Veterinary Developmental Anatomy (Embryology) Module 2 PDF

Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation MODULE 2 EARLY DEVELOPMENT IN BIRDS AND MAMMALS AND PLACENTATION OVERVIEW The...

Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation MODULE 2 EARLY DEVELOPMENT IN BIRDS AND MAMMALS AND PLACENTATION OVERVIEW The mammalian body is composed of an array of organs, tissues, and individual cells that function in specialized and highly coordinated manner. The cells, tissues, and organs exhibit considerable diversity in both structure and function, but they are derived from a single cell, a fertilized ovum. The fertilized ovum is a product of the fusion of the two specialized reproductive cells, gametes, of male and female origin. Following fertilization, the ovum undergoes a series of division which ultimately lead to formation of pluripotent stem cells, from which all cells, tissues and organs of the body arise. This module is the discussion of the different processes that involves in the early development of birds and mammals. This process involves division, growth and differentiation of cells (mitosis and meiosis) gametogenesis, fertilization, cleavage, gastrulation, neurulation and placenta formations, implantations and twinning. INDICATIVE CONTENT ∙ Division, Growth and Differentiation of the Cells ∙ Gametogenesis ∙ Fertilization ∙ Cleavage stage ∙ Gastrulation ∙ Neurulation ∙ Foetal Membranes (Birds and Mammals) ∙ Forms of Implantation and Placentation ∙ Twinning LEARNING OUTCOMES ∙ Understand and discuss the division, growth and differentiation of the cells (cell cycle, mitosis and meiosis). ∙ Identify and describe the two types of gametogenesis – spermatogenesis and oogenesis. ∙ Understand and identify the different processes that involves in fertilization of sperm cell and egg cell. ∙ Describe the processes that take place during cleavage, gastrulation and neurulation stage of embryonic development. ∙ Identify and describe the different foetal membranes of birds and mammals. ∙ Understand the processes during implantations and determine the different types of placentation in different animals. ∙ Identify and describe the different types of twinning. DISCUSSION DIVISION, GROWTH AND DIFFERENTIATION OF THE CELLS Somatic cells – cells associated with tissue formation and regeneration. Specialized reproductive cells – also known as germ cells, includes gametes of male and female origin  8 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation (sperm cell and egg cell) and their precursors. The Cell Cycle Cell cycle is the series of growth and development steps a cell undergoes between its “birth”— formation by the division of a mother cell—and reproduction—division to make two new daughter cells. Stages of the cell cycle ∙ Interphase - cell grows and makes a copy of its DNA G1 phase – “resting phase”, first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps. S phase - DNA synthesis occurs prior to chromosomal replication. The cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase. G2 phase - second gap phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. B. Mitotic (M) phase - the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells. ∙ The cell divides its copied DNA and cytoplasm to make two new cells. ∙ M phase involves two distinct division-related processes: mitosis and cytokinesis. ∙ In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis takes place in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase. ∙ In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cell Cycle Exit and G0 ∙ Some types of cells divide rapidly, and in these cases, the daughter cells may immediately undergo another round of cell division. Ex. Cells in early embryo and tumor. ∙ Cells may exit the G1 phase and enter a resting state called G0 phase. ∙ G0 phase, a cell is not actively preparing to divide, it’s just doing its job. ∙ Ex. conduct signals as a neuron or store carbohydrates as a liver cell. ∙ G0 is a permanent state for some cells, while others may re-start division if they get the right signals. ∙ Neurons do not divide and continue to function permanently in a G0 state.  9 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Mitosis ∙ Describe the nuclear division of somatic cells, a process which usually results in the production of two cells, with the same chromosome compliment as progenitor cell from which they derived. ∙ Mitosis is essential in the embryonic growth and development and for repair and replacement of tissue throughout life. ∙ Mitosis is the phase of the cell cycle where chromosomes in the nucleus are evenly divided between two cells. ∙ When the cell division process is complete, two daughter cells with identical genetic material are produced. ∙ Somatic cells are full compliment of chromosomes is referred to as Diploid and given the designation of 2n. Number of chromosomes in Diploid in Human and Animal Cells Species Number of Chromosomes (2n)  1. Human 46 2. Cats 38 3. Cattle 60 4. Chickens 78 5. Dogs 78 6. Donkeys 62 7. Goats 60 8. Horses 64 9. Pigs 38 10. Rabbits 44 11. Rats 42 12. Sheep 54 Stages of Mitosis 1. Prophase - the first stage of mitosis. The chromosomes consisting of closely associated sister chromatids, condense. ∙ The chromatin condenses into discrete chromosomes. ∙ The nuclear envelope breaks down and spindles form at opposite poles of the cell. 10 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation ∙ Chromatin fibers become coiled into chromosomes, with each chromosome having two chromatids joined at a centromere. ∙ The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm. ∙ The two pairs of centrioles (formed from the replication of one pair in Interphase) move away from one another toward opposite ends of the cell due to the lengthening of the microtubules that form between them. ∙ Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator. ∙ Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers. ∙ The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers. ∙ The chromosomes begin to migrate toward the cell center. 2. Metaphase ∙ The nuclear membrane disappears completely. ∙ Polar fibers (microtubules that make up the spindle fibers) continue to extend from the poles to the center of the cell. ∙ Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both  sides of their centromeres. ∙ Chromosomes align at the metaphase plate at right angles to the spindle poles. ∙ Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes. 3. Anaphase ∙ The paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell. ∙ At the end of anaphase, each pole contains a complete compilation of chromosomes. 4. Telophase The chromosomes are cordoned off into distinct new nuclei in the emerging daughter cells. The following changes occur: The polar fibers continue to lengthen. Nuclei begin to form at opposite poles. The nuclear envelopes of these nuclei form from remnant pieces of the parent cell's nuclear envelope and from pieces of the endomembrane system. Nucleoli also reappear. Chromatin fibers of chromosomes uncoil. After these changes, telophase/mitosis is largely complete. The genetic contents of one cell have been divided equally into two. Cytokinesis ∙ Cytokinesis is the division of the cell's cytoplasm. ∙ It begins prior to the end of mitosis in anaphase and completes shortly after telophase/mitosis. ∙ At the end of cytokinesis, two genetically identical daughter cells are produced. ∙ These are diploid cells, with each cell containing a full complement of chromosomes. 11 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  Cells produced through mitosis are different from those produced through meiosis. In meiosis, four daughter cells are produced. These cells are haploid cells, containing one-half the number of chromosomes as the original cell. Sex cells undergo meiosis. When sex cells unite during fertilization, these haploid cells become a diploid cell. Meiosis ∙ Greek word meiosis, which means “lessening” ∙ special type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell that gave rise to them. ∙ DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. ∙ The two meiotic divisions are known as Meiosis I and Meiosis II. Meiosis I ∙ Meiosis I segregates homologous chromosomes, which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). ∙ Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. ∙ Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). 1. Prophase I ∙ Prophase I is typically the longest phase of meiosis. ∙ During prophase I, homologous chromosomes pair and exchange DNA (homologous recombination). ∙ This often results in chromosomal crossover. ∙ This process is critical for pairing between homologous chromosomes and hence for accurate segregation of the chromosomes at the first meiosis division. ∙ The new combinations of DNA created during crossover are a significant source of genetic variation, and result in new combinations of alleles, which may be beneficial. ∙ The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. 12 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation ∙ The process of pairing the homologous chromosomes is called synapsis. ∙ At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma). Sub Stages Of Prophase I a. Leptotene ∙ leptonema, from Greek words meaning "thin threads". ∙ In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids— become "individualized" to form visible strands within the nucleus. ∙ The two sister chromatids closely associate and are visually indistinguishable from one another. During leptotene, lateral elements of the synaptonemal complex assemble. ∙ Leptotene is of very short duration and progressive condensation and coiling of chromosome fibers takes place. b. Zygotene ∙ zygonema, from Greek words meaning "paired threads", ∙ occurs as the chromosomes approximately line up with each other into homologous chromosome pairs. ∙ In some organisms, this is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. ∙ At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place, facilitated by assembly of central element of the synaptonemal complex. ∙ Pairing is brought about in a zipper-like fashion and may start at the centromere (procentric), at the chromosome ends (proterminal), or at any other portion (intermediate). ∙ Individuals of a pair are equal in length and in position of the centromere. ∙ Thus pairing is highly specific and exact. The paired chromosomes are called bivalent or tetrad chromosomes. c. Pachytene ∙ pachynema, from Greek words meaning "thick threads",. ∙ At this point a tetrad of the chromosomes has formed known as a bivalent. ∙ This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. ∙ Nonsister chromatids of homologous chromosomes may exchange segments over regions of homology. ∙ Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology. ∙ At the sites where exchange happens, chiasmata form. ∙ The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. d. Diplotene ∙ diplonema, from Greek words meaning "two threads",the synaptonemal complex degrades and homologous chromosomes separate from one another a little. ∙ The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I. ∙ In human fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth. ∙ This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.  13 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation e. Diakinesis ∙ Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through. ∙ This is the first point in meiosis where the four parts of the tetrads are actually visible. ∙ Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. ∙ Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. f. Synchronous processes ∙ During these stages, two centrosomes, containing a pair of centrioles in animal cells, migrate to the two poles of the cell. ∙ These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, which are essentially cellular ropes and poles. ∙ The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centrosome, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I. ∙ Microtubules that attach to the kinetochores are known as kinetochore microtubules. ∙ Other microtubules will interact with microtubules from the opposite centrosome: these are called nonkinetochore microtubules or polar microtubules. ∙ A third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton 2. Metaphase I ∙ Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrosomes attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. ∙ This attachment is referred to as a bipolar attachment. ∙ The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. 3. Anaphase I ∙ Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. ∙ Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. ∙ The cell elongates in preparation for division down the center. ∙ Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents chromatids to apart. ∙ This allows the sister chromatids to remain together while homologs are segregated. 4. Telophase I ∙ The first meiotic division effectively ends when the chromosomes arrive at the poles. ∙ Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids.  ∙ The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. 14 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation ∙ The chromosomes uncoil back into chromatin. ∙ Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. ∙ Sister chromatids remain attached during telophase I. ∙ Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Meiosis II ∙ Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. ∙ Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. ∙ The end result is production of four haploid cells (n chromosomes, 23 in humans) from the two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. ∙ The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II. 1. Prophase II ∙ Disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. ∙ Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division. 2. Metaphase II ∙ The centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. ∙ The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. 3. Anaphase II ∙ The remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. ∙ The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. 4. Telophase II ∙ Similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. ∙ Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. ∙ Meiosis is now complete and ends up with four new daughter cells.  15 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation GAMETOGENESIS Gametogenesis is the development and maturation of sex cells called gametes which occurs in organs known gonads; specifically ovaries in females and testes in males. The gametes arise from primordial cells outside the embryonic body, in the yolk sac tissue. The cells migrate to and enter the embryonic gonad where they proliferate and eventually develop and mature. Gametes differ from other cells in their number of chromosome. The non- reproductive cells (somatic) are diploid containing 2 sets of chromosomes, one set from each male and female parent. The reproductive cells (germ cells) carry only one complete set of chromosomes. The reduction in number of chromosome occur when the germ cells undergo two stages of specialized cell divisions – meiosis. ∙ Gametes = mature reproductive cells capable of fertilization o Ovum = product of oogenesis  o Spermatozoon = product of spermatogenesis ∙ Diploid germ cells also known as Primordial germ cells = spermatogonia and oogonia ∙ Gametogenesis starts at neurula stage. Cells of the body: 1. Somatic cells = undergo mitosis 2. Germ cells = undergo both mitosis and meiosis Major Phases of Gametogenesis: 1. The origin of the germ cells and their migration to the gonads – primordial germ cell stage (during neurula). 2. Multiplication of the germ cells in the gonads through the process of mitosis – primordial germ cells become spermatogonia. 3. Reduction of the numbers of chromosomes by one-half by meiosis – primary spermatocyte produced. 4. Final stages of maturation and differentiation of the gametes into spermatozoa or ova that are capable of fertilizing or being fertilized 16 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation SPERMATOGENESIS ∙ Formation of spermatozoa ∙ Duration is from before birth to sometime between sexual maturity and death of the individual ∙ Spermatocytogenesis = formation of spermatids in Seminiferous tubules. ∙ Spermiogenesis = transformation of spermatids to spermatozoa. Part of Spermatid Part of Spermatozoon Nucleus Head Centrosome Axial filament of tail Centriole Located in Neck of sperm  Cytoplasm Reduced and become envelope around head, mid piece and tail Golgi Apparatus Acrosome Mitochondria Spiral filament of mid piece 17 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Sperm-producing capacity Bull Boar Ram Stallion  Sperm 16 27 25 20 X106/gram of testis/day Testis weight (grams) 350 360 275 200 Total sperm 11 19 14 8 production (x109 sperm) Length of 61 34 49 49 spermatogenesis Semen and ejaculation characteristics of farm species 18 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Cattle Dairy Beef Sheep Swine Horses Volume (ml) 6 4 1-2 225-400 60-100 Sperm 1.2 1.0 3.0 0.2 0.15 concentration (billion/ml) Total sperm 7 4 3 45 9 (billion) Motile sperm 70 65 75 60 70 (%) Morphologicall 80 80 90 60 70 y normal sperm (%) pH 6.5-7.0 6.5-7.0 5.9-7.3 6.8-7.3 6.2-7.8 Sperm Numbers in regions of the male tract (x 109) Sexually Depleted Bull Bull on Rested (Bull) (20 6 ejaculation/5hr) ejaculation/wee k Caput 19.4 16.2 22.6 Corpus 4.7 3.0 5.2  Cauda 37.6 13.7 26.4 Vas Deferens 7.9 2.1 3.3 and Ampulla Movement through the epididymis Ram – 14-20 days Bull – 8-11 days Boar – 10 days Fate of unejaculated sperm 1. ½ of produced sperm not available for ejaculation 2. Reabsorbed by excurrent duct system 3. Some selective removal of abnormal sperm in the epididymis (macrophages) 4. Sperm lost in urine Abnormalities Of Spermatozoa 1. Dwarf 19 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation 2. Abnormal head - round, very large or very small, double-headed 3. Abnormal neck and body - crooked neck 4. Abnormal tail - short tail, tailless, double tail, coiled tail OOGENESIS ∙ Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. ∙ It is developed from the primary oocyte by maturation. ∙ Initiated in the embryonic stage. ∙ Duration is from before birth to sometime between sexual maturity until loss of fertility ∙ Primary oocyte (2N) remains in PROPHASE I until it is ovulated 🡪 Secondary oocyte (1N) and 1st Polar Body (1N) ∙ If fertilized by sperm = Secondary oocyte completes MEIOSIS II = fertilized ovum (zygote = 2N) and 2nd Polar body (1N) ∙ In mammals, the first part of oogenesis starts in the germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary. ∙ Oogenesis consists of several sub-processes: oocytogenesis, ootidogenesis, and finally maturation to form an ovum (oogenesis proper).  20 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  21 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Oocyte Structure 22 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  ∙ Primordial Follicle - squamous epithelial cells that surrounds the primary oocytes. ∙ Primary Follicle - epithelial follicular cells that surrounds the oocyte during puberty stage. ∙ Secondary Follicle - maturing ovarian follicle consisting of an oocyte surrounded by two or more layers of tall, supporting granulosa cells. ∙ Graafian Follicle Cells - A mature follicle. A fluid-filled structure in the mammalian ovary within which an ovum develops before ovulation. It provides for the maturation and release of a fertilizable oocyte. It also forms the corpus luteum, which promotes and maintains implantation of the embryo. ∙ Zona pellucida - The thick transparent membrane surrounding a mammalian ovum before implantation. Glyco proteins secreted by the oocyte that forms the translucent a cellular layer between vitelline membrane and the follicular cells. Supports communication between oocytes and follicle cells during oogenesis; protects oocytes, eggs, and embryos during development, and regulates interactions between ovulated eggs and free-swimming sperm during and following fertilization. ∙ Antrum - a fluid filled cavity in the of an ovarian follicle filled with follicular fluid. Appearance of the follicular antrum during follicular maturation is the first sign that a follicle has reached the next stage of maturation. It has changed from a primary follicle to a secondary follicle. ∙ Granulosa Cells - stratified layer of the follicular cells and surrounds each oocyte. ∙ Thecal Cells - Group of ovarian follicles divided into two layers, the theca interna and the theca externa. Endocrine cells in the ovary made up of connective tissue surrounding the follicle that has many diverse functions during folliculogenesis. These roles include synthesizing androgens, providing signal transduction between granulosa cells and oocytes during development by the establishment of a vascular system, providing nutrients, and providing structure and support to the follicle as it matures. ∙ Cumulus Oophorus - are accumulated granulosa cells that attached the oocyte to follicular wall. 23 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  ∙ Corona Radiata - an outer layer of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation. Its main purpose in many animals is to supply vital proteins to the cell. Fate of follicles and oocytes (in cows) Total # of follicles At birth 100,000 3 months 75,000 11/2 – 3 year 21,000 Aged cow 2,500 Life span of sperm and oocyte Ova Sperm (hours) (hours) Cattle 20 - 24 30 – 48 Horse 6-8 72 – 120  24 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Sheep 16 - 24 30 – 48 Swine 8 –10 24 - 72 Types of Ovum Type of Ovum Amount of Yolk Yolk Distribution Animal Microlecithal Scanty Isolecithal Placental Mammals Mesolecithal Moderate amount Increasing Amphibians gradient from Animal pole to Vegetal pole Macrolecithal Large amount Telolecithal (at Birds the vegetal pole of the egg) Types Of Eggs Based On Yolk Content And Distribution 1. Oligolecithal - with little yolk 2. isolecithal - yolk dispersed uniforml 3. Mesolecithal - moderate amounts of yolk 4. Telolecithal - large amount of yolk, concentrated at one end 5. Centrolecithal - yolk concentrated in center of egg Ovum vs Spermatozoon Characteristics Ovum Spermatozoon Store Food Has stored food (yolk) No stored food Motility Relatively non motile Highly mobile # of gamete One ovum per 1 4 sperms per 1 produced/1 primary oocyte primary gametogenesis spermatocyte # of gametes produced Definite number Indefinite number by 1 individual Sex Determination Birds - Sex is determined by sex chromosome of ovum (Z or W) w/c will be fertilized by sperm (Z). ZW = female, ZZ = male Mammals - XY = male, XX = female  FERTILIZATION Fertilization is the process whereby the spermatozoon and the ovum fuse to form a single celled zygote. Following the penetration of the vitelline membrane by the spermatozoon, the activated ovum completes meiosis and extrudes the second polar body. The chromosomes contained in the haploid male pro nucleus align with the corresponding chromosomes in the female pronucleus. The paternal and maternal chromososmes condense, become attached to mitotic spindles and align themselves centrally. The first mitotic division of cleavage follows. The integration of the paternal and maternal genetic material, which occurs during this process is referred as syngamy. 25 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation ∙ 1N ovum + 1N sperm = 2N zygote ∙ Site of fertilization: Fallopian Tube ∙ START: gamete fusion ∙ END: start of cleavage ∙ Secondary oocyte completes Meiosis II, ∙ if unfertilized = degenerates ✔ Many aquatic animals release ova and spermatozoa into the water and fertilization takes place in this aqueous environment. A consequence of courtship and mutual chemical attraction between male and female gametes increases the probability of courtship. ✔ Internal Fertilization – fertilization of the ova that is retained within the female reproductive tract. This is affected with the high numbers of spermatozoa released at copulation and relatively large size of the ovum. ✔ Millions of spermatozoa are deposited in the female tract, only hundreds of spermatozoa reach the site of fertilization. ✔ Involvement of more than one spermatozoon in fertilization (polyspermy) is an abnormal occurrence in mammals and leads to embryonic death. ✔ Spermatozoa are deposited in the vagina or uterus in female reproductive tract after coitus. ✔ They are transported to uterine tube. 3 regions – infundibulum (fimbriae – funnel shaped that captures the ova, ampulla – fertilization takes place and isthmus – important reservoir for spermatozoa). ✔ Transportation of spermatozoa occurs in two phases: rapid phase – associated with muscular contraction of the tract following coitus, with spermatozoa present in the ampulla of the uterus within five to 15 minutes, and slow sustained phase – it continues for some hours, the sperm move from vagina or uterus to the isthmus, bind to mucosal epithelium resulting in the suppression of their motility. Volume of ejaculate, number of spermatozoa per ml, and site of deposition of spermatozoa in the female reproductive tract of domestic animals  Capacitation - process of biochemical and physiological modifications of the spermatozoa to fertilized ova. It involve the removal or alteration of the inhibiting factors derived from the seminal plasma, which coat the spermatozoa in the epididymis. Cellular events in the process of fertilization. For entry into the ovum, the spermatozoon mist first pass the cells of corona radiate, penetrate the zona pellucida and fuse with the oocyte cell membrane. Passage 26 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation of the sperm through the corona radiate is through vigorous motility and the release of the hyaluronidase, which breaks down the hyaluronic acid binding the corona cells together. Sperm bind in the species specific interaction with a glycoprotein receptor molecule, ZP3, in the zona. Acromosome reaction – penetration of the spermatozoon in the zona release enzymes hyalurinadase and acrosin. The release of the enzymes and the inherent motility of the spermatozoon facilitate the penetration of the zona pellucida. The vitelline membrane of the ovum fuses with the cell membrane of the spermatozoon thus incorporating it within the ovum. Barriers to polyspermy. Entry of more than one spermatozoon into mammalian ovum (polyspermy) leads to death of the zygote. This is prevented by the natural anatomical barriers of the female reproductive tract (cervix and uteru tubal junction) and Zona reaction –ovum defense against polyspermy which prevents the entry of more than one spermatozoon, the release of an ezyme from the zona when the head the spermatozoon comes in contact with the surface of the oocyte and it prevents the penetration of additional spermatozoa. Zona reaction is effective in humans, cattle, sheep and dogs; less effective in pigs, cats, rats and mice and in effective in rabbits. In avian, polyspermy does not endanger zygote survival because the other spermatozoa degenerates without any adverse effect to the fertilized ovum. In vitro Fertilization (IVF). The process whereby the secondary oocytes are fertilized with capacitated spermatozoa outside the body. In this procedure, under appropriate laboratory conditions, spermatozoa fertilize oocytes and the resulting embryo can be cultured to the cleavage stage prior to transfer to a female of the same species. Successfully practiced to cattle, sheep, pigs and humans. It promotes the increased production of offspring of genetically superior breeding stock and enhancement of the breeding rate of endangered species. Comparative Fertilization Rates. Fertilization rate refers to the ova released at ovulation which are fertilized following the natural or artificial insemination. Polytocous animals (dogs and cats) 85% - 100% and monotocous animals (cattle and sheep) 85% - 95%. In horses 60%.  Sex Determination. Every normal nucleated cell in the animal body contains a fixed number of chromosomes. Mammals – Female XX, Male XY Avian, Fish, Amphibians and Reptiles –Female ZW, Male ZZ. An ovum fertilized by the X bearing chromosomes is destined to be female (XX) and Y bearing chromosomes will become male (XY). ∙ Turtles and crocodiles – sex is determined by the incubation temperature (Male – 16oC to 28 oC, Female - 32 oC. Parthenogenesis. The development of an embryo from an ovum that has been activated by means other than spermatozoon. This occurs naturally to insects and lower animals. Can be induced experimentally in amphibians, birds and mammals.Occurs naturally in turkeys and rarely in chickens, produced are always male (ZZ). Chromosomes of Domestic Animals. Species Primary Sex Ratio Primary Sex Ratio Male Female Male Female Humans 50 50 51 49 Cattle 50 50 52 48 Dogs 50 50 54 46 Horses 50 50 52 48 Pigs 50 50 52 48 Sheep 50 50 50 50 27 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Events Leading To Fertilization 1. Insemination - in cervix of uterus 2. Sperm capacitation - removal of surface proteins that cover the sperm, enzymatic alteration in cell membrane of sperm increasing sperm motility 3. Migration of spermatozoa to the fallopian tube 4. Spermatozoon binds to specific Glycoprotein (GP) on Zona Pellucida (ZP) that surrounds the oocyte 5. Acrosome reaction = release of lytic enzymes hyaluronidase 🡪 corona radiata (CR) acrosin or zona lysis 🡪 zona pellucida 6. Spermatozoon and oocyte plasma membranes FUSE (secondary oocyte completes Meiosis II) 7. Ovum forms FERTILIZATION CONE 8. Spermatozoon penetrates corona radiate and zona pellucida 9. Ovum forms FERTILIZATION MEMBRANE 10. Formation of Female and Male PRONUCLEUS 11. Female and Male 1N pronucleus make contact and begin MITOSIS - DNA synthesis takes place before mitosis, mitosis begins 12 hrs after fusion 12. UNION of female and male CHROMOSOMES - centriole of sperm move towards opposite poles  of ovum and form spindle fibers 28 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  CLEAVAGE Cleavage refers to the initial series of mitotic divisions by which the large zygote is fractionated into numerous “normal size” cells. Each daughter cell of the cleavage process is termed a blastomere. Cleavage begins with a zygote, progresses through compaction to a morula stage and terminates at the start of the blastocyst (blastula) stage. The first eight blastomeres are undifferentiated and have identical potential in mammals; thereafter, blastomeres differentiate into inner & outer cells with different missions. The first cleavage division occurs 1 to 5 days following ovulation (depending on species), thereafter cells divide about once every 12 hours. As many as eight generations of mitoses may occur without intervening cell growth (cytoplasmic increase). Thus, e.g., one 150 micron diameter zygote can becomes a collection of 256 cells, each about 7 microns in diameter. The fertilized ovum with a diameter of 80 -120µm is one of the largest mammalian cells and has a large amount of cytoplasm relative to the size of its nucleus. As cleavage proceeds, division of the cytoplasm follows nuclear division and two daughter cell produced and referred as blastomeres. The two blastomeres divide repeatedly, producing four, eight, 16 and 32 cells, and divison continues until spherical mass of cells, termed, morula, is formed. 29 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Morula [L.= small mulberry] is a solid ball of blastomeres within a zona pellucida (typically consisting of 16 to 64 blastomeres). blastomeres become compacted; cells on the inside differentiate from those along the surface of the morula: outer blastomeres become flattened and form tight junctions (reducing fluid permeability); they develop the capacity to secrete fluid (internally); they are destined to become trophoblasts which form the chorion & amnion (fetal membranes) of the conceptus and inner blastomeres form gap junctions to maximize intercellular communication; they are destined to become inner cell mass which forms the embryo itself (plus two fetal membranes). As few as three inner blastomeres are sufficient to produce an entire embryo (and adult). When a morula leaves the uterine tube and enters the uterus (uterine horn) it is at about the 16-cell stage, around 4 to 7 days after fertilization (depending on species). The 32-cell stage morula (5-7 days post ovulation) is ideal for embryo transfer in cattle. The yolk content of the fertilized ovum determines the pattern of cleavage in individual species: Miolecithal (Isolecithal) ova – are ova with small amount of evenly distributed yolk. Produced by the primitive chordates and placental mammals, blastomeres are equal size. Megalecithal (Telolecithal) ova – are ova with amount of yolk present displaces the embryo – forming cytoplasm into small area at the animal pole. Applies in the fish, reptiles and birds. Mitosi is restricted to animal pole where cytoplasm is devoid of yolk. Type of division is partial or meroblastic cleavage. The type of cleavage is also known as discoidal. Medialecithal (mesolecithal) ova – are ova with a moderate amount of yolk. Produce by the amphibians. The yolk accumulation at the vegetal pole retards mitosis, and blastomeres of un equal size are produced. Holoblastic cleavage is the divisions in which the entire ovum divides and the blastomeres produced are either equal or un equal size. The final stage of cleavage is marked by the formation of the blastula which consist of the single layer of cells lining a central cavity known as blastocoele. Blastocyst (or Blastula) develops during the second week, after the zona pellucida ruptures. It consists of a large number of blastomeres arranged to form a hollow, fluid-filled, spherical or cylindrical structure. It contains an inner cell mass (embryoblast), evident as a collection of cells localized inside one polar end of the blastula. The surface cells of the blastocyst are designated trophoblasts (future chorion of the conceptus). The cavity of the blastocyst is called a blastocoele. Eventually the blastocyst attaches to or implants within the uterine wall (pending species). CLEAVAGE IN THE PRIMITIVE CHORDATES, AMPHIBIANS, AND AVIAN SPECIES AND MAMMALS Primitive Chordates. Cleavage is holoblastic and blastomeres produced are almost equal size. Cleavage furrows –morula – blastula (surrounded by the blastocoele). Amphibians. Cleavage is holoblastic and un equal. 30 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Avian species. Megalicithal ovum. The 3 mm diameter blastodisc, which the embryo develops. Fertilsed ovum passes down female tract and acquires albumen and shell membrane. The ovalbumin, provides inert food in the developing embryo. Its journey is 24 to 26 hours, cleavage is complete and consequently gastrulation had commenced. The region of the clear yolk is termed as subgerminal cavity. The blastodisc (blastoderm) is composed of two regions: area pellucida - is the central region and which the embryo develops and area opaca – is the peripheral region which digest the underlying yolk which nourishes the embryo. Mammals. Cleavage is holoblastic. Cleavage takes place within zona pellucida, and taking 24 hours for the first cleavage divisions. Subsequent divisions take place with 12 hours interval. Early cleavage divisions are reported to be control of the mRNA derived from the maternal gamete. Compaction. The process that gives the blastomeres a defined orientation for the first time as each cell has a fixed contact area with adjacent cells and free outer surface. This occurs at human and mice at 8 cell stage, 16 cell stage in sheep and 32 cell stage in cattle. Embryonic Stem Cells. Cells in the embryo which have the ability to differentiate into all the cell types required for the formation of the tissues, organs and systems. 31 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  GASTRULATION Gastrulation is the morphogenic process that gives rise to three germ layers: ectoderm, mesoderm, and endoderm. (In some species, evidence of primitive gut formation can be seen [gastrula Gr.= little stomach].) It is the critical period of developmental because potential of the tissues because potential of tissue to develop into organs begins to be restricted and developing organs most sensitive to external agents (teratogens). Gastrulation includes the following sequence, beginning with a blastocyst: — A thickened embryonic disc becomes evident at the blastocyst surface, due to cell proliferation of the inner cell mass cells. Trophoblast cells overlaying the inner cell mass degenerate in domestic mammals (in the mouse and human, trophoblast cells overlaying the inner cell mass separate and, instead of degenerating, become amnionic wall.) — From the inner cell mass, cells proliferate, break loose (delaminate), and migrate to form a new cell layer inside the trophoblast layer. The new layer of cells, called the hypoblast, will form a yolk sac. The remaining inner cell mass may be called the epiblast. — On the epiblast surface, a primitive streak forms as differential cell growth generates a pair of ridges separated by a depression. [NOTE: The primitive streak defines the longitudinal axis of the embryo and indicates the start of germ layer formation. — Deep to the primitive streak, a space (coelom/celom) becomes evident between the hypoblast layer and epiblast. Subsequently, the coelom is filled by mesoderm that undergoes cavitation and gives rise to body cavities. — Epiblast cells proliferate along primitive streak margins and migrate through the streak into the coelom. The migrating cells form endoderm & mesoderm layers. — Initial migrating cells join the hypoblast layer, forming embryonic endoderm (hypoblast cells constitutes yolk sac endoderm). — The majority of migrating cells enter the coelom as primary mesenchyme and become mesoderm. The primary mesenchyme migrates laterally and cranially (but not along the midline region directly cranial to the primitive streak where notochord will form). Note: Mesoderm divides into: paraxial, intermediate, and lateral mesodermal regions. — Within the lateral mesoderm, cavitation re-establishes a coelom (hoseshoe-shaped). The mesoderm splits into two layers bordering the coelom—somatic mesoderm is attached to the ectoderm and 32 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  splanchnic mesoderm is joined to endoderm. — The remaining epiblast becomes ectoderm which forms skin epidermis & nervous system. Note: Migrating epiblast (INVOLUTION) cells displacing the hypoblast became the intraembryonic endoderm; Displaced hypoblast 🡪 extraembryonic (yolk sac) endoderm; 33 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  migrating epiblast entering the coelom 🡪 mesenchyme 🡪 mesoderm and remaining epiblast 🡪 ectoderm Division of Mesoderm - Paraxial Mesoderm, intermediate Mesoderm and lateral Mesoderm Division of Lateral Mesoderm Somatic mesoderm = beneath the ectoderm – Ectoderm + Somatic mesoderm = – Somatopleure Splanchnic mesoderm = overlies the endoderm – Endoderm + Splanchnic mesoderm = Splanchnopleure Coelom = space between 2 mesodermal layers 🡪 body cavities Mesoderm can exist in two morphologic forms: mesenchyme and epithelioid: Mesenchyme features aggregates of stellate cells within an abundant extracellular matrix composed of fluid and macromolecules (polymers). Epithelioid refers to organized cells having distinct apical and basal surfaces; the latter commonly rests on a basal lamina produced by epithelioid secretion. Mesoderm can transform from a mesenchyme to epithelioid and vice versa: The mesoderm that streams through the primitive streak is primary mesenchyme. Somatic, splanchnic, and somite mesoderm can be temporarily epithelioid. The temporary epithelioid transforms to a secondary mesenchyme which ultimately forms muscle and connective tissue (including cartilage, bone, ligaments, tendons, dermis, fascia, and adipose tissue). Thus, the term “mesenchyme” refers to the morphologic appearance of embryonic tissue. Although most mesenchyme is mesoderm, the other germ layers can also form mesenchyme, e.g., ectomesenchyme from neural crest ectoderm. Formation of the Notochord The notochord is a rod-shaped aggregate of cells located between ectoderm and endoderm anterior to the primitive streak of the embryo. It occupies the midline coelomic space that was not invaded by migrating primary mesenchyme. The notochord is important because it induces: formation of the head process, development of the nervous system, and formation of somites The notochord marks the future location of the vertebral column and the base of the cranium. The ultimate fate of the notochord is to become nucleus pulposus of intervertebral discs. Note: The notochord develops from the primitive node located at the cranial end of the primitive streak. From the node, mesoderm-forming cells proliferate and migrate forward into the future head region where they become the rod-shaped notochord.  34 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Gastrulation in Birds. Start: 6hrs after egg is laid (1st day of incubation) and end: 48th hr (2nd day of incubation). Formation of primitive streak - maximum length: 18 hrs of incubation and disappears: 60 hrs of incubation. NEURULATION Neurulation refers to notochord-induced transformation of ectoderm into nervous tissue. The process begins during the third week in the region of the future brain and then progresses caudally into the region of the future spinal cord. The neurulation process involves the following steps: — ectodermal cells overlaying the notochord become tall columnar (neuroectoderm); they form a thickened area designated the neural plate. The other ectodermal epithelium is flattened. — a neural groove is formed as edges of the  neural plate become raised on each side of a midline depression. (Apical ends of individual neuroectodermal cells constrict.) — a neural tube is then formed as the neural groove undergoes midline merger of its dorsal edges. The tube separates from non-neural ectoderm which unites dorsal to it. (Tube formation begins in the cranial cervical region of the central nervous system and progresses cranially and caudally until anterior and posterior neuropores, the last openings, finally close.) — bilaterally, where the neural groove is joined to non-neural ectoderm, cells detach as the neural groove closes; the cells proliferate and assume a position dorsolateral to the neural tube—forming neural crest. Neural tube becomes the central nervous system, i.e., the brain and spinal cord. Neural crest cells are remarkable for the range of structures they form. Some cells migrate dorsally and become pigment cells in skin. Other cells migrate ventrally and become neurons and glial cells of the peripheral nervous system, or adrenal medulla cells. In the head, neural crest forms mesenchyme (ectomesenchyme) which becomes meninges, bone, fascia, and teeth. In the early stage of development during neurulation, the following occurs: – Early development of CNS = formation of neural tube – Formation of head process = because of cephalic folding 35 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation – Formation of Somites = from paraxial mesoderm – Initial development of Heart = formation of cardiac tube – Others: formation of foregut, tail process, hindgut and lateral body folds = cylindrical body Neural crest - from detached neural ectodermal cells gives rise to the following: – Dorsally: pigment cells in skin – Ventrally: neurons and glial cells of PNS – In the head region 🡪 ectomesenchyme 🡪 meninges, bone, fascia, and teeth – Formation of Somites. Collectively, the somites constitute the vertebral plate. Out of the somites arise the sclerotome, forerunner of the bodies and neural arches of the vertebrae; the dermatome, precursor of the connective tissue of the skin; and the myotome, or primitive muscle, from which the major muscles of vertebrates are derived. The term somite is also used more generally to refer to a body segment, or metamere, of a segmented animal. It starts during the 20th hour of incubation. Different Regions of Somite Dermatome (lateral) 🡪 skin DERMIS – Surface ectoderm 🡪 skin EPIDERMIS Myotome (middle) 🡪 skeletal musculature Sclerotome (medial) 🡪 most of AXIAL SKELETON (vertebrae, ribs, bone of skull) Somites develop as follows: — mesoderm, designated paraxial mesoderm, accumulates on each side of the notochord — progressing from rostral to caudal over time, transverse fissures divide the paraxial mesoderm into blocks — each block becomes a somite (epithelioid cells within a somite block re-orient 90°, from transverse to the notochord to longitudinal) — head (occipital) somites develop from proliferation of local mesenchyme lateral to the cranial end of the notochord — rostral to the notochord, mesenchyme forms less-developed somites, called somitomeres; these migrate into pharyngeal arches and form muscles of the jaw, face, pharynx, & larynx.  FOETAL MEMBRANES (BIRDS AND MAMMALS) The fetal membranes are membranes associated with the developing fetus. It surround the developing embryo and form the fetal-maternal interface. Structures or tissues which develop from the zygote and which do not from part of the embryo itself and are functional importance only in the embryonic life and are called extra embryonic or foetal membranes. Their function is the supply or storage of nutrients, respiratory exchange, excretion and mechanical protection of the embryo. They are also concerned with the transfer of immunoglobulins from mother to the embryo which confer passive immunity. In mammals, foetal membranes are involved in the hormone production and formation of the placenta. As these membranes are solely required from embryological development, they are either shed or absorbed at hatching or birth. Fetal membranes developed during neurula stage. Extraembryonic ectoderm + Somatic Mesoderm = Somatopleure 🡪 CHORION and AMNION Extraembryonic endoderm + Splanchnic Mesoderm = Splanchnopleure 🡪 YOLK SAC and ALLANTOIS Four Fetal Membranes Yolk Sac. 1st to be formed. Connected to midgut via yolk stalk. Supplied by vitelline vessels (omphalomesenteric). Most important in egg laying vertebrates. Serves as early nutrition for embryo, later shrinks and non functional 🡪 Meckel’s diverticulum (outpocketing of small intestine) and vestigial in 36 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Mammals. Amnion. The amnion arises by a folding of a mass of extra-embryonic tissue called the somatopleure. Lined with ectoderm and covered with mesoderm (both are germ layers), the amnion contains a thin, transparent fluid in which the embryo is suspended, thus providing a cushion against mechanical injury. The amnion also provides protection against fluid loss from the embryo itself and against tissue adhesions. Inner sac of extraembryonic somatopleure (CHORIOAMNIOTIC FOLD). Filled with amniotic fluid = mechanical protection = shock-absorbing environment for the fragile embryo. Merges with umbilical cord = epithelial layer of umbilical cord. Chorion. The chorion is one of the membranes that surround the fetus while it is still being formed. In mammals, the fetus lies in the amniotic sac, which is formed by the chorion and the amnion and separates the embryo from the mother’s endometrium. The chorion in turn comprises two layers: a double layer of trophoblasts on the outer side and the mesoderm on the inner side, in contact with the amnion. The outer layer of the chorion is made of trophoblasts (also known as the trophoblast), which are the first cells to differentiate once the mammalian egg has been fertilized. They first form the outer layer of the blastocyst and eventually develop into most extraembryonic tissues, including a part of the chorion referred to as the chorion trophoblast cells, also known as the extraembryonic ectoderm. The inner layer of the chorion is the mesoderm, which is one of the first layers to develop in the embryo and lies between the endoderm and the ectoderm. The chorion has two main functions: protect the embryo and nurture the embryo. To protect the embryo, the chorion produces a fluid known as chorionic fluid. The chorionic fluid lies in the chorionic cavity, which is the space between the chorion and the amnion. The chorionic fluid protects the embryo by absorbing shock originating from forces such as movement. To nurture the embryo, the chorion grows chorionic villi, which are extensions of the chorion that pass through the uterine decidua (endometrium) and eventually connect with the mother’s blood vessels.  Outersac = outer boundary of entire conceptus Derived from extraembryonic somatopleure Contains the extraembryonic coelom = Chorioamniotic cavity Fuses with allantois = CHORIOALLANTOIC MEMBRANE = mediates gas and water exchange = wall of chorionic vesicle Allantois. The allantois (plural allantoides or allantoises) is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extra embryonic tissues). It helps the embryo exchange gases and handle liquid waste. The allantois, along with the amnion and chorion (other extraembryonic membranes), identify humans and other mammals as well as reptiles (including birds) as amniotes. Of the vertebrates, only the anamniotes (amphibians and non-tetrapod fish) lack this structure. This sac-like structure is primarily involved in nutrition and excretion, and is webbed with blood vessels. The function of the allantois is to collect liquid waste from the embryo, as well as to exchange gases used by the embryo. Derived from extraembryonic splanchnopleure Ventral diverticulum of hindgut splanchnopleure Highly vascular; functional vessels of placenta = Allantoic blood vessels🡪 umbilical blood vessels Urachus ligament is the remnant of allantois that connects the belly button to the bladder 37 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  38 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Birds Fetal Membrane Mammals Fetal Membrance 39 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  FORMS OF IMPLANTATION AND PLACENTATION Implantation The term implantation is used to describe the attachment of the developing embryo to the endometrium. This process, which occurs in three stages in domestic animals, is gradual, with apposition of the blastocyst or foetal membranes to the uterine epithelium followed by adhesion. Depending on the species, the final stage may involve firm attachment or actual invasion of the endometrium. As an embryo remains relatively independent of maternal influences prior to implantation, it can be grown to the blastocyst stage in vitro. However, from the time of implantation onwards, the viability of the conceptus is greatly influenced by maternal factors, with embryonic survival dependent on hormonal and immunological adaptation of the dam to pregnancy. The form of implantation differs from one species to another. In primates and guinea pigs, the blastocyst burrows through the uterine epithelium to the uterine stroma where the embryo develops. This form of implantation is referred to as interstitial implantation. In rodents, implantation involves the blastocyst becoming lodged in a uterine cleft with proliferation of the surrounding uterine mucosa. This form of implantation is known as eccentric implantation. In horses, cattle, sheep, pigs, dogs, cats and rabbits, the fluid‐filled sacs surrounding the embryo expand so that the extra‐embryonic membranes become apposed to the endometrium and attach to it. This form of implantation, the most common form of attachment in mammals, is referred to as centric or superficial implantation. In animals with either interstitial or eccentric implantation, these three stages of attachment occur within a short time interval and it is possible to estimate accurately the time of implantation. With centric or superficial implantation, the stages of attachment extend over a longer time period than in interstitial implantation and wide variation has been reported for the time of implantation in ruminants and horses. The interval between fertilisation and implantation in humans and in selected domestic animals. l Time Rodents Humans Rabbits  40 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Cats 12 to 14 Pigs 12 to 16 Dogs 14 to 18 Sheep 14 to 18 Cattle 17 to 35 Horse 17 to 56 s In utero spacing and embryo orientation After reaching the uterus, blastocysts move to their implantation sites. In cattle and sheep, when a single oocyte is fertilised, the blastocyst attaches to the middle or upper third of the uterine horn adjacent to the ovulating ovary. In sheep, when two blastocysts are derived from one ovary, one blastocyst usually migrates to the contralateral horn, where it becomes implanted. As intra‐uterine migration is rare in cattle, when twins arise from ovulation involving one ovary, both embryos usually develop in the same horn. In mares, ultrasonography has demonstrated that, irrespective of which ovary ovulates, the blastocyst migrates between the left and right uterine horns from the 11th to the 17th day. After this time, mobility ceases and the blastocyst implants in either the left or right horn close to the body of the uterus. In polytocous animals, those producing litters, the blastocysts are evenly spaced within the uterine horns. Although the underlying mechanism responsible for the spacing of implanting blastocysts is unclear, oestrogen produced by the developing blastocyst is considered to have an important role in embryo spacing. Endocrine control of implantation Implantation requires cooperative interaction between the dam and the blastocyst. The high levels of oestrogen produced during the follicular stage of the oestrous cycle cause proliferation of the endometrium and, in addition, progesterone produced during the luteal stage renders the endometrium receptive to the blastocyst. In all mammals, progesterone is essential for both the establishment and maintenance of pregnancy. For maintenance of pregnancy in domestic mammals, continued functioning of the cyclical corpus luteum is a requirement and this is achieved through the production of anti‐luteolysin by the conceptus which inhibits the production of luteolytic uterine secretions. This response to the presence of the conceptus is referred to as maternal recognition of pregnancy. While the basic strategy is to maintain and prolong the cyclical corpus luteum by inhibiting or reducing the secretion of prostaglandin F2α (PGF2α), the factors which control the process show species variation. In species in which the life span of the corpus luteum is similar in pregnant and non‐pregnant animals, recognition of pregnancy may occur by different means. Delayed implantation In a number of species, there is an unusually long delay between the entry of the blastocyst into the uterus and the time at which implantation occurs. In these species, the blastocyst enters a period of decreased cell division and metabolic quiescence, referred to as diapause, a state characterised by decreased protein  and nucleic acid synthesis and a decline in carbon dioxide output. In mink and ferrets, the interval is comparatively short, usually a matter of weeks, whereas in roe deer, bears, badgers and seals, the interval may be substantially longer, up to four months in some instances. Delayed implantation increases the probability that offspring are born at a time of year favourable for survival. Although there is limited information on the underlying mechanisms which operate in delayed implantation, both uterine and 41 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation hypothalamic factors are implicated. When blastocyst development is slowed as a consequence of seasonal influences, this type of diapause is referred to as seasonal or obligative delayed implantation. In addition to those animals in which delayed implantation is a normal occurrence, a similar but shorter delay may occur in certain species of rodents and insectivores. The delay in implantation in these species is attributed to the influence of stress factors, such as lactation, which inhibit implantation. If rodents become pregnant during a post‐partum oestrus, blastocyst implantation is delayed until weaning occurs. This delay is influenced by litter size. With a litter size of one or two, implantation is not delayed, whereas with six or more offspring there may be a delay of up to six days. This mechanism, which ensures that the dam does not have to support two litters contemporaneously, is referred to as facultative or lactational delayed implantation. Ectopic pregnancy Implantation and subsequent embryonic development in an extra‐uterine location is referred to as ectopic pregnancy. Sites of abnormal implantation include the ovary, the uterine tube and the peritoneal cavity. Ectopic pregnancy, which occurs more frequently in humans than in domestic animals, usually leads to death of the embryo or foetus and may be accompanied by severe maternal haemorrhage and sometimes death. Embryonic mortality In the absence of infectious diseases, and despite optimal nutrition, early embryonic mortality is a frequent occurrence in all domestic species. Most of these early embryonic deaths, which occur around the time of maternal recognition of pregnancy or the time of implantation, are attributed to defective interaction between the conceptus and the dam. Survival of the developing embryo depends on the establishment of a placenta, the formation of which, in turn, depends on cooperative interactions between the blastocyst and the uterus. These interactions are affected by complex factors, which involve adequate hormonal stimulation of the endometrium, environmental stimuli and the nutritional status of the mother. Factors which may contribute to early embryonic mortality are hormonal imbalance, maternal rejection and chromosomal abnormalities in the developing embryo. Placentation Present only in PLACENTAL MAMMALS. When the blastocyst reaches the uterus, it is initially sustained by uterine secretions and, after a short delay, it attaches to the endometrium with the subsequent formation of a placenta. This complex structure allows selective exchange of nutrients, gases and waste products. It also functions as a site of hormone production. Based on the relationship between foetal membranes and maternal tissue, two basic types of placentae are recognised, choriovitelline and chorioallantoic. When the fused vascular choriovitelline membranes become attached to the endometrium, the resulting placenta is known as a choriovitelline or yolk sac placenta. This type of placentation is commonly encountered in marsupials. When the chorioallantoic membrane becomes attached to the  endometrium, the resulting placenta is referred to as a chorioallantoic placenta. While this is the definitive form of placentation in higher mammals, it may be preceded by and co‐exist with a temporary choriovitelline placenta. Parts of Placenta The placenta is a fetal organ made up of its parenchyma, chorion, amnion, and umbilical cord. The fetal structures form from the zygote and therefore separate the fetus from the endometrium. The fetal tissues form from the chorionic sac - which includes the amnion, chorion, yolk sac, and allantois. 42 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation 3 Layers of Fetal Placenta 1. Endothelium of allantoic vessels 2. Connective tissue (c.t.) = somatic mesoderm of chorion + splanchnopleure of allantois 3. Epithelium = chorionic ectoderm 3 Layers of Maternal Placenta 1. Epithelium of Endometrium 2. Connective tissue of endometrium 3. Endothelium of uterine blood vessels Types of Placenta According to Number of Layers 1. Epitheliochorial = all 6 layers intact Species: Horse, Pig, Cattle Partially: Sheep and Goat 2. Synepitheliochorial = only 5 intact layers 3 from fetal, 2 from maternal Epithelium of endometrium degenerates Species: Partially: Sheep and Goat  Also known as SYNDESMOCHORIAL 3. Endotheliochorial = only 4 intact layers 3 from fetal, 1 from maternal epithelium and c.t. of endometrium degen Species: carnivores 4. Hemochorial = only 3 intact layers from fetal all 3 layers of maternal placenta degen *ectoderm of chorion contacts with blood in the uterine b.v. Species: Rodents, Primates, Human Types of Placenta according to Shape and Distribution of apposition areas 1. Diffuse Placenta = villi distributed throughout chorionic vesicle except undilated ends 43 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation - Species: Horse and Pig - Horse: Microcotyledonary = microplacentomes are distributed diffusely 2. Cotyledonary placenta = cotyledons (fetal) + caruncles (maternal) = placentome - Species: Ruminants = placenta consist of rows of relatively large placentomes 3. Zonary placenta = villi concentrated in a wide zone = belt around chorionic vesicle a. Complete: surrounding entire vesicle - Species: Dog, Cat b. Incomplete: doesn’t completely surround the vesicle - Species: Raccoon, Bear, Skunk, Mink, Ferret  4. Discoid Placenta = villi concentrated in oval-shaped disk a. Single = Human b. Double = Monkey 44 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Type of Placenta according to status of endometrium 1. Deciduate = uterine endometrium is sloughed off during parturition - Species: Carnivores, Human, Rodents 1. Adeciduate = uterine endometrium remains intact during parturition - Species: Horse and Pig *RUMINANTS = partially deciduate Implantation of Placenta Attachment of trophoblast (ectoderm) of chorion to epithelium of uterine endometrium Rupture of Zona Pellucida 🡪 free Blastocyst 🡪 Apposition 🡪 Adhesion Types: 1. Noninvasive implantation = trophoblast don’t grow into the epithelium of uterine endometrium. Occurs mostly in large domestic animals. 2. Interstitial implantation = trophoblast invade and partially destroy the epithelium of uterine endomentrium 🡪 endometrium close over 🡪 form nest over embryo = nidation Anomalies of Placentation 1. Hydrops of the amnion or allantois - excessive fluid in amniotic or allantoic cavities that results in fetal death and uterine and prepubic tendon rupture. Clinical Signs: progressive bilat abd distension, anorexia, recumbency 2. Strangulation by Umbilical cord = in species with long umbilical cords such as swine = neck or limb strangulation in varying degree may occur TWINNING Twinning is the development of 2 or more embryos in a dam that normally gives birth to 1 offspring/gestation Causes: Fertilization of separate ova  Complete/partial separation of blastomeres and blastocyst during cleavage Duplication after gastrula stage when specific organ-forming regions called “fields” are being organized One blood supply shared by completely separated twins Terminologies Free = fetuses are not attached from each other Conjoined = fused at certain region Symmetrical = equal in size Asymmetrical = unequal in size Monozygotic = from 1 zygote Dizygotic = from 2 zygote 45 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Types of Twins 1. Free symmetrical MONOZYGOTIC twins = IDENTICAL TWINS - One zygote - Same sex, same genetic composition - Zygote duplicates itself - Twins separate completely 2. Conjoined (fused) symmetrical MONOzygotic twins = - Incomplete separation of blastomeres later in embryonic devt, zygote incompletely divides A.) Diplopagus (2-fold joined) or SIAMESE TWINS - Classified as: a. Thoracopagus = sternal b. Abdominopagus = abdomen c. Pygopagus = pelvis/sacrum (back to back) d. Cephalopagus (Craniopagus) = head B.) Monster = abnormal twins wherein axial structures are duplicated a. Dicephalus = 2 heads b. Diprosopus = 2 faces c. Dicaudatus = 2 tails d. Tetrabrachius = 4 thoracic limbs e. Tetrascelus = 4 pelvic limbs 46 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation  3. Free Asymmetrical twins - From monozygotic or dizygotic twins - Separate twins: one normal, one rudimentary - Abnormal twin survives by being attached to blood supply of fetal membranes of normal twin - Abnormal Twin = amorphous globosus, anidian (formless) fetus, acardiac fetus or holocardius = no body form, only hair, bone, teeth, muscle and rudimentary digestive organs 4. Conjoined asymmetrical twins - Occur after gastrulation wherein specific fields are organized 47 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation - Unequal size (Heteropagus) - One normal individual = autosite, with an extra body part attached to it = parasite - E.g. Notomelus twin = extra pelvic limb attached to the back of the animal 5. Free symmetrical Dizygotic (or POLYzygotic) twins = FRATERNAL TWINS - 2 or more zygote develop separately, separate fetal membranes and placenta during same pregnancy  - Same sex or different sexes - If the twin are of different sexes, the female twin = abnormally developed genital system = freemartin * Fraternal twin comes from two zygote with separate placenta. Congenital malformations or Congenital defects Abnormalities present at birth that result from errors arising during development. Critical Period: - Time wherein organ system in the body is being formed and inductive tissue interactions and morphogenesis are already occurring - Blastula and Gastrula stage - Neurula stage = few organ systems are disrupted by environmental factors Neurula stage = less likely for congenital malformations to occur because specific fields-forming organ/structure have already been laid out Causes of Congenital Malformations 1. Hereditary or genetic or intrinsic factors = deleterious genes, associated with breed predilection e.g. Patent Ductus Arteriosus in Poodle 2. Environmental or extrinsic factors = teratogens (drug, plant, virus, pesticide, etc.) Classifications of Teratogens 1. Transmitted directly to embryo (maternal-placental-fetal interactions) a. Radiation = miscarriage, malformations 48 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation b. Viruses = e.g. Bovine Viral Disease c. Steroid hormones = autism d. Low MW compounds (vitamins, ethanol, heavy metals) - e.g. Mercury = brachygnathia/shortened jaws; scoliosis; hypoplasia of kidney, gonad, and spleen 2. Catabolic by-products of metabolism e.g. antineoplastic drug like cyclophosphamide and carcinogens 3. Substances that activate maternal metabolic enzymes which at high lvls produce toxic intermediates that often kill entire litters e.g. Phenobarbital and related barbiturates and Dioxin Aminoglycosides = ototoxicity Chloramphenicol = Gray baby syndrome Tetracycline = enamel hypoplasia Trimethoprim = increase risk of neural tube defects Phenobarbital = congenital malformations  Cyclophosphamide = congenital malformations Doxorubicin Vincristine Propylthiouracil = fetal goiter and maternal hepatotoxicity and agranulocytosis Triiodothyronine = fetal goiter 4. Deficiencies of maternal substances = Vit. A, E, B6, B2, Folic acid, and several cations like Cu, Mg, Mn, Se, and Zn e.g. Hyper- and Hypo- vitaminosis A = induce birth defects during closure of neural tube resulting in eye, brain and heart defects 5. Teratogens that attack the placenta e.g. Brucella abortus Thalidomide = highly teratogenic in primates and man = severe limb defects Aspirin = extremely teratogenic in rodents but not in primates = cleft palate, facial and tail shortening and CV defects in dog embryo EXERCISES/ DRILLS Activity No. 1 Directions: In a separate sheet, draw and label pictures of the following. 1. Stages of cell cycle. 2. Stages of mitosis. 3. Stages of meiosis. 4. Sperm cell and its parts. 5. Egg cell and its parts. 6. Ovarian cells Activity No. 2 Directions: In a separate sheet, draw and label pictures of the steps/ process that occur during fertilization. 49 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation Activity No. 3 Directions: Watch videos of gametogenesis, fertilization, cleavage, gastrulation, twinning from you tube. Write what you have learned from the videos. EVALUATION TEST I. MULTIPLE CHOICES Directions: Read the questions carefully. Choose the letter of the correct answer and encircle your answer. 1. How many chromosomes are present in goat? a. 60 b. 23 c. 46  d. 50 2. Which of the following is the first stage in mitosis? a. Anaphase b. Telophase c. Prophase d. Metaphase 3. Which of the following refers to a pair threads? a. Leptonema b. Zygonema c. Pachynema d. None of the choice 4. What is the longest phase of meiosis? a. Metaphase I b. Anaphase II c. Prophase I d. Telophase I 5. Which of the following refers to a reductional division? a. Meiosis I b. Meiosis II c. Mitosis 6. What will become of golgi apparatus and mitochondria of spermatid in the spermatozoa? a. Spiral filament of mid piece, acrosome b. Acrosome, spiral filament of mid piece c. Neck of sperm, axial filament of tail d. Axial filament of tail, neck of sperm 7. Female offspring of a hen will most likely to have a _______ set of sex chromosome. a. ZW b. ZZ c. XY d. XX 8. How many sperms produced per 1 primary spermatocyte? a. 1 b. 2 c. 3 d. 4 9. How many ovum produced per 1 primary oocyte? 50 Veterinary Developmental Anatomy (Embryology) Module 2: Early Development in Birds and Mammals and Placentation a. 1 b. 2 c. 3 d. 4 10. Which species of animals has a macrolecithal type of ovum? a. Pig b. Bird c. Fish d. Cat  11. What is the offspring of the turtle if the incubation period is 32oC? a. Female b. Male c. Both 12. What is the fertilization rates in horses? a. 85% b. 60% c. 95% d. 100% 13. Which part of the ovum where the spermatozoon must first penetrated before it enters inside the ovum? a. Zona pellucida b. Nucleus c. Corona radiate d. A&C 14. Which of the following is a glycoprotein found in ovum which is known as the species specific receptor? a. ZP3 b. AP3 c. ZP1 d. AP1 15. What is the site of spermatozoa deposition in pigs? a. Vagina b. Uterus c. Vulva d. Ovary 16. What is the volume of ejaculate per ml of the cattle in every mating? a. 1ml b. 2ml c. 3ml d. 4ml 17. Which part of the female reproductive tract is the important reservoir of the spermatozoa? a. Infundibulum b. Ampulla c. Isthmus d. Fimbriae 18. What do you call the integration of the paternal and maternal genetic material? a. IVF b. Syngamy c. Capacitation d. Synchron