Oogenesis - Gametogenesis
The maturing procedure in oogenesis leading to the formation of ovum begins before birth but is not completed until after puberty. The primary oocytes form previous to birth; these get surrounded by a single layer of flattened follicular cells making the primordial follicle. When these primordial follicles enlarge at puberty the flattened follicular cells become cuboidal and after that columnar forming the primary follicle. The primary oocytes begin the process of first meiotic division previous to birth and stay in suspended prophase till the reproductive cycle begins at puberty. The follicular cells surrounding the primary oocyte secrete an inhibiting substance termed as oocyte maturation inhibitor (OMI) which keeps the meiotic division at a standstill.
This suspended meiotic division might account to some extent for the large number of errors such as non-disjunction that occur as the mother's age increases. It is significant to know that new primary oocytes are not formed after birth. In difference primary spermatocytes keep forming in the male. After puberty 36 to 48 hours before every ovulation the primary oocyte completes the first meiotic division producing the secondary oocyte and the first polar body. The division of cytoplasm is not equal; the secondary oocyte receives all the cytoplasm whereas the first polar body receives hardly any and soon degenerates. At ovulation the 2nd meiotic division begins in the secondary oocyte and progresses up to metaphase; it is completed only if fertilization takes place. Again most of the cytoplasm is retained by just only one of the two resultant cells the ovum, the other cell or the 2nd polar body degenerates. The secondary oocyte which is released at ovulation is surrounded by the thin non- cellular envelope, the zona pellucida and a layer of follicular cells called corona radiata. In comparison to another body cells, it is a large cell but to the unaided eye, it appears only as a tiny speck.