Gametogenesis PDF

 Common embryologic terms 1- Embryology: It is the science which studies the prenatal (before birth) development of embryo and fetus; however, the term can also refer to postnatal development. Embryology includes both embryogenesis (general embryology) and morphogenesis {special...

 Common embryologic terms 1- Embryology: It is the science which studies the prenatal (before birth) development of embryo and fetus; however, the term can also refer to postnatal development. Embryology includes both embryogenesis (general embryology) and morphogenesis {special embryology which includes the formation of body tissues (histogenesis) and organs (organogenesis)}. 2- Embryo (embryonic period): It is the time from fertilization to the earliest (primordial) stages of organ development (about 30 days in dog, cat, sheep, pig; almost 60 days in horse, cattle, and human). 3- Fetus (fetal period): It is the time after the embryonic period and until birth in which differentiation and growth of the tissues and organs formed during the embryonic period occur. Zygote: Cell results from the union of an oocyte and a sperm after fertilization. It carries the full chromosomes (diploid, 2N). Morula: It is a solid mass of approximately 8 (early morula) – 16 (compact morula) blastomeres (according to species) which is formed by cleavage of the zygote. Sperm (1N) Oocyte (1N) Fertilization Zygote (2N) Cleavage Morula Cleavage Blastula (bastocyst / blastoderm) Gastrulation Gastrula Neurulation Neurula Differentiation Fetus  Initial development of the mammalian embryo: A: Zygote; B: 2-cell embryo; C: 4-cell embryo; D: Early morula; E: Compact morula; F: Blastocyst; G: Expanded blastocyst; H: Blastocyst in the process of hatching from the zona pellucida; I: Ovoid blastocyst with embryonic disc; J: Elongated blastocyst; K: Embryonic disc in the process of gastrulation. 1: Inner cell mass; 2: Trophectoderm; 3: Epiblast; 4: Hypoblast; 5: Embryonic disc; 6: Amniotic folds; 7: Ectoderm; 8: Mesoderm; 9: Endoderm.  Cell division (proliferation) A) Mitosis - This type of cell division occurs in all somatic cells and results in two new (daughter) diploid (2N) cells similar to each other and to their parent. - Mitosis is preceded by a preparatory phase called interphase in which the DNA (chromosomes) are replicated (formation of sister chromatids, dyads) and the centrioles duplicated to form centrosomes.  Phase Events Prophase - Pairing, shortening and thickening of the chromosomes. - Lysis of nuclear membrane. -Formation of microtubules spindles (asters) by the two centrosomes. Metaphase -Complete disappearance of the nuclear membrane (prometaphase). - Attachment of spindles to the centromeres of the chromosomes - Equatorial (midway) arrangement of the chromosomes Anaphase - The centromeres split and the attached spindles shorten - The pairs of conjoined sister chromatids separated and draw to the opposite cell pole Telophase The two groups of identical chromosomes cluster at their respective pole, de-condense and a nuclear membrane formed around each set. B) Meiosis - It takes place in germ (sex) cells only. - It results in formation of haploid (1N) gametes (sperms and oocytes). - It leads to formation of daughter cells not genetically identical due to exchange of genetic material between non-sister chromatids (crossover). -Before meiosis: Germ cell enters meiosis with diploid chromosomes (2N). - There are two successive meiotic divisions (meiosis I and meiosis II) The second meiotic division (Meiosis II) - It follows the first division without a normal interphase (i.e., without an intervening step of DNA replication). - Meiosis II is similar to mitosis except that the chromosome number of the cell entering the second meiotic division is haploid. Species Number of chromosomes in pairs Chicken, dog 39 Horse 32 Cattle, goat 30 Sheep 27 Human 23 Pig, cat 19  Diagrammatic representation of meiosis. A to D, Stages of prophase of the first meiotic division. The homologous chromosomes approach each other and pair; each member of the pair consists of two chromatids. Observe the single crossover in one pair of chromosomes, resulting in the interchange of chromatid segments. E, Metaphase. The two members of each pair become oriented on the meiotic spindle. F, Anaphase. G, Telophase. The chromosomes migrate to opposite poles. H, Distribution of parental chromosome pairs at the end of the first meiotic division. I to K, Second meiotic division. It is similar to mitosis except that the cells are haploid.  Gametogenesis - Gametogenesis is the process of formation and development of male gamete (sperm = spermatozon) and female gamete (oocyte = ovum = egg). - Gametogenesis is called spermatogenesis in males and oogenesis in females. - The gametes are originated from the primordial germ cells (PGCs) which are diploid (2N) cells that originate from the primary ectoderm (epiblast) and terminate in the gonadal ridge. Emigration (migration) of PCGs In mammals; - PGCs move (by pseudopodia, in an amoeboid manner) from the primary ectoderm into endoderm of yolk sac and then migrate up through the intestinal wall and the dorsal mesentery to finally colonize the gonadal ridge. - The gonadal ridge together with PCGs form the gonad which is the primordium of either the testis or the ovary. In birds - The PGC have no pseudopodia and are unable to move. - Therefore, they re-enter the body via blood circulation of the yolk sac. - They then reach the gonadal ridge passively through blood.  Spermatogenesis - Spermatogenesis is the process of formation and maturation of the spermatozoa (male sex cells). - It occurs in seminiferous tubules of the testis from prenatal life to death. - During spermatogenesis the germ cells move towards the lumen (centripetal movement). A. Spermatocytogenesis: - It starts from primordial germ cell (PGC) to spermatid - It consists of two periods; A1) Period of proliferation (from PGC to spermatogonia type B) A2) Period of maturation (From primary spermatocyte to spermatid) B. Spermiogenesis (spermiohistogenesis): From spermatid to spermatozon A1) Proliferation period of spermatocytogenesis (clonal expansion) - During fetal life; The primordial germ cells (2N) give rise to spermatogonia type A (2N) which remain dormant in the basal layer of seminiferous tubules of the testis. - At puberty; The spermatogonia type A proliferate by mitosis with homonymous division (complete separation of cells) to give rise to two types of cells; a) One dormant cell which acts as stem cell to renew the stock of type A cells. This dormant spermatogonia A stay to ensure a continuous supply of spermatogonia when depleted. This can explain why the male can produce sperm for ever (until death). b) The other cell becomes type B spermatogonia (2N) which also multiplicate by mitosis but with heteronymous division (incomplete separation of cells due to presence of cytoplasmic bridges. This ensures that all cells undergo the same morphological changes at the same time and so produce identical sperms). A2) Maturation period of spermatocytogenesis - Spermatogonia type B divide by mitosis giving rise to primary spermatocyte (2N) which characterized by larger nucleus size. - Primary spermatocyte divides by first meiotic division (meiosis I) into two equal size secondary spermatocytes each of them contain haploid (1N). - In mammals, one secondary spermatocyte contains X chromosome and the other contains Y chromosome. - Each secondary spermatocyte (1N) divides by second meiotic division (meiosis II) to produce two spermatids (1N). B) Spermiogenesis (spermiohistogenesis) - It is the differentiation of the spermatids into sperm cells. 1- Nuclear condensation: The nucleus becomes smaller, denser, oval and eccentric. 2- Acrosome formation: - The Golgi complex produces small vesicles, which then fuse into a single large acrosomal vesicle close to the nucleus. - When the vesicle covers the cranial aspect of the condensed nucleus, it is called acrosome. 3- Flagellum formation: The centrioles which migrate to the pole of nucleus opposite to acrosome, form the axial filament (axoneme) from which mid piece and tail of the sperm develop. 4- Spiral filament formation: The mitochondria arrange spirally around the flagellum in the mid piece. 5- Cytoplasmic reduction: - Elimination of all unnecessary cytoplasm (collectively called residual body) by Sertoli phagocytosis. - Some cytoplasmic remnants may be left and hang to the mid piece of the immature sperm as protoplasmic droplet. - Once the sperm becomes mature, it free itself from the Sertoli and go to the lumen of seminiferous tubule, this process is called spermiation.  6 5 4 11 Diagram showing Spermiogenesis 3 2 1 stages Three differing stages of spermiogenesis: on the left a fresh spermatid, on the right an immature sperm cell, and in the middle an in-between stage. 1. Axonemal structure, first flagellar primordium, 2. Golgi complex, 3. Acrosomal vesicle, 4.Pair of centrioles (distal and proximal), 5. Mitochondrion, 6. Nucleus, 7. Flagellar primordium, 8. Microtubules, 9. Sperm cells tail, 10. Acrosomal cap, 11. protoplasmic droblet  Sperm structure Mature sperms are free-swimming, actively motile cells consisting of a head, neck, mid piece and tail. (1) The head - forms most of the bulk of the sperm and contains the haploid nucleus. - The anterior two thirds of the nucleus is covered by the acrosome, a cap like saccular organelle containing several enzymes (hyaloronidase and others) which help the sperm to penetrate of the ova during fertilization. - The posterior one third covered only by post acrosomal plasma membrane. (2) The neck - contains the two centrioles (proximal and distal, the distal one forms the flagellum). - The weakest and fragile part of the sperm  (3) The mid (middle) piece - consists of ring-shaped mitochondria forming spiral filament around the sheath of the thick outer fibers and the axoneme. - provides the energy for the flagellar movement. - The terminal ring; presents at the junction with the tail to prevent caudal displacement of the mitochondria during motility. (4) The tail - The tail provides the motility of the sperm that assists its transport to the site of fertilization. It composes of two pieces: A- Principal piece - Has a sheath of ring fibers around the axoneme. -The axoneme or axial filament is composed of 9 microtubules doublets plus 2 central microtubules. B- End piece. - Consists of only the 9+2 structure of the axoneme.  Oogenesis - Oogenesis is the sequence of events by which oogonia are transformed into mature oocytes. - It begins before birth and is completed after puberty. - It continues to menopause, which is permanent cessation of the menstrual cycle (in woman only). - It is divided into two periods; period of prenatal maturation and period of maturation of oocytes. A) Period of Prenatal Maturation - Occurs only in the prenatal life (before birth). - Starts with primordial germ cells (2N) which divide mitotically to give rise to oogonia (2N). - Oogonia proliferate by mitosis and when enlarge; they form primary oocytes (2N). - Primary oocytes begin the first meiotic division before birth but arrested in prophase I. - The primary oocyte becomes surrounded by flat follicular cells and together they form the primordial follicle. - In contrast to the continuous production of primary spermatocytes in males, no primary oocytes form after birth in females and so each female born with limited numbers of primary oocytes. This can explain why women have menopause when they become older (between 45 and 50 years).  B) Period of Postnatal Maturation of Oocytes Time Changes After primary oocytes (arrested in prophase I) birth remain dormant within primordial follicles. NB: In contrast to continuous production of primary spermatocytes in males, no primary oocytes form after birth in females and so each female born with limited numbers of primary oocytes. This can explain why women have menopause when they become older (between 45 and 50 years). At primary oocyte enlarges and primordial puberty follicle becomes primary follicle.  Hours Primary oocyte completes the first before meiotic division to give rise to a ovulation secondary oocyte (1N, large) and the first polar body (1N, small, soon degenerates). NB: The follicular cells surrounding the primary oocyte secrete oocyte maturation inhibitor factor which arrests meiosis I in dictyotene stage of prophase I during the period from prenatal life until puberty. Before ovulation, this factor blocked by ovarian cycle regulating hormones and thus mieosis I completed. At ovulation Secondary oocyte begins Meiosis II, but arrested in metaphase II. After Secondary oocyte completes Meiosis II fertilization and the fertilized oocyte and the second polar body formed.  Folliculogenesis - The growth and development of ovarian follicles from the primordial to the ovulatory stages. - The female mammal is born with a fixed number of follicles i.e. at birth the ovary contains all the follicles it is ever going to have. 1. Primordial follicle - consists of one primary oocyte surrounded by a single layer of squamous (flat) follicular cells. 2. Primary follicle - formed when the surrounding of primary oocyte becomes a single layer of cuboidal follicular cells 3. Secondary follicle - It is characterized by formation of; a. Granulosa cells: - Formed when the follicular cells proliferate into multiple layers (stratified granulosum). b. Zona pellucida: - A fenestrated translucent glycoprotein layer between the oocyte and follicular epithelium. NB: Cytoplasmic processes of the granulosa cells pass through the fenestration of the zona pellucida to connect with the oocyte, thereby assure their communication and supply the primary oocyte by yolk. c. Theca follicular cells: - Formed by the ovarian stroma surrounding the basal lamina of granulosa cells. 4. Tertiary (antral) follicle - Formed when granulosa cells secrete fluid which leads to formation of fluid-filled spaces, which fuse to form a single large cavity, the antrum, which contains follicular fluid. - The primary oocyte reaches full size and is pushed to one side of the follicle, - The inner follicular cells surrounding the oocyte are called the cumulus oophorus. - The cumulus oophorus cells radially arranged on the cell membrane of the oocyte is called the corona radiata. - The theca layer becomes organized into the theca interna (vascular and glandular layer) and theca externa (fibrous). 5. Graafian (mature) follicle - It is a selected tertiary follicle that continues to enlarge until reaches maturity and produces a swelling on the surface of the ovary. Atresia occurs for the other non selected tertiary follicles. - Changes occur in Graafian follicle before ovulation: - The spindle formed and oriented itself radially to the cellular surface. - Further, the processes of the granulosa cells retracted from the oocyte surface into the zona pellucida. This leads to the formation of the perivitelline space. In this space the ejection of the first polar body takes place as a sign that the first meiosis ended and the secondary oocyte formed. Diagram showing changes occur in Graafian follicle before ovulation 1. Theca interna and externa, 2.basal membrane, 3.Granulosa cells, 4. Graafian follicle, 5. Primary oocyte, 6. Cumulus oophorus, 7. Ovarian tissue, 8.Tunica albuginea, 9. Abdominal space, 10. oocyte plasma membrane (oolemma), 11. diakinesis stage of prophase I, 12 corona radiata, 13. processes of granulosa cells, 14. Zona pellucida. 1. zona pellucida, 2. perivetilline space, 3. mitotic spindle, 4. retracted processes of granulosa cells, 5. oolemma, 6. granulosa cells, 7. first polar body.  Types of ova A) Amount of yolk 1. Oligolecithal 2. 3. Polylecithal Mesolecithal little amount of yolk moderate large amount of yolk and smaller ova amount of and larger egg because embryo yolk because embryo develops inside develops outside mother and feeds by mother body and so placenta needs high yolk to feed on Example; Mammals Fishes and Birds and reptiles frogs B) Distribution of yolk in the cytoplasm 1. Isolecithal ova; They contain few amount of yolk which is equally (uniformly) distributed in the cytoplasm. Example: oligolecithal ova (Mammals) 2. Anisolecithal ova; They contain moderate or larger amount of yolk which displace the small amount of cytoplasm and nucleus to the animal pole and so the yolk is unequally distributed. Example: meso-and polylecithal ova.  Telolecithal ova Centrolecithal ova The yolk accumulates in the vegetable The yolk accumulates in pole. the center of ova and so They are subdivided into: the nucleus and Micro- Macro- telolecithal cytoplasm are pushed to telolecithal the periphery. Incomplete complete isolation Example. Insects. isolation of yolk (there is a from cytoplasm demarcation line) (no demarcation line between yolk and cytoplasm) Examples: (lower Polylecithal (birds fishes and frogs) and reptiles)  animal pole Nucleus Vegetable Yolk uniformly pole distributed with cytoplasm Chalaza Corona radiata Bird ovum (polylecithal, macro-telo-lecithal) Mammalian ovum (oligo-isolecithal) Jelly coat animal pole Yolk Nucleus Cytoplasm Vitelline membrane Vegetable pole Yolk mixed with few cytoplasm Insect ovum (centrolecithal) Frog ovum (mesolecithal, micro- telolecithal Egg (ovum) coverings Animal pole Vegetable pole The part containing cytoplasm Contains yolk and nucleus Characterised by active, rapid Slow rate of growth growth rate Cleavage divisions occur rapidly Slowly or lack divisions Form the embryo Form yolk sac, placenta and allantois  Spermatogenesis Oogenesis Occurance In seminiferous tubules of testis In the ovary cortex Start time before birth before birth Direction of When the cells mature, they When the cells mature, cell movement move toward the center (lumen), they centripetal movement move toward the periphery (surface), centrifugal movement Proliferation Start late (at puberty) but Start and end before (mitosis) continue for ever (until death) so birth so the number of unlimited cell number oocyte is limited Produce spermatogonia type A & Only one type of B oogonia Spermatogonia can give rise to All primary oocytes are primary spermatocyte after birth formed before birth Start with homonymous then Only homonymous continue as heteronymous division division (complete separation of cells)  Meiosis I Occurs at any time from Start before birth and puberty completed to death few hours before ovulation Lead to formation of two One big cell (secondary equal oogonium,1N) and one size cells (secondary small spermatocyte, 1N) polar body Meiosis II Occurs at any time from Begin at ovulation and puberty completed in uterine to death tube after fertilization Transformation Present (spermiogenesis) No End result From one spermatogonium,From one oogonium, 4 one ova and three sperms are formed with the smaller polar bodies are same formed each has 1N size and each has 1N and and only X either X chromosome or Y chromosome  Sperm Ovum Size Small Large Shape Elongated Spherical Quantity Large number (million) 1- 25 (acc. to the animal) Sex X and Y chromosome Only in birds determination Motility Vigorous (by the tail) Lack (no tail) Protection Only plasma membrane Plasma membrane + other membranes + follicles Nucleus Condensed and form sperm Not condensed head Yolk Non Little to much Golgi Acrosomal cap Diffused complex Mitochondria Spiral filament in mid piece Diffused Centrioles Retained and form the axial Disappeared filament