: General embryology Contents Page 7  Foundation I B: General embryology Structure of female genital system The uterine wall is composed of three layers; endometrium, myometrium and perimetrium from inside outward. The fallopian tube has three parts: isthmus (most medial part), Ampulla and infundibulum. Contents Page 8  Foundation I B: General embryology Contents Page 9  Foundation I B: General embryology Gametogenesis TOPIC ILOS Gametogenesis Describe the phases of cell division Define the gametogenesis Differentiate the female oogenesis from the male spermatogenesis Compare between the male and female gametes Definition: It is the process of gametes formation through cell division in the primary sex organs. Cell division: There are two distinct types of cell division: mitosis and meiosis, each one of them is preceded by the interphase. During the interphase, the DNA is replicated converting the single stranded chromosomes into double stranded chromosomes. 1- Mitosis gives rise 2 daughter cells; each one is genetically identical to the mother cell. It has four phases: prophase, metaphase, anaphase and telophase. 2- Meiosis gives rise to four daughter cells (haploid gametes) through two divisions (each division has four phases). A. First meiotic division (meiosis I): Contents Page 10  Foundation I B: General embryology Results: 2 daughter cells; each cell contains:23 d-chromosomes (double-stranded). B. Second meiotic division (meiosis II): it is characterized by Results of meiotic division: 4 daughter cells; each cell contains 23 s- chromosomes (single-stranded). Contents Page 11  Foundation I B: General embryology For more information about Cell division Click here Contents Page 12  Foundation I B: General embryology Male gametogenesis (spermatogenesis): Definition: It is the process of transformation of spermatogonia (diploid; 46 chromosomes cell) into spermatozoa (haploid; 23 chromosomes), the male gamete for fertilization. Site: It occurs in the seminiferous tubules of the testis. Duration: is 2 months (roughly 60 days). Onset: started from puberty (13 –16 Y) till old age. Stages: It is composed of four stages (proliferation, growth, maturation and spermiogenesis). Contents Page 13  Foundation I B: General embryology Stages of spermatogenesis 1- Proliferation: (mitosis) increase in number of spermatogonia. 2- Growth: increase in size of spermatogonia to become primary spermatocyte. 3- Maturation: (meiosis) Meiosis I…..1ry spermatocytes(46 d ) gives 2 2ry spermatocytes(23 d) Meiosis II…..2ry spermatocytes(23 d) gives 2 spermatids (23 s) 4- Spermiogenesis: Spermatid…………sperm Spermiogenesis Definition: morphological transformation of the spermatids to mature sperms (spermatozoa) Process: The nucleus: condensed and forms most of head of the sperm. Golgi apparatus covers the anterior 2/3 of the nucleus to form the head cap or acrosomal cap of the sperm. Centrioles: directed towards the opposite side of the nucleus and elongate gradually to form flagellum that forms the tail of the sperm. Mitochondria (sheath): surround the first part of the tail and it is concerned with energy production for movement. The cytoplasm: most of the cytoplasm of the head is removed by the Sertoli cells. Contents Page 14  Foundation I B: General embryology Sperm structure: Its length is about 60 microns and is composed of: 1- Head 2- Neck 3- Flagellum: midpiece, principal piece and end piece The sperm takes minutes (variable from 6-60 minutes) till reach the site of fertilization. About 600 million of sperms are present in each ejaculate. The sperm has the fertilization power for 1-2 days and can live for maximum 5 days in the female genital tract. Contents Page 15  Foundation I B: General embryology Female gametogenesis (oogenesis) Definition: It is the process of transformation of oogonia (diploid; 46 chromosomes cell) into mature ovum (haploid; 23 chromosomes), the female gamete for fertilization. Site: In the outer cortex of the ovary Stages: 3 stages (proliferation, growth and maturation) st  During intrauterine life (proliferation, growth & 1 meiotic till prophase I) st  At puberty (Immediately before 1st ovulation, completion of the 1 nd meiotic division, the secondary oocyte starts the 2 meiotic division and arrested in the metaphase II) nd  Immediately after fertilization (completion of the 2 meiotic division to give the mature ovum and the second polar body) N.B. There are 2 million 1ry oocytes in the ovaries of the newborn females. There are only about 480 becomes 2ry oocytes and are expelled at ovulation during reproductive life (about 40 years), the remaining become atretic. Contents Page 16  Foundation I B: General embryology The spermatogenesis results in four sperms while the oogenesis results in one mature ovum and three polar bodies. Activity Compare between spermatogenesis and oogenesis List the different stages of spermatogenesis Contents Page 17  Foundation I B: General embryology Female reproductive Cycles TOPIC ILOS Female Describe the phases of ovarian cycle reproductive Explain the hormonal regulation of each phase cycles Classify the ovarian follicles Describe the phases of menstrual cycles They include the ovarian and uterine cycles 1- Ovarian cycle: Definition: cyclic changes in the ovaries. Time: Begins at puberty (8-14 year) and ends at menopause (45-55 year) and stops during pregnancy. Duration: 28 days Cause: FSH and LH. Phases: three phases: Follicular phase (preovulatory phase) Ovulation Luteal phase (postovulatory phase) Contents Page 18  Foundation I B: General embryology Follicular phase: it includes the maturation of the oocyte and development of the follicular cells that surround it forming different types of follicles: A. Primordial follicle: 1ry oocyte surrounded by a single layer of follicular cells which are at first flat cells (before birth), but later they become cuboidal (during puberty). B. Growing follicles: The follicle gradually increases in size due to: Two membranes are added by the follicular cells : zona pellucida & basement membrane Follicular cells: that are interrupted by follicular cavity (antrum). Theca cells: externa and interna. C. Mature Graafian follicle: Site: in the cortex of the ovary. Time: at 14th day Shape: rounded Size: 1-2 cm in diameter. Contents Page 19  Foundation I B: General embryology Structure: from outside inwards: 1) Theca externa & interna layers 2) Follicular cells: Many layers and classified into: Corona radiata: few layers of cells surrounding the ovum. Granulosa cells: all around the follicle. Cumulus oophorus: cells connecting corona with granulosa cells. 3) The follicular cavity: filled with the follicular fluid. 4) Zona pellucida: becomes thicker. 5) Immature ovum: (100-200 micron) and immediately before ovulation becomes 2ry oocyte. Hormonal regulation of ovarian cycle A. The role of follicle stimulating hormone (FSH): It stimulates the maturation of oocyte. It stimulates follicular growth, specifically affecting granulosa cells and stimulates them to secret estrogen. Contents Page 20  Foundation I B: General embryology B. The role of Luteinizing hormone (LH): It supports theca cells that provide androgens. Its surge triggers ovulation, also initiating the conversion of the residual follicle into a corpus luteum that, in turn, produces progesterone to prepare the endometrium for a possible implantation. It is necessary to maintain luteal function for the second two weeks of the menstrual cycle. If pregnancy occurs, LH levels will decrease, and replaced by the action of HCG (human chorionic gonadotropin), a hormone very similar to LH but secreted from the new placenta. Ovulation: Definition: rupture of the mature Graafian follicle and liberation of the ovum which surrounded by the zona pellucida and corona radiata. Fate: it enters the uterine tube where it is either fertilized or discharged from the uterus during the menstrual period. th Time: mid-cycle (14 day in an average 28-day menstrual cycle) Causes: FSH and LH hormones. Prostaglandins: produces contraction of the theca externa cells. Luteal phase: After ovulation, the wall of the Graafian follicle collapse then the following events occur: ❑ The follicular cavity becomes filled with blood and forms the corpus haemorrhagicum. ❑ After three days, the bleeding stopped, the granulosa cells enlarged and became distended with the yellow pigment (lipochrome). Contents Page 21  Foundation I B: General embryology ❑ The follicle is transformed into a yellow body, known as corpus luteum which: Secretes progesterone and little estrogen. Lives for 9 days then degenerate if pregnancy doesn’t occur. ❑ Fate of corpus luteum 1. If fertilization occur The corpus luteum enlarges (by human chorionic gonadotrophin hormone) and continues to secrete progesterone till 4th month of pregnancy and called corpus luteum of pregnancy. 2. If fertilization doesn’t occur: The corpus luteum begins to degenerate about 10 to 12 days after ovulation, the corpus luteum is, then, transformed into white fibrous tissue in the ovary called the corpus albicans. Contents Page 22  Foundation I B: General embryology Contents Page 23  Foundation I B: General embryology 2- Menstrual (Uterine) cycle Definition: Cyclic and structural changes in the endometrium of the uterus every 28 days Onset: from puberty till menopause. Cause: estrogen and progesterone. Phases: three main phases. Menstrual (bleeding) phase. Proliferative (estrogenic) phase. Secretory (luteal, progestronic) phase. Menstrual (bleeding) phase Duration: usually lasts 4 to 5 days. Cause: Degeneration of the corpus luteum causes sudden drop of progesterone level leading to Contents Page 24  Foundation I B: General embryology spasm of the spiral arteries of the endometrium resulting in ischemia, necrosis, and degeneration of superficial layer of endometrium ending into shedding of superficial layer and bleeding. Characters of menstrual phase: It is the bleeding from vagina. Amount: 50-60 cc. Contents: 1) Unclotted blood. 2) 2ry oocyte and its surrounding (zona pelllucida and corona radiate) 3) Functional layer of endometrium (stratum compactum and stratum spongiosum) Proliferative (estrogenic) phase Duration: about 9 days (from the end of the menstrual phase till 14th day of the cycle Corresponding to the growth of ovarian follicles. Cause: estrogen secreted by growing follicles. Characters of the proliferative phase: 1) Endometrium: it is 2-3 mm in thickness. 2) The glands: grow up and tortuous with some secretions inside them. 3) The arteries; elongated and coiled. Contents Page 25  Foundation I B: General embryology Secretory (luteal, progestronic) phase Duration: about 14 days, extends from the time of ovulation. Corresponding to the formation and growth of the corpus luteum Cause: progesterone and estrogen hormones produced by corpus luteum. Characters of the secretory phase: 1) The endometrium: 5-7 mm in thickness. 2) The glands: grow more & become wide, tortuous and full of glycogen & mucin secretions. 3) The spiral arteries: more coiled and reach the superficial layer. 4) The connective tissue stroma: increase fluid content of the connective tissue. N.B: The endometrium in secretory phase divided into 3 layers: Superficial layer (stratum compactum): contains neck of gland. Middle layer (stratum spongiosum): contains body of gland. Deep layer (stratum basalis): contains base of gland. Contents Page 26  Foundation I B: General embryology First week of pregnancy At the end of lecture, the student should be able to answer these questions TOPIC ILOS First week of Define the fertilization. pregnancy Explain how the sperm is prepared to do fertilization List the steps and results of fertilization Describe the journey of the ovum till reach the uterine cavity Identify the normal and abnormal sites of implantation The sperm to be capable of fertilizing the oocyte undergoes: 1. Capacitation. 2. Acrosome reaction. Capacitation: Definition: process by which the sperms gain the capability to fertilize the ovum. Site: inside the female genital tract starting from the cervix, uterine cavity, and fallopian tube. Mechanism: removal of the proteins from plasma membrane around head of the sperm by uterine proteolytic enzymes. N.B. these proteins are released by the seminal vesicle to protect the acrosomal cap of the sperm. Contents Page 27  Foundation I B: General embryology Acrosome reaction: Definition: it is the process by which the sperms release the contents of the acrosomal cap in response to the zonal proteins. Only capacitated sperms can do this process once they get in contact with the corona radiata. Site: in the area surrounding the oocyte. Mechanism: the following substances are released through multiple perforations in the acrosomal cap, the perforations are produced by the zonal proteins: Hyaluronidase enzyme, Trypsin-like substances & Zona lysine Contents Page 28  Foundation I B: General embryology Fertilization: Definition: meeting and union between male gamete (sperm) and female gamete (secondary oocyte) Site: ampulla of the uterine tube (widest part of the tube( Steps of Fertilization: 1) Penetration of the corona radiata: by hyaluronidase 2) Penetration of the zona pellucida: by zona lysine 3) Fusion of plasma membranes of the oocyte and sperm. 4) Zona (cortical) reaction: Once the sperm penetrates the zona pellucida, cortical granules present under the cell membrane of the ovum release their contents outside the oocyte which make the zona pellucida firmer and makes them impermeable to other sperms. 5) Completion of second meiotic division of oocyte: Contents Page 29  Foundation I B: General embryology forms the mature ovum (female pronucleus) and the second polar body. 6) Formation of male pronucleus : the nucleus of the sperm enlarges to form the male pronucleus and the tail degenerates. 7) Union of the pronuclei: this will lead to formation of zygote with 46 chromosomes. Contents Page 30  Foundation I B: General embryology Results of Fertilization: 1. The 2ry oocyte completes the second meiotic division. 2. Restores the normal diploid number of chromosomes (46) in the zygote (the newly formed cell). 3. Initiation of cell division of zygote. 4. Determines the sex of the embryo. 5. Determines the state of health and disease of the embryo. 6. Stoppage of ovulation and menstrual cycles due to negative feedback inhibition by high level of estrogen and progesterone. 7. Corpus luteum enlarge to become corpus L. of pregnancy. 8. The endometrium grows forming the decidua of pregnancy. Cleavage: Definition: repeated mitotic division of the zygote, resulting in a rapid increase in the number of cells (blastomeres) within the zona pellucida Site: occurs along the uterine tube toward the uterus. Process of segmentation: First the zygote divides into two blastomeres then 4 then 8. When there are 16 blastomeres, is called a morula. It is formed about 3 days after fertilization then it enters the uterine cavity The zona pellucida keeps the cells together during cleavage and prevents adhesion of these cells to the uterine tubes. Small cavities appear between the cells that unite together to form larger cavities. Contents Page 31  Foundation I B: General embryology About 4 days after fertilization, a fluid - filled cavity called the blastocele appears inside the morula. The fluid passes from the uterine cavity, through the degenerated zona pellucid to fill this space. The blastocele separates the blastomeres into two groups: 1. A thin, outer cell layer called the outer cell mass (trophoblast), which gives rise to the embryonic part of the placenta. 2. An inner group of blastomeres called the inner cell mass (embryoblast) which gives rise to the embryo. Contents Page 32  Foundation I B: General embryology Implantation Definition: process of embedding of the blastocyst into the endometrium. st th nd Time: Begins at the end of the 1 week (7 day) and completed by the end of the 2 week. Normal site of implantation: In the endometrium of the uterus, in the upper part of the body near the fundus. Process of implantation: 1- Differentiation of the trophoblast into two layers, Syncytiotrophoblast and cytotrophoblast. 2- The sycytiotrophoblast adheres itself to the endometrial tissues then it forms a defect through which blastocyst is gradually embedded in endometrium. 3- A fibrin clot fills the defect in the endometrium that repaired gradually. Contents Page 33  Foundation I B: General embryology Abnormal sites of implantation: A) Inside the inferior segment (third) of the uterus (Placenta praevia) B) Outside the uterus (ectopic pregnancies) 1) Tubal pregnancy  The tube ruptures early lead to internal hemorrhage and abdominal pain.  May be confused with appendicitis if the pregnancy is in the right uterine tube. 2) Peritoneal pregnancy (Abdominal): On any abdominal or pelvic organ. Secondary to rupture tubal pregnancy. 3) Ovarian pregnancy: It is either primary or secondary to rupture tubal pregnancy. The decidua: is the endometrium of the pregnant uterus. It consists of three parts: 1) The decidua basalis: lies between the fetus and the muscle wall of the uterus, this part will form the placenta. 2) The decidua capsularis: overlies the fetus. 3) The decidua parietalis: lines the rest of the uterine cavity. Structure: The endometrium: increase in thickness. The glands: become wide, tortuous and full of glycogen & mucin secretions. The spiral arteries: more coiled and reach the superficial layer. The connective tissue stroma: increase fluid content Decidual cells: enlarged pale stained, loaded with glycogen & lipids Contents Page 34  Foundation I B: General embryology Activity List 2 abnormal site of implantation Describe the stages of fertilization Define capacitation For more information about first week of pregnancy click here Contents Page 35  Foundation I B: General embryology Second week of pregnancy At the end of lecture, the student should be able to answer these questions TOPIC ILOS Second week of Justify the followings: pregnancy 1- The second week is the week of twos 2- Testing for pregnancy The events that occur during the second week: 1- Completion of trophoblast (outer cell mass) differentiation into 2 layers: syncytiotrophoblast and cytotrophoblast. 2-Differentiation of inner cell mass into 2 layers: Epiblast (primary ectoderm) and hypoblast (primary endoderm) forming a bilaminar embryonic disc. 3- Development of 2 cavities: Amniotic cavity and yolk sac. 4- Development and differentiation of extraembryonic mesoderm into 2 layers: somatic and visceral Contents Page 36  Foundation I B: General embryology 1- Completion of trophoblast (outer cell mass) differentiation. The cells of trophoblast undergo repeated mitotic division & differentiated into 2 layers: Cytotrophoblast: inner layer of cells. Sycytiotrophoblast: outer multinucleated mass Isolated cavities (lacunae) appear in the syncytiotrophoblast which become filled with maternal blood. The syncytiotrophoblast begins to produce a hormone (human chrionic gonadotrophin) (HCG) which enters the maternal blood in the lacunae. By the end of the second week, quantities of this hormone are sufficient to be detected, which serve as the basis for pregnancy testing. Contents Page 37  Foundation I B: General embryology 2-Development of two cavities. 1- The yolk sac replaces the blastocele and bounded by a thin membrane, called the Heuser’s membrane that is derived from the hypoblast. 2- The epiblast gives rise to the amniogenic cells that surround the amniotic cavity. 3-Development and differentiation of extraembryonic mesoderm. Origin: the mesodermal cells arise from Hypoblast. Cytotrophoblast. Development: Isolated spaces appear within the extra embryonic mesoderm. These spaces rapidly fuse to form a large cavity, which is the extra– embryonic coelom. The extra – embryonic mesoderm becomes divided into two layers: Contents Page 38  Foundation I B: General embryology 1) Somatic mesoderm: lining the trophoblast and covering the amniotic cavity. 2) Splanchnic mesoderm: covering the primary yolk sac. The two layers connected at the caudal end of the embryo and are called the connecting stalk (the future umbilical cord) The trophoblast together with somatic layer of the extra– embryonic mesoderm form the chorion, the blastocyst is then called, chorionic vesicle Activity The 2nd week of pregnancy is a week of 2 (Explain) Contents Page 39  Foundation I B: General embryology Third week of pregnancy TOPIOC ILOS Third week of Outline the development of the followings: pregnancy 1- Prechordal plate 2- Notochord 3- Neural tube On starting the third week (15th day), the embryonic disc becomes oval shaped which is broad cranially and narrow caudally. Also, currently a group of structures appear on the midline of the embryonic disc. The midline structures: 1- Prechordal plate 2- Primitive node and streak 3- Notochord 4- Cloacal membrane Contents Page 40  Foundation I B: General embryology 1- Development of prechordal plate: Near the cranial end of the embryonic disc, the cubical hypoblastic cells (primary endoderm) in the middle line increase in height and fuse with the overlying epiblast forming a plate gives rise to the buccopharyngeal membrane (future mouth). 2- Development of primitive streak, node, groove and pit: Primitive streak and node: both result from the proliferation of cells of the epiblast in the median plane of embryonic disc. Primitive groove and pit: A depression in the middle of the primitive streak and node due to migration of their cells. Contents Page 41  Foundation I B: General embryology 3- Development of notochord: On 17th day, some cells of primitive node grow cranially between the epiblast and hypoblast until reach prechordal plate forming notochord process which undergoes many changes to form definite notochord which represents the body axis. Contents Page 42  Foundation I B: General embryology Fate of Notochord: 1) Part of notochord in the center of bodies of vertebra: degenerates and disappear. 2) Part of notochord in the intervertebral disc: undergoes mucoid degeneration and forms nucleus bulbosus. Development of neural tube Time: it is formed at 3rd week. Stages: Increase height of ectoderm cells which form (neural plate) Elevation of the lateral margin of neural plate to form (neural folds) Depression of the median part of neural plate to form (neural groove) Fusion of neural folds converting neural plate into (neural tube) Neural tube separates from ectodermal surface. Some neuroectodermal cells separate and lie between neural tube and ectoderm (neural crest) Contents Page 43  Foundation I B: General embryology At first cranial and caudal ends of neural tube are open then are closed Fate: Cranial end of tube dilates to form brain and the caudal end forms spinal cord. Derivatives of neural crest Neuroblasts which gives:  Nerve cells of sensory ganglia (5th, 7th, 9th, 10th cranial nerve) Contents Page 44  Foundation I B: General embryology  Nerve cells of dorsal root ganglia, sympathetic ganglia and th parasympathetic ganglia of cranial nerves (3rd, 7th, 9th, 10 ) Spongioblasts which gives:  pia and arachnoids mater  Schwan cells. Chromaffin cells which give:  Suprarenal medulla.  Carotid and aortic bodies’ cells. Melanocytes: pigment cells of skin. Gastrulation Definition: is the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc. Time: During the 3rd week Origin of the three layers: 1) Some epiblastic cells of the primitive streak displace the hypoblast, forming the intraembryonic endoderm (the hypoblastic layer disappears). 2) Cells remaining in the epiblast form the intraembryonic ectoderm. 3) Some cells from the sides of the primitive streak migrate laterally and cranially between the ectoderm and endoderm. This third layer of embryo is known as the intraembryonic mesoderm 4) By the middle of the 3rd week, intraembryonic mesoderm separates the ectoderm from endoderm everywhere except: A. At the buccopharyngeal membrane (prechordal plate) cranially. B. In the median plane cranial to the primitive node where the notochordal process is located. C. At the cloacal membrane caudally. Contents Page 45  Foundation I B: General embryology Contents Page 46  Foundation I B: General embryology Intraembryonic mesoderm (IEM) Origin: primitive streak and node Development: the migrating epiblastic cells from (primitive streak and node) fill the space between the ectoderm and the endoderm on each side of notochord that extended to be in the middle of the most embryonic disc, IEM on each side is divided into three parts: i. Paraxial part: On each side of notochord ii. Intermediate cell mass: Lateral to the paraxial part iii. Lateral plate: Lateral to the intermediate cell mass. Contents Page 47  Foundation I B: General embryology i. Paraxial mesoderm. The paraxial mesoderm lies on each side of notochord and divides into 42- 44 pairs of segments or somites The somites correspond roughly to the vertebrae. The somites are arranged as occipital (4), cervical (8), thoracic (12), lumbar (5), sacral (5) & coccygeal (8-10) segments. Fate: somites are divided into three parts: 1) Sclerotome: ventromedial part which gives axial skeleton (vertebral column and ribs) 2) Myotome: intermediate part and gives skeletal muscles 3) Dermatome: superficial (dorsolateral) part and gives dermis of skin and fascia. ii. Intermediate cell mass The intermediate part of intraembryonic mesoderm. Fate: it gives ❖ The urogenital system including: 1) Testis or ovary 2) Male and female duct system Contents Page 48  Foundation I B: General embryology 3) Kidney ❖ Cortex of suprarenal gland (its medulla is ectodermal in origin) iii. Lateral plate mesoderm lateral part of intraembryonic mesoderm. Fate: ❖ Small cavities appear in lateral plate mesoderm. ❖ These cavities unite to form single horseshoe cavity (intra-embryonic coelom) that is divided into pleura, peritoneum & pericardium cavities ❖ The intra-embryonic coelom divides lateral plate mesoderm into: 1) Somatic (parietal) layer: gives  Muscles of abdominal wall and chest wall )Muscles of the trunk).  Somatic layer of pleura, peritoneum and pericardium. 1) Visceral (splanchnic) layer: gives  Muscles of heart, smooth muscles of bronchial tree and gut.  Visceral layer of pleura, peritoneum and pericardium. Endoderm: it gives rise to 1) Epithelial lining of: Gastro-Intestinal Tract. Respiratory Tract. Urinary Bladder and urethra. Tympanic cavity, antrum and auditory tube. 2) Parenchyma of: Tonsil, thyroid, parathyroid glands, thymus, liver and pancreas Ectoderm: it gives rise to Neural tube and crest derivatives. Epidermis of the skin. Eye: conjunctiva, outer layer of cornea, lens, lacrimal gland and nasolacrimal duct. Contents Page 49  Foundation I B: General embryology Ear: external auditory meatus and outer layer of tympanic membrane. Nose: epithelium of nasal cavity and paranasal sinuses. Mouth: enamel of teeth, anterior of oral cavity, gums and salivary glands. Lower half of anal canal and terminal part of male urethra. For more information about third week of pregnancy click here Contents Page 50  Foundation I B: General embryology Fourth week of pregnancy TOPIC ILOS Fourth week of Give reason of the followings: pregnancy 1- The central nervous system is ectodermal in origin 2- The midline of embryonic disc is devoid from intraembryonic mesoderm Folding of the embryo Definition: folding of flat trilaminar embryonic disc upon itself ventrally to form cylindrical embryo Time: 4th week. Causes: 1. Rapid longitudinal growth of central nervous system is cause of (head and tail folds) 2. Shape of rapidly growing somites is the cause of lateral folding. 3. Rapid enlargement of amniotic fluid Process: 1. Folding of the ends of the embryo produces: head fold at cephalic end and tail fold at caudal end. 2. Folding of the sides of embryo produces right and left lateral folds. Contents Page 51  Foundation I B: General embryology Results of folding: Shape: embryo becomes cylindrical. Amniotic cavity: expanded and enlarge at the expense of yolk sac to surround embryo completely. Yolk sac: constricted into two parts 1. Part enclosed within embryo form primitive gut (foregut, midgut and hind gut) 2. Part outside embryo becomes definitive yolk sac or yolk sac proper. Primitive gut differentiated into:  Part enclosed in head fold forms foregut.  Part enclosed in tail fold forms hind gut.  Part enclosed in two lateral folds forms midgut.  Midgut remains opens with yolk sac by broad duct (vitello-intestinal duct) Results of formation of head fold  Part of yolk sac in head fold forms foregut  Forebrain at most cephalic end of embryo. Contents Page 52  Foundation I B: General embryology  Buccopharyngeal membrane (on ventral aspect of embryo cranial to pericardium)  Septum transversum (in the ventral aspect of embryo caudal to pericardium). Results of formation of tail fold  Part of yolk sac in tail fold forms hindgut  Cloacal membrane and connecting stalk (in the ventral aspect of the embryo). Results of formation of head fold  Part of yolk sac in lateral folds forms midgut  Rounded appearance of embryo.  Anterior abdominal wall is formed  Primitive gut becomes tube like structure. Contents Page 53  Foundation I B: General embryology Activity Define folding Enumerate 4 results of embryo folding For more information about folding click here Contents Page 54  Foundation I B: General embryology Fetal membranes TIOIC ILOS Fetal membrane I Describe the fate of allantois. List the congenital anomalies of allantois. Describe the development and congenital anomalies of amniotc cavity. Fetal membrane Describe the development and congenital anomalies of yolk sac. II Describe the development and congenital anomalies of umbilical cord. Describe the development and congenital anomalies of placenta They include allantois, amnion, yolk sac, umbilical cord, chorion and placenta These structures develop from the zygote but don‘t form parts of the embryo except yolk sac which forms the gut and allantois which shares in the formation of urinary bladder. Contents Page 55  Foundation I B: General embryology 1- Allantois Definition: Diverticulum from caudal wall of yolk sac extends into connecting stalk Fate: after folding, it extends from hindgut (urinary bladder) to umbilical ring (umbilicus) and then it is differentiated into: 1) Proximal part of allantois: that will enlarge to form the apex of urinary bladder. 2) Distal part of allantois: that will form the urachus which is a tube, after birth, it becomes a fibrous band and forms median umbilical ligament which extends from apex of urinary to umbilicus. Function: 1) Its blood vessels persist as umbilical veins and arteries. 2) Its proximal part forms the apex of urinary bladder. Contents Page 56  Foundation I B: General embryology Congenital anomalies of Allantois: 1)Patent Urachus (Urachal fistula): It is due to failure of obliteration of whole urachus. Feature: There is a communication between (umbilicus and urinary bladder) and urine may be discharged at the umbilicus. 2) Urachal sinus: it is due to persistence of distal part of urachus. 3) Urachal diverticulum: it is due to persistence of proximal part of urachus. 4) Urachal cyst: it is due to persistence of middle part of urachus. 2- Amnion and amniotic sac ❖ It includes the amniogenic cells and the somatic extraembryonic mesoderm surrounding it. ❖ Amniotic sac is the sac that contains the amniotic fluid and surrounded by the amnion. Contents Page 57  Foundation I B: General embryology Development: Small cavity, its floor is formed by epiblast (ectoderm), while its sides and roof are formed by amniogenic cells. It becomes surrounded by somatic layer of extraembryonic mesoderm. After folding: the amniotic sac expanded and enlarges at the expense of yolk sac to surround embryo completely and is reflected to cover the umbilical cord. At birth: the amniotic sac ruptures by uterine contraction and the amniotic fluid passes through the cervical canal. Amniotic fluid Definition: fluid fills amniotic sac, one liter at birth. Composition: 99% water and 1% (desquamated epithelial cells, organic constituents as CHO, enzymes, fats, hormones, pigments and inorganic salts). Contents Page 58  Foundation I B: General embryology Functions of amnion: 1) Allows fetal movements and symmetrical growth of fetus. 2) Prevent adhesion between different parts of fetus. 3) Control body temperature of fetus. 4) Barrier against infection. 5) Maintain fluid and electrolytes balance. 6) Protection against trauma and external shock. 7) During birth :Dilatation of cervix and antiseptic against infection. Circulation of Amniotic Fluid: ❖ Formation of Amniotic fluid is by amniogenic cells, fetal kidney and placenta. ❖ Fetus swallows the fluid, then it is absorbed from digestive tract of the fetus, then fluid passes to (fetal blood, across the placenta it enters maternal blood, placental exchange of wasted blood by filtered blood that passes to fetus then fetal kidney) at which the fluid is formed then it passes through urinary tract and returns to amniotic sac. Congenital anomalies of amnion: Oligo-hydraminos Poly-hydramnios Definition Volume of fluid is less Volume of fluid is more than than 0.5liter 2liters. Causes a) Placental insufficiency. a) 60 %idiopathic. b) Failure of kidney formation. b) 20 %fetal factors: c) Urinary tract obstruction. esophageal Artesia. c) 20 %maternal factors as diabetes mellitus Contents Page 59  Foundation I B: General embryology Character 1) Fetal abnormalities (due to 1) Premature labor. impaired growth) 2) Distress to mother and 2) Adhesion between amnion fetus. and fetus 3) Excess fetal movements and cause true knots of umbilical cord 3- Yolk sac Development: nd 1) At 2 weeks: Roof of primary yolk sac: is the endoderm of embryonic disc. The walls: by Heuser's membrane and surrounded from outside by a visceral layer of EEM. 2) At 3rd week: Yolk sac gets smaller becoming secondary yolk sac and allantois arises from it as a caudal diverticulum 3) During 4th week: Yolk sac constricted into two parts after folding: a. Part enclosed within embryo form primitive gut Contents Page 60  Foundation I B: General embryology b. Part outside embryo becomes definitive yolk sac or yolk sac proper. Primitive gut differentiated into Part enclosed in head fold forms foregut, part enclosed in tail fold forms hindgut, part enclosed in two lateral folds forms midgut. Midgut remains temporally opens with yolk sac by broad duct (vitellointestinal duct, VID) th 1) During 5 week: Vitellointestinal duct elongates, atrophy and detach, definitive yolk sac starts to degenerate. Contents Page 61  Foundation I B: General embryology Function of yolk sac 1) Transfer nutrients to embryo before function of placenta. 2) Development of blood cells and blood vessels first occur in extraembryonic mesoderm covering wall of yolk sac. 3) Endoderm of yolk sac is incorporated into embryo as primitive gut. 4) Its endoderm gives epithelium of respiratory tract and digestive tract. 5) Primordial germ cells appear in endoderm of wall of yolk sac and migrates to developing sex organ. Congenital anomalies of yolk sac (Vitellointestinal duct) 1) Vitelline fistula (Patent VID): Cause: failure of obliteration of whole VID. Feature: There is a communication between (umbilicus and midgut) and fecal matter may be discharged on the umbilicus 2) Vitelline sinus: Cause: Persistence of distal part of VID. 3) Vitelline diverticulum: (Meckel’s diverticulum) Cause: Persistence of proximal part of VID, its inflammation gives a similar picture to the appendicitis 4) Vitelline cyst: Cause: Persistence of middle part of VID. 5) Vitelline ligament: the VID remains as a fibrous cord Contents Page 62  Foundation I B: General embryology 4- Umbilical cord Definition: cord connects mother with fetus containing umbilical vessels. Length: 50-60 cm. Diameter: one cm. Shape: Twisted because the vein is shorter than the arteries. False knots along its length may present. Amnion covers both surfaces. Attachment to placenta: near center of fetal surface of placenta Development of umbilical cord: The 2 Parts (covering and lining) of extraembryonic mesoderm are connected at caudal end of embryo by connecting stalk. Allantois appears as diverticulum from caudal wall of the yolk sac and extends into connecting stalk. Contents Page 63  Foundation I B: General embryology After folding the connecting stalk becomes on the ventral aspect of the embryo. Structure of umbilical cord: 1) Amniotic sheath. 2) Extra embryonic mesoderm of connecting stalk become loose and gelatinous (Wharton's jelly) 3) Umbilical vessels: Two umbilical arteries: carry non oxygenated blood. Left umbilical vein: carry oxygenated blood, the right one disappeared. th 4) Remnant of vitellointestinal duct: disappear by (6 week) 5) The urachus (distal part of allantois) th 6) Intestinal loop of midgut (till 10 week) Changes of umbilical cord after birth: Two umbilical arteries become occluded to form medial umbilical ligaments. Left umbilical vein become occluded to form ligamentum teres of liver. Distal part of allantois (urachus) is obliterated to form median umbilical ligament. Congenital anomalies of umbilical cord: 1) Length: Very short cord: cause premature separation of placenta. Very long umbilical cord: form a Loop around the neck which is fatal. 2) Persistent physiological umbilical hernia (omphalocele) th Physiological umbilical hernia is not reduced after 10 week. 3) Number: Two (double), three (triple). Contents Page 64  Foundation I B: General embryology 4) Attachment of umbilical cord Marginal attachment: cord attached to margin of placenta. Velamentous attachment: cord attached to amniotic membrane and its vessels pass through this membrane to reach placenta. 1) Knots of umbilical cord: False knots: no significance. True knots: due to long umbilical cord and may tighten and cause fetal death. 5- Chorion Definition: formed of somatic layer of extra embryonic mesoderm and two layers of trophoblast. Development: ❖ Firstly, chorionic villi cover the whole chorionic vesicle. ❖ As chorionic vesicle grows: The villi associated with decidua capsularis are compressed. These villi degenerates producing bare area (Chorion leave( ❖ Villi associated with decidua basalis increase in number, branch and enlarge (Chorion frondosum) ❖ The chorionic cavity replaces the extraembryonic coelom Chorionic villi ❖ Definition: finger like processes from trophoblast into decidua basalis and capsularis covers chorionic vesicle. ❖ Types according to structure: 1) Primary chorionic villi: formed of:  Outer syncytiotrophoblast.  Inner cytotrophoblast. 2) Secondary chorionic villi: formed of:  Outer syncytiotrophoblast.  Inner cytotrophoblast.  Core of extraembryonic mesoderm. Contents Page 65  Foundation I B: General embryology 3) Tertiary chorionic villi: formed of :  Outer syncytiotrophoblast.  Inner cytotrophoblast.  Core of extraembryonic mesoderm.  Blood vessels develop in the core of villi. ❖ Types according to function: 1) Stem villi (anchoring villi) Tertiary villi branch and penetrate decidua basalis and fix vesicle to wall of uterus (anchoring villi) 2) Terminal (free or absorbing) villi Villi grow from sides of anchoring villi and projecting freely into maternal blood in intervillous space (absorbing villi) 6- Placenta Definition: primary site of nutrient and gas exchange between mother and fetus. Structure: Contents Page 66  Foundation I B: General embryology 1) Fetal part: develops from chorion frondosum. 2) Maternal part: derived from endometrium (decidua basalis and maternal blood in intervillous space) Gross appearance: Shape: discoid. Diameter: 15-20cm. Thickness: 2-3cm. Wight: 500-600gm Surfaces:  Fetal surface of placenta: o Smooth-shiny and covered by amnion. o Umbilical cord attached near its center. o Umbilical vessels branch on its surface to form chorionic vessels which enter chorionic villi.  Maternal surface of placenta: shows cotyledons with Cobblestone appearance produced by bulging villi. Contents Page 67  Foundation I B: General embryology Microscopic structure: 1) Intervillous space Contain maternal blood derived from lacuna developed in syncytiotrophoblast during 2nd week of development. 2) Placental barrier or membrane: Definition: membrane separating maternal from fetal blood. Structure: a) Until 20 weeks: 4 layers: ❖ Syncytiotrophoblast. ❖ Cytotrophoblast. ❖ Extra embryonic mesoderm. ❖ Endothelium of fetal capillaries. b) After 20 weeks: cytortrophoblast is lost so the barrier becomes 3 layers: ❖ Syncytiotrophoblast. ❖ Extra embryonic mesoderm ❖ Endothelium of fetal capillaries. c) Late in pregnancy: extraembryonic mesoderm is lost; the barrier becomes formed of 2 layers: ❖ Syncytiotrophoblast. ❖ Enodothelium of fetal capillaries Function of placenta: 1) Respiratory function: Fetus receives O2 and gets rid of CO2 by simple diffusion. 2) Nutritive function: Transfer nutrient as glucose by simple diffusion and amino and fatty acids by selective absorption. 3) Excretory function: Transfer urea, uric acid, billirubin through placental membrane by simple diffusion. Contents Page 68  Foundation I B: General embryology 4) Protective function Transfer maternal antibodies as diphtheria, smallpox, and measles. Barrier against some infection from mother to fetus but some viruses as German measles and syphilis pass through it. 5) Metabolism: Synthesizes glycogen, cholesterol and fatty acid and transfer drugs by simple diffusion. 6) Endocrine function: A- Chorionic gonadotrophin hormones (early in pregnancy) Trophoblast produces it which maintain corpus luteum and stimulate corpus luteum to secret progesterone. B- Progestron (later in pregnancy > 4th month) Trophoblast produces progesterone from 4th month which is essential for maintenance of pregnancy. C- Estrogen (near labour) by trophoblast which increase sensitivity of myometrium to oxytocin helping delivery Congenital anomalies of placenta: 1) Anomalies in position Placenta previa: in the lower segment of the uterus and has 3 types: (lateralis, marginalis and centralis). Placenta accreta: chorionic villi penetrate myometrium. 2) Anomalies in attachment of umbilical cord to placenta. 3) Anomalies in shape Zonary placenta: empty in middle. Bipartite placenta: placenta is divided into 2 parts with one umbilical cord. Contents Page 69  Foundation I B: General embryology 4) Anomalies in number Placenta succentoriata: There is one or more small accessory placenta in addition to main placenta which is retained in uterus after delivery and cause postpartum hemorrhage. Activity Differentiate between oligohydramnios and polyhydramnios Enumerate 5 functions of placenta Differentiate between the primary, 2nr and 3ry chorionic villi To complete your activities, please scan QR code and answer the questions Contents Page 70  Foundation I B: General embryology References 1. Moore Clinically Oriented Anatomy 7th Edition. 2. Langman's Medical Embryology. Contents Page 71