Grossly, a corpus luteum is large compared to the rest of the ovary. There is a bright yellow (luteum = yellow) corrugated outline.
The bulk of the corpus luteum are granulosa lutein cells, which contain abundant pink cytoplasm. Connective tissue with blood vessels extend into the folds of the corpus luteum.
Theca lutein cells (formerly theca interna cells) are darker smaller cells that lines the folds of granulosa lutein cells. They are difficult to detect in this image at low power. The theca externa forms a poorly defined capsule around the corpus luteum. This capsule merges with the connective tissue which extend as thin septae into the undulating folds.
The former follicular cavity is filled with blood and may be termed corpus hemorrhagicum. This is normal after ovulation as the follicle ruptures and the oocyte is extruded.
The corpus luteum develops from the ovarian follicle after it has an extruded the oocyte during ovulation. Basically, it is a temporary endocrine gland that produces estrogen and progesterone to sustain the endometrium. The LH secretion induces hypertrophy of the granulosa cells and theca interna cells of the ruptured follicle to become granulosa lutein cells and theca lutein cells. The bulk of the corpus luteum is comprised of granulosa lutein cells, which form thick undulating folds that are rimmed by theca interna cells.
The granulosa lutein cells are large polygonal cells with round vesicular nuclei. Their cytoplasm stains lightly because of high lipid content. The theca lutein cells are located at the periphery; they are smaller and darker than the granulosa lutein cells.
During the 'luteal' phase of the menstrual cycle, LH stimulates the corpus lutein cells to secrete estrogen and progesterone, which further stimulates the endometrium in preparation for implantation of a fertilized egg. If the egg is not fertilized, the corpus luteum regresses to become a scarred structure called the corpus albicans.
1 Eroschenko VP. diFiore's Altas of Histology with Functional Correlations. 10th Ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005: 384.