Oogenesis • The formation of female gametes is called oogenesis. • The terms oogenesis refers to the entire sequence of events by which oogonia are transformed into ova.
It is the maturation of female gametes in the ovaries, the primary sex organs of female partner. • This maturation process begins before birth, but it is not completed until after puberty.
Ooogonia • Once the primordial germ cells have arrived in the gonad of a genetic female, they differentiate into oogonia. • Oogonium [Gr. oon an egg + Gr. gone generation] is singular and oogonia are plural. • The oogonia are the stem cells from which other oogenic cells develop.
Month 3 • These oogonia undergo a number of mitotic divisions, and by the end of the 3rd month of intrauterine life they form clusters of cells. • So there are a lot of clusters of oogonia by the end of 3rd month of intrauterine life.
Primary oocytes • Majority of oogonia continue to divide by mitosis, but some of them differentiate into much larger cells, called primary oocytes. • The primary oocytes immediately enter into the prophase of meiosis I.
Month 5 • By the 5th month of intrauterine life the total number of germ cells in the ovary reaches its maximum. • They are now seven million 7,000,000. • The oogonia also start degenerating.
DDD • Now oogonia are multiplying by mitosis, differentiating into primary spermatocytes and degenerating. • The division and differentiation along with degeneration of oogonia continues throughout intrauterine life.
Month 7 • Majority of oogonia have degenerated by 7th month, except for a few near the surface. • The primary oocytes are also forming and degenerating. • All surviving primary oocytes have entered first meiotic division, and most of them are individually surrounded by a layer of flat epithelial cells forming primordial follicle.
At Birth • At birth there is not a single oogonium present in the ovary. • They have either degenerated or differentiated into primary oocytes. • So at birth the cortex of the ovary contains only primary oocytes. • The total number of primary oocytes, present in the cortex of the ovary, at birth is 2 million (2,000,000).
It must be stressed that no primary oocyte forms after birth, in contrast to the continuous production of primary spermatocytes in male after puberty.
Puberty • All the primary oocytes are in primordial follicles. • They have entered into prophase of meiotic-I and resting at diplotene stage. • The degeneration of primary oocytes continues and only 400,000 are present at the time of puberty. • Out of them only 500 ovulate.
Ovary • Epithelium • Tunica albuginea • Cortex • Medulla
Primordial follicle • At birth the cortex of the ovary is nothing but cellular connective tissue with almost 2,000,000 primordial follicles. • At birth, the cortex contains a superficial zone of primordial follicles; these consist of primary oocytes about 25 mm in diameter, each surrounded by a single layer of flat follicular cells.
With the onset of puberty, 5 to 12 primordial follicles begin to mature with each reproductive cycle. • They pass through the stages of primary follicle, secondary follicle, tertiary follicle and Gra’afian follicle but only one matures fully and ovulates. • As soon as one ovulates the others start degenerating.
Every primary oocyte completes its first meiotic division shortly before ovulation (36 to 48 hours). • It is important to realize that some of the oocytes, reaching maturity late in life, remain dormant in the dictyotene phase for 40 years or more.
Just before ovulation the primary oocytes in mature follicle resumes its meiosis I and forms two daughter cells of unequal size, secondary oocyte and first polar body. • The secondary oocyte receives most of the cytoplasm and polar body receives practically none.
Primary follicle and Secondary follicle • The first sign of follicular activation (primordial follicle) is the growth of the follicle cells from squamous to cuboidal, followed by their multiplication to form a multilayered mass, the membrana granulosa, surrounded by a thick basal lamina (membrana limitans sterna).
Zona pellucida • The primary oocyte secretes a thick layer of deeply staining a cellular, homogeneous membrane of glycoproteins, between its surface and the surrounding granulosa cells called zona pellucida.
Theca folliculi • As the follicle enlarges, the cellular connective tissue of the cortex adjacent to the follicle form theca folliculi which later on becomes organized into inner layer of spindle-shaped secretory cells called theca interna and an outer layer of fibers with fibroblast like cells called theca externa . • Theca interna produces estrogen.
The tertiary follicle continues to enlarge. The follicular cells proliferate, the follicular antrum becomes large and the follicle starts pressing the surface of the ovary. • The primary oocyte is towards the surface and it is about to rupture. This fully mature follicle is called Preovulatory follicle or Gra’afian follicle.
Follicle Stimulating Hormone (FSH) induces early development of follicles, but the final stages of maturation require Luteinizing Hormone (LH) as well.
Secondary oocyte • As soon as the follicle is mature, the primary oocyte resumes its first meiotic division, leading to the formation of two daughter cells (secondary oocyte and first polar body), of unequal size, but each with 23 (double structured) chromosomes. • The surge in the release of LH also seems to induce resumption of the first meiotic division of primary oocyte and it is converted into secondary oocyte.
Secondary oocytes enter into meiosis II. Meiosis II is completed only if secondary oocyte is fertilized. • Otherwise the cell degenerates within 24 hours after ovulation. • So the ovum or ootid develops from a fertilized secondary oocyte.
The nucleus does not return to its resting stage after first meiotic division. • Immediately after completion of first maturation division the secondary oocyte enters the second maturation division without DNA replication.
The moment secondary oocyte shows the spindle formation with the chromosomes aligned on the metaphase plate, ovulation occurs and the oocyte is shed from the ovary.
Ovulation is the discharge of a secondary oocyte from a Gra’afian follicle of ovary.