Fig. 3. Regulation of progesterone secretion. Low-density lipoprotein (LDL) cholesterol is the lipoprotein substrate that is converted by specific enzymes into progesterone. This substrate is available only to the granulosa cells and the follicle after basement membrane breakdown following ovulation. The angiogenesis that accompanies the formation of the corpus luteum enables the vascular system to supply LDL cholesterol to the corpus luteum. (Carr BR, MacDonald PC, Simpson ER: The role of lipoproteins in the regulation of progesterone secretion by the human corpus luteum. Fertil Steril 38:303, 1982.)