Figure 1-5. The process of oogenesis.
(c) As a primary oocyte begins dividing, two different cells are
produced, each containing 23 unpaired chromosomes. One of the cells is called a
secondary oocyte and the other is called the first polar body. The secondary oocyte is
the larger cell and is capable of being fertilized. The first polar body is very small, is
nonfunctional, and incapable of being fertilized.
(d) By the time follicles have matured to the graafian follicle stage, they
contain secondary oocytes and can be seen bulging from the surface of the ovary.
Follicle development to this stage takes about 14 days. Ovulation (ejection of the
mature egg from the ovary) occurs at this 14-day point in response to the luteinizing
hormone (LH), which is released by the anterior pituitary gland.
(e) The follicle at the proper stage of maturity when the LH is secreted
will rupture and release its oocyte into the peritoneal cavity. The motion of the fimbriae
draws the oocyte into the fallopian tube. The luteinizing hormone also causes the
ruptured follicle to change into a granular structure called corpus luteum, which secretes
estrogen and progesterone.
(f) If the secondary oocyte is penetrated by a sperm, a secondary
division occurs that produces another polar body and an ovum, which combines its 23