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Gametogenesis is the process of formation of gametes by primary sex organs (Testis in males and Ovary in females). Gametes are sperms and ovum in males and females, respectively. So, testis produce sperms in males and ovary produce ovum in females.

• Gametogenesis is of two types: Spermatogenesis and Oogenesis.
• Oogenesis is the process of formation of a mature female gamete (ovum) in the ovary.
• In contrast to spermatogenesis, which begins in males at puberty, oogenesis begins in females before they are even born, i.e. during embryonic development.
• Millions of gamete mother cells (oogonia) are formed when oogenesis is initiated during the embryonic development within each fetal ovary. After birth, no more oogonia are formed and added.
• During embryonic development only, oogonia (diploid, 2n) divide by mitotic division to produce million of cells. Most of these cells degenerate in a process known as atresia, but few develop into larger cells called primary oocytes (2n).
• Primary oocytes start division and enter into prophase of meiosis-I during fetal development and get temporarily arrested at this stage until after puberty.
• So, at the time of birth, primary oocytes are present in prophase of meiosis-I.
• In this stage, each primary oocyte is surrounded by a single layer of flat follicular cells, and the entire structure is called a Primordial follicle. These follicles are present in the ovarian cortex.

Representation of Primordial Follicle

• During childhood, no development of follicles occur. At the onset of puberty, these follicles start developing each month till menopause.
• During menstrual cycle each month, few primordial follicles start to grow and develop into primary follicles. Primary follicle consists of a primary oocyte surrounded by layers of cuboidal and low-columnar cells called Granulosa cells.

Representation of Primary Follicle

• At puberty only 60,000-80,000 primary follicles are left in each ovary, others get degenerate till puberty and around only 400 will be able to mature and ovulate during a woman’s reproductive lifetime.
• As the primary follicle further grows in size, it forms a glycoprotein layer between the primary oocyte and the granulosa cells called as Zona pellucida. Also the stromal cells (present in ovarian cortex; for learning refer topic Ovary under Female Reproductive System) form an organized layer called as Theca folliculi around primary follicle. At this stage, primary follicle is said to develop into a Secondary follicle.

Representation of Secondary Follicle

• Now, in a secondary follicle, theca gets differentiated into two layers: (1) Theca interna, (2) Theca externa. Theca interna is highly vascular internal layer of cuboidal secretory cells that secrete estrogen, while theca externa is outer layer of stromal cells and collagen fibers.

• In addition to Theca, granulosa cells make a fluid-filled cavity called as Antrum in the center of the secondary follicle. Antrum is filled with follicular fluid.
• As the cavity is made, innermost layer of granulosa cells of antrum gets firmly attached to the zona pellucida and is now called as Corona radiata. So, Corona radiata is present between Antrum and Zona pellucida.
• At this stage, secondary follicle with antrum, theca interna, theca externa, corona radiata is said to become a Tertiary follicle or Mature (Graafian) follicle.

Representation of Mature (Graafian) Follicle

• It is at this stage that the diploid (2n) primary oocyte within the tertiary follicle or mature graafian follicle grows in size and completes meiosis-I (reduction division), producing two haploid (n) cells each with 23 chromosomes. Also, it is an unequal division resulting in the formation of a large cell called Secondary oocyte (haploid, n) and a smaller cell called First polar body (haploid, n).
• So, meiosis-I produces Secondary oocyte and First polar body.
• Secondary oocyte receives most of the nutrient rich cytoplasm of the primary oocyte during division so it is larger in size.
• Once a secondary oocyte is formed, it begins meiosis II but then stops in metaphase.
• Mature (graafian) follicle ruptures at this stage (i.e. metaphase of meiosis-II or metaphase-II) to release its secondary oocyte from the ovary, a process known as Ovulation.
• First polar body may or may not divide again. With secondary oocyte, first polar body also gets ovulated from mature follicle.
• Secondary oocyte after ovulation from ovary reach the fallopian tube and further future of secondary oocyte depends on fertilization.
• If fertilization does not occur, the cells degenerate.
• But if the sperm is present in fallopian tube and gets in contact with secondary oocyte fertilization takes place, and meiosis-II resumes. Meiosis-II of secondary oocyte only gets complete in case of fertilization.
• Secondary oocyte then gets split into two haploid cells again of unequal size. The larger cell is the Ovum (n), or Mature Egg; smaller one is the Second polar body (n).
• Finally, the nuclei of the sperm cell (n) and the ovum (n) unite to form a diploid zygote (2n).
• If the first polar body also undergoes division, it produce two polar bodies, then the primary oocyte results in total three haploid polar bodies, together with single ovum. All the 3 polar bodies get degenerate.
• On comparison to Spermatogenesis in males, one primary spermatocyte produces four gametes (sperm), while in oogenesis in females one primary oocyte gives rise to a single gamete (an ovum).

Diagramatic representation of Oogenesis