After ovulation, the ruptured follicle collapses and fills with a blood clot (corpus haemorrhagicum) which then forms the corpus luteum. The granulosa cells enlarge, and become vesicular, and are now called the granulosa lutein cells. these become folded, as you can see here.

The spaces between the folds are filled with theca interna cells, which also enlarge and become glandular, and are now known as the theca lutein cells.

The zona granulosa cells begin to secrete progesterone (granulosa lutein cells). The corpus luteum also secretes oestrogen (which inhibits FSH) and relaxin (which relaxed the fibrocartilage of the pubic symphysis).

If pregnancy does not occur, then the corpus luteum degenerates into the corpus albicans, and levels of oestrogen and progesterone fall, allowing release of FSH and LH.