A segment of the plicated corpus luteum is shown in figure on left at higher magnification. As noted above, the main cell mass is composed of granulosa lutein cells (GLC). On one side of this cell mass is the connective tissue (CT) within the former follicular cavity; on the other side are the theca lutein cells. The granulosa lutein cells contain a large spherical nucleus (see, also, GLC, in figure on right) and a large amount of cytoplasm. The cytoplasm contains yellow pigment (usually not evident in routine H&E sections), hence the name, corpus luteum. Theca lutein cells (TLC) also contain a spherical nucleus, but the cells are smaller than the granulosa lutein cells. Thus, when identifying the two-cell types, aside from location, note that the nuclei of adjacent theca lutein cells generally appear to be closer to each continues as the corpus luteum develops. As the corpus luteum becomes more plicated, the former follicular cavity becomes reduced in size. At the same time, blood vessels (BV) from the theca of the follicle invade the former cavity and the transforming membrana granulosa cells. Cells of the theca interna follow the blood vessels into the outermost depressions of the plicated structure. These theca interna cells become transformed into cells of the corpus luteum called theca lutein cells.
Corpus luteum, ovary, human, H&E ×20.
A portion of a fully formed corpus luteum is shown here. Most endocrine cells are the granulosa lutein cells (GLC). These form a folded cell mass that surrounds the remains of the former follicular cavity (FC). External to the corpus luteum is the connective tissue of the ovary (CT). Keep in mind that the theca interna was derived from the connective tissue stroma of the ovary. The location of theca lutein cells (TLC) reflects this origin, and these cells can be found in the deep outer recesses of the glandular mass, adjacent to the surrounding connective tissue.
other than nuclei of adjacent granulosa lutein cells. The connective tissue (CT) and small blood vessels that  invaded the mass of granulosa lutein cells can be identified as the flattened and elongated components between the granulosa lutein cells.
The changes whereby the ruptured ovarian follicle is transformed into a corpus luteum occur under the influence of pituitary luteinizing hormone. In turn, the corpus luteum itself secretes progesterone, which has a profound effect on the estrogen-primed uterus. If pregnancy occurs, the corpus luteum remains functional; if pregnancy does not occur, the corpus luteum regresses after having reached a point of peak development, roughly 2 weeks after ovulation. The regressing cellular components of the corpus luteum are replaced by fibrous connective tissue, and the structure is then called a corpus albicans.