Chapter 4. Gametogenesis: Oogenesis - Review of Medical Embryology Book

PRENATAL MATURATION: the ova are formed in the ovary from cells called oogonia which proliferate by mitotic division. All of the oogonia enlarge to form primary oocytes, of which about 2 million are present at birth. No primary oocytes form after birth (in contrast to the continuous production of primary spermatocytes in the male after puberty)

Ovarian stromal cells surround the developing primary oocyte to form a single layer of flattened follicular cells. The primary oocyte and its follicular cells constitute the primordial follicle

The follicular cell layer becomes cuboidal and then columnar as the primary oocyte enlarges at puberty and a primary follicle is formed

A primary follicle with more than one layer of cuboidal follicular cells is called a growing follicle

Primary oocytes begin the first meiotic division before birth but do not complete prophase until after puberty (arrested in the dictyotene stage until before ovulation)

Long duration of the meiotic division may account for the high frequency of meiotic errors such as nondisjunction

POSTNATAL MATURATION: the primary oocytes stay dormant in the ovaries until puberty

The primary oocyte increases in size and a membrane, the zona pellucida, forms around it as the follicle matures
Just before ovulation, the primary oocyte completes the first meiotic division, but unlike its male counterpart, the division of cytoplasm is unequal

The secondary oocyte gets almost all the cytoplasm
The first polar body receives little cytoplasm and is a small, nonfunctional cell that degenerates

At ovulation, the nucleus of the secondary oocyte begins the second meiotic division progressing only to metaphase, then division arrests

If fertilization occurs, the second meiotic division is completed, and the mature oocyte retains most of the cytoplasm, whereas the second polar body is small and degenerates

The secondary oocyte released at ovulation is surrounded by the zona pellucida and a follicular cell layer, the corona radiata. It is a large cell
About 2 million primary oocytes are found in the ovaries of a newborn femal Many regress during childhood so that at puberty about 30-40 thousand remain. Only about 200-400 of these ever reach full maturity after puberty and are expelled at ovulation during the female's reproductive life

EVERY MENSTRUAL CYCLE corresponds to the maturation of an oocyte, which becomes an ovum through division, yielding cells of unequal size (oocytes and polar bodies). This unequal division produces

Oocyte is immobile and massive when compared to the highly motile sperm
Oocyte contains much cytoplasm with yolk granules for nutrition during early development, whereas the sperm has sparse cytoplasm and is specialized for motility
There are 2 kinds of normal sperm, with respect to sex chromosomes: 23,X and 23,Y; there is but 1 kind of normal oocyte: 23,X (refers to 23 chromosomes in the complement, made up of 22 autosomes and 1 sex chromosome, X or Y)