Chapter 9. Reproductive Cycles: The Ovarian Cycle and The Corpus Luteum - Review of Medical Embryology Book

Introduction: at the onset of each ovarian cycle, a number of primary follicles begin to grow and mature. Usually, only 1 follicle reaches full maturity and only 1 oocyte is discharged. The others degenerate and become atretic. In the next cycle, another group of primary follicles matures and again only 1 develops. When a follicle becomes atretic, the oocyte and follicular cells degenerate and are gradually replaced by connective tissue, forming the corpora atretica
Corpus luteum (CL) formation

AFTER OVULATION: the follicular walls and theca folliculi collapse and become folded and under LH influence develop into the corpus luteum, a glandular structure which secretes mostly progesterone (also some estrogen)

Progesterone causes the endometrial glands to secrete, preparing the endometrium for blastocyst implantation
If the oocyte is fertilized, the corpus luteum enlarges to form a corpus luteum of pregnancy and increases hormone production

With pregnancy, chorionic gonadotropin, secreted by the trophoblast of the chorion, prevents degeneration of the corpus luteum

If the oocyte is not fertilized, the corpus luteum degenerates in 10-12 days after ovulation and forms the corpus luteum of menstruation and is eventually transformed into a white scar, the corpus albicans

AFTER OVULATION: when the wall of the ruptured follicle collapses, the rest of the follicular cells gradually become vascularized by vessels growing in from the periphery. The follicular cells begin to hypertrophy, become polyhedral, and develop a yellowish pigment. The modified yellowish cells are called luteal cells

The corpus luteum of pregnancy, by the end of the third month of pregnancy, may constitute as much as one-third to one-half the total size of the ovary
The luteal cells continue to secrete progesterone until the end of the fourth month, but thereafter regress slowly
Whether during this period new luteal cells are added to the periphery by differentiation of the surrounding stroma cells, or by active division of the existing luteal cells, is unknown
Removal of the corpus luteum of pregnancy before the fourth month usually results in abortion