FIGURE 23.7 • Secondary follicle in a late stage of development. a. Schematic drawing of a mature (Graafian) follicle with a large antrum containing an oocyte embedded within the cumulus oophorus. The cells of the cumulus oophorus immediately surrounding the oocyte remain with it after ovulation and are referred to as the corona radiata. b. Photomicrograph of a mature secondary follicle. Note the large fluid-filled antrum (A) and the cumulus oophorus (CO) containing the oocyte. The remaining cells that surround the lumen of the antrum make up the membrana granulosa (stratum granulosum, SG). The surface of the ovary is visible on the right. Note the presence of two primary follicles (upper right). TI, theca interna. 45.
granulosa cells increases and is correlated with an increased number of LH receptors on the free antral surface. Extracellular, densely staining, PAS-positive material called Call-Exner bodies (see Fig. 23.6a) may be seen between the granulosa cells. These bodies are secreted by granulosa cells and contain hyaluronan and proteoglycans.
The mature or Graafian follicle contains the mature secondary oocyte.
The mature follicle, also known as a Graafan follicle, has a diameter of 10 mm or more. Because of its large size, it extends through the full thickness of the ovarian cortex and causes a bulge on the surface of the ovary. As the follicle nears its maximum size, the mitotic activity of the granulosa cells decreases. The stratum granulosum appears to become thinner as the antrum increases in size. As the spaces between the granulosa cells continue to enlarge, the oocyte and cumulus cells are gradually loosened from the rest of the granulosa cells in preparation for ovulation. The cumulus cells immediately surrounding the oocyte now form a single layer of cells of the corona radiata. These cells and loosely attached cumulus cells remain with the oocyte at ovulation.
During this period of follicle maturation, the thecal layers become more prominent. Lipid droplets appear in the cytoplasm of the theca interna cells, and the cells demonstrate ultrastructural features associated with steroid-producing cells. In humans, LH stimulates the cells of the theca interna to secrete androgens, which serve as estrogen precursors. Some androgens are transported to the smooth-surfaced endoplasmic reticulum (sER) in the granulosa cells. In response to FSH, the granulosa cells catalyze the conversion of androgens to estrogens, which in turn stimulate the granulosa cells to proliferate and thereby increase the size of the follicle. Increased estrogen levels from both follicular and systemic sources are correlated with increased sensitization of gonadotropes to gonadotropin-releasing hormone. A surge in the release of FSH or LH is induced in the adenohypophysis approximately 24 hours before ovulation. In response to the LH surge, LH receptors on granulosa cells are downregulated (desensitized), and granulosa cells no longer produce estrogens in response to LH. Triggered by this surge, the first meiotic division of the primary oocyte resumes. This event occurs between 12 and 24 hours after the LH surge, resulting in the formation of the secondary oocyte and the first polar body. The granulosa and thecal cells then undergo luteinization and produce progesterone (see page 839, section on the corpus luteum).
Ovulation is a hormone-mediated process resulting in the release of the secondary oocyte.
Ovulation is the process by which a secondary oocyte is released from the Graafian follicle. The follicle destined to ovulate in any menstrual cycle is recruited from a cohort of several primary follicles in the first few days of the cycle. During ovulation, the oocyte traverses the entire follicular wall, including the germinal epithelium.