Gametogenesis Course Materials

MADE BY APHRODITE PASCOE Biology “Gametogenesis” Gametogenesis: Formation of specialised sex cells (gametes) whether male or female through meiosis ➡ The two gametes fertilise to form a new individual. ➡ The process of the male gametes is called spermatogenesis ‣ Male gamete = sperm ‣ Gamete precursor cells = spermatogonia ‣ Mature cells = spermatozoa ➡ The female process is called oogenesis ‣ Female gamete = ovum ‣ Gamete precursor cells = oogonia ‣ Mature cells = de nitive oocyte Germ Line: cell line that will give rise to the gamete ‣ Arises from primary ectoderm ‣ Distinct at the 4th week of development -> cells are called primordial germ cells (PGC) ‣ Migrate to posterior body wall to populate the developing gonad Germ cell migration and characteristics: ➡ ➡ They’re aasily recognised due to the pale cytoplasm and ovoid shape They migrate through ameboid yolk sac to the gut tube between 4th to 6th week of development ➡ From the gut tube via the mesentery to the dorsal body wall ➡ They go through mitosis as they continue migrating ➡ If they get stuck along the way, they give rise to a teratoma The timing of gametogenesis: fi Males: ➡ Germ cells are dormant until puberty ➡ At puberty, the seminiferous tubules mature -> germ cells become spermatogonia ➡ Spermatogonia which undergo meiosis mature into spermatozoa ➡ The production of sperms is continuous until death Females: ➡ Germ cells undergo mitotic divisions to become oogonia ➡ Meiosis begins at the fourth moth of fetal development ➡ They then remain dormant and under meiotic arrest as primary oocytes until maturity ➡ ➡ Beginning at puberty, some ovarian follicles resume meiosis, this is a response to monthly pituitary gonadotropic hormones Each month, primary oocyte matures to secondary oocytes and is ovulate Spermatogenesis: ➡ Testosterone hormone is secreted at puberty E ects: ➡ Development of secondary sexual characteristic ➡ Growth of testes ➡ Seminiferous tubules mature ➡ ➡ Spermatogonium is located after basement membrane around seminiferous tubules ➡ Each one is connected to Sertoli cells through specialized junctions ➡ ➡ ff Spermatogenesis begins- -> the dormant primordial germ cell continues developing and divides through mitosis and develop into a spermatogonia Sertoli cells are connected through tight junctional complexes that isolate spermatogonia from the lumen of the tubule The cells that will go through spermatogenesis arise from mitosis ➡ Spermatocyte produce two secondary spermatocytes and then four spermatids ➡ Spermatids morphologically covert to a mature sperm ➡ Tubulobulbar processes helps reabsorb the excess cytoplasm ➡ When the connection between spermatids and Sertoli cells break, spermiation takes place after the spermatozoa is released to the lumen Sperm (Spermatozoon): ➡ Has the head, mid-piece and tail ➡ Head contains nucleus and apical vessel called acrosome ‣ The acrosome is lled with hydrolytic enzymes and plays a vital role during fertilisation ➡ Mid-piece has mitochondria and energy for swimming ➡ The tail contains microtubules which helps in propulsion Continuous wave of sperm production: ➡ ➡ In germinal epithelium -> gametes are produced in synchronous waves The daughter cells remain attached (by small cytoplasmic bridges) to allow passage of molecules ➡ Each spermatogenesis cycle takes about 64 days ‣ Spermatogonial mitosis takes about 16 days ‣ First meiotic division takes about 8 days ‣ Second meiotic division takes about 16 days ‣ Spermiogenesis takes about 24 days ➡ Spermatozoa undergoes maturation called capacitation ➡ The sperm continues moving from seminiferous tubules to ampulla of the oviduct Sperms are stored in the epididymis fi ➡ ➡ During ejaculation, the sperm is propelled through vas deferens ➡ There are more than 200 million spermatozoa per ejaculate ➡ In the ampulla, the sperm can fertilise an egg in 1-3 days ➡ In IVF, the sperm are arti cially capacitated Oogenesis: ➡ It is discontinuous ➡ At week 16 oogonia enter meiosis and become dormant ➡ The primary oocytes nucleus becomes large and watery and is called germinal vesicle ➡ The germinal vesicle protects DNA during meiotic arrest ➡ ➡ Primary oocyte is enclosed by epithelial follicle cells -> primary oocyte is called primordial follicle At puberty -> 400.000 million oocytes Hormones of the female cycle: ➡ ➡ At puberty, hypothalamus, pituitary and the ovary hormones control menstruation The hormones produce female gametes and prepare the uterus for the embryo cycle contains of: ‣ Single oocyte ‣ Proliferation of the uterine walls ‣ Oocyte produced through ovulation ‣ Follicle develops to corpus luteum ‣ Unless fertilised, the sloughing of the uterine walls and involution to corpus luteum ➡ The cycle takes 28 days fi ➡ The The cycle: ➡ Begins with menstruation ➡ There is an increase in the secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus in the 5th day ➡ Secretion of gonadotropic hormones (gonadotropins), luteinizing hormone (LH) and follicle stimulating hormone (FSH) also increases (GnRH causes rst menstrual cycle) ➡ Pituitary gonadotropins control folliculogenesis and proliferative phase of the uterine wall. 5-12 primary follicles each month: Before menstruation cycle, the epithelium of primordial follicle thickens -> it becomes cuboidal-> is called primary follicle ➡ The oocyte and the follicle cells together secrete a thin layer of acellular material (layer called zone pelucida) ➡ ➡ Epithelium of 5-12 follicles proliferate to form a capsule around the oocyte -> growing follicle ➡ Some follicles become dormant at this point ➡ Some enlarge as a response to increasing FSH levels -> antral or vesicular follicles fi Single follicle becomes dominant: One follicle becomes dominant and absorbs uid (graa an follicle), others degenerate ➡ At this point, oocyte does not resume meiosis Resumption of meiosis and ovulation: ➡ On 13th -14th of the cycle, FSH and LH sharply rise -> ovulatory surge stimulates oocyte to resume meiosis ➡ By 20 hours, the chromosomes are lined up in the metaphase plate ➡ Three hours before ovulation, the secondary oocyte undergoe meiotic arrest ➡ The oocytes nucleus breaks down and the cumulus cells loose cell to cell connection and detaches from the antral cavity ➡ In the next few hours, cumulus cells secrete hyaluronic acid causing them to expand (cumulus expansion) -> the cumulus cells around the oviduct transport oocyte to the oviduct ➡ The follicle is then expulsed from the follicle hence ovulation ➡ After the surge, follicle is displaced to the ovary surface ➡ With the combination of tension and collagen degrading enzymes, the follicle ruptures 38 hours after the ovulation surge After ovulation: ➡ The granulose cells of the raptured follicular proliferate -> form the luteal cells of the corpus luteum ➡ Corpus luteum (endocrine structure) maintains uterine endometrium to be ready to receive embryo ➡ If no embryo implants -> corpus luteum degenerates into a scar structure -> corpus albicans Fertilisation: fi fl ➡ ➡ Occurs when viable spermatozoa encounter an oocyte -> interact with speci c glycoproteins when they reach zone pellucida ➡ When the nucleus interacts with the oocyte -> cortical granules are released ➡ The fusion between the sperm and the egg causes the oocyte to resume meiotic arrest The fertilized oocyte is the zygote fi ➡