In figure on right, a later stage in the growth of the secondary follicle is shown. The antrum (FA) is larger, and the oocyte is off to one side, surrounded by a mound of follicular cells called the cumulus oophorus. The remaining follicular cells that surround the antral cavity are referred to as the membrana granulosa (MG), or simply granulosa cells.
Atretic follicle, ovary, monkey, H&E ×65. marked by arrows in adjacent figure on right. The two larger, more advanced follicles do not display the remains of a zona pellucida, but they do display
Atretic follicles (AF) are shown here and at higher magnificaother features of follicular atresia.
tion in adjacent figure on right. The two smaller atretic follicles can be identified by virtue of the retained zona pellucida (ZP)
Atretic follicles, ovary, monkey, H&E ×120.
In atresia of a more advanced follicle, the follicular cells tend to degenerate more rapidly than the cells of the theca interna, and the basement membrane separating the two becomes thickened to form a hyalinized membrane, the glassy membrane.
Thus, the glassy membrane (arrows) separates an outer layer of remaining theca interna cells from the degenerating inner follicular cells. The remaining theca interna cells may show cytologic integrity (RTI); these intact theca cells remain temporarily functional in steroid secretion.
AF, atretic follicle FA, antrum of follicle FC, follicle cells MG, membrana granulosa RTI, remaining theca interna cells TE, theca externa TI, theca interna ZP, zona pellucida arrowhead, persisting theca interna cells arrows, glassy membrane KEY
After the oocyte and its immediately surrounding cells (i.e., the cells of the cumulus oophorus) are discharged from the mature ovarian follicle (ovulation), the remaining follicle cells (membrana granulosa) and the adjacent theca interna cells differentiate into a new functional unit, the corpus luteum. The cells of the corpus luteum, luteal cells, rapidly increase in size and become filled with lipid droplets. A lipid-soluble pigment in the cytoplasm of the cells, lipochrome, gives them their yellow appearance in fresh tissue. Electron micrographs of the luteal cells demonstrate that they have features typical of steroid-secreting cells, namely, abundant smooth endoplasmic reticulum and mitochondria with tubular cristae. Two types of luteal cells are identified: Large, centrally located granulosa lutein cells are derived from the granulosa cells; smaller, peripherally located theca lutein cells are derived from the theca interna. A rich vascular network is established in the corpus luteum into which progesterone and estrogen are secreted by the lutein cells. These hormones stimulate growth and differentiation of the uterine endometrium to prepare it for implantation of a fertilized ovum.
Corpus luteum, ovary, human, H&E ×20.
This figure shows ovarian cortex shortly after ovulation. The arrowhead points toward the surface of the ovary at the site of ovulation. The cavity (FC) of the former follicle has been invaded by connective tissue (CT). The membrana granulosa has become plicated, and the granulosa cells, now transforming into cells of the corpus luteum, are called granulosa lutein cells (TC). The plication of the membrana granulosa begins just before ovulation and
Corpus luteum, ovary, human, H&E ×65 (on left) and ×240 (on right).