•Fig. 44.22 Steroidogenicpathwaysinthecorpusluteum(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.)LH LH receptor LDLR and HDLR-mediated uptake Acetate Cholesterol Stimulation of steroidogenesis LH LH receptor LDLR and HDLR-mediated uptake Cholesterol Pregnenolone Stimulation of steroidogenesis StAR protein CYP11A1 Androstenedione Androstenedione Progesterone Estradiol-17˜3˜-HSD
The Corpus Luteum
After ovulation the remnant of the antral cavity fills with blood from damaged blood vessels in the vicinity of the stigma. This gives rise to a corpus hemorrhagicum. Within a few days, red blood cells and debris are removed by macrophages, and fibroblasts fill in the antral cavity with a hyaline-like extracellular matrix. In the mature corpus luteum the granulosa cells, now called granulosa lutein cells, enlarge and become filled with lipid (cholesterol esters). The enlarged granulosa lutein cells collapse into and partially fill in the old antral cavity. The theca along with blood vessels, mast cells, macrophages, leukocytes, and other resident connective tissue cells infiltrate the granulosa layer at multiple sites.
The human corpus luteum is programmed to live for 14 days, plus or minus 2 days (corpus luteum of menstruation), unless “rescued” by the LH-like hormone human chorionic gonadotropin (hCG), which originates from an implanting embryo. If rescued, the corpus luteum of pregnancy will remain viable during the pregnancy (usually ≈ 9 months). The mechanism by which the corpus luteum of menstruation regresses in 14 days is not fully understood. The corpus luteum appears to become progressively less sensitive to basal levels of LH, so that hCG binding to the LH receptor is needed for continued health and function of the corpus luteum. Regression appears to involve release of the prostaglandin PGF2α from both granulosa lutein cells and the uterus in response to declining levels of progesterone during the second week of the luteal phase. Several paracrine factors (endothelin, monocyte chemotactic protein-1) from immune and vascular cells are likely to play a role in the demise and removal of granulosa lutein cells. The corpus luteum is ultimately turned into a scarlike body called the corpus albicans, which sinks into the medulla of the ovary and is slowly absorbed.
The Gamete
The LH surge induces two parallel events, ovulation and luteinization. If ovulation occurs normally, the corpus luteum is devoid of a gamete.
Before the LH surge, the granulosa cells have very low capacity to convert cholesterol into a steroid hormone. The LH surge induces the onset of expression of CYP11A1, 3β-HSD and StAR protein, allowing granulosa lutein cells to convert cholesterol into progesterone. Because CYP17 expression is extremely low, progesterone accumulates and moves out of the granulosa lutein cells and enters the vasculature. Progesterone production by the corpus luteum (see