Oogenesis PART 7
- 1. Ms.Anu Sebastian: B.Pharm; M.Pharm Assistant Professor, Dept. Of Pharmacology Nirmala College Of Pharmacy Muvattupuzha,Ernakulam,Kerala
- 2. The formation of gametes in the ovaries is termed as oogenesis. It begins in females before birth. During early fetal development ,Primordial germ cells migrate from the yolk sac to the ovaries. These germ cells differentiate within the ovaries into oogonia. Oogonia are diploid(2n) stem cells that divide mitotically to produce millions of germ cells. Most of these germ cells degenerate in a process known as atresia. Remaining oogonia undergo meosis- I to form primary oocyte; but cell division stops at Prophase-I in meosis-I
- 3. At birth, approx. 200,000 to 2000,000 primary oocytes remain in each ovary. Of these , about 40,000 are still present at puberty, and around 400 will mature & ovulate during a woman’s entire lifespan. Each month after puberty gonadotropins(FSH & LH) secreted by the anterior pituitary further stimulate the development of several primordial follicles. A few primordial follicles start to grow, developing into primary follicles. Primary follicle consists of a primary oocyte that is surrounded in a later stage of development by several layers of cuboidal & low- cuboidal columnar cells called granulosa cells.
- 4. The outermost granulosa cells are covered by basement membrane. It forms a clear glycoprotein layer called the Zona pellucida between the primary oocyte & the granulosa cells. The stromal cells surrounding the basement membrane begin to form an organized layer called the theca folliculi. A primary follicle develops into a secondary follicle. In a secondary follicle,the theca differentiates into two layers: (1)The theca interna: a highly vascularized internal layer of stromal cells & collagen fibres. (2)The theca externa: an outer layer of stromal cells & Collagen fibres.
- 5. In addition, the granulosa cells begin to secrete follicular fluid, which builds up in a cavity called the antrum in the center of the secondary follicle. The innermost layer of granulosa cells becomes firmly attached to the Zona pellucida & is now called the corona radiata. The secondary follicle eventually becomes larger, turning into a mature(Graafian )follicle. This follicle just before ovulation, the diploid primary oocyte completes meiosis – I, producing two haploid(n) cells of unequal size-each with 23 chromosomes;The smaller cell produced by meiosis –I ,called the first polar body , consists of a packet of discarded nuclear material.
- 6. The larger cell, known as the secondary oocyte , receives most of the cytoplasm. Once a secondary oocyte is formed, it begins meiosis- II but then stops in metaphase. The mature (graafian) follicle soon ruptures & releases its secondary oocyte, a process known as ovulation. At ovulation, the secondary oocyte is expelled into the pelvic cavity together with the first polar body 7 corona radiate. If fertilization does not occur, the cells degenerate. If sperms are present in the uterine tube & one penetrats the secondary oocyte, meiosis –II resumes
- 7. The secondary oocyte splits into two haploid cells, again of unequal size. The larger cell is he ovum, or mature egg ; the smaller one is the second polar body. The nuclei of the sperm cell & the ovum then unite, forming a diploid Zygote. If the first polar body undergoes another division to produce two polar bodies, then the primary oocyte ultimately gives rise to three haploid polar bodies, which all degenerate & a single haploid ovum.
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