basal lamina microvilli of oocyte gap junction process of granulosa cell the basal lamina (Fig. 22.5). The theca folliculi further differentiates into two layers:

• The theca interna is the inner, highly vascularized layer of cuboidal secretory cells. The fully differentiated cells of the theca interna possess ultrastructural features characteristic of steroid-producing cells. Cells of the theca interna possess a large number of luteinizing hormone (LIT) receptors. In response to LIT stimulation, they synthesize and secrete the androgens that are the precursors of estrogen. In addition to secretory cells, the theca interna contains fibroblasts, collagen bundles, and a rich network of small vessels typical of endocrine organs.

• The theca externa is the outer layer of connective tissue cells. It contains mainly smooth muscle cells and bundles of collagen fibers.

Boundaries between the thecal layers and between the theca externa and surrounding stroma are not distinct. However, the basal lamina between the granulosa layer and the theca interna establishes a distinct boundary between these layers. It separates the rich capillary bed of the theca interna from the granulosa layer, which is avascular during the period of follicular growth.

Maturation of the oocyte occurs in the primary follicle

The distribution of organelles changes as the oocyte matures. Multiple, dispersed Golgi elements derived from the single Balbiani body of the primordial oocyte become scattered in the cytoplasm. The number of free ribosomes, mitochondria, small vesicles, and multivesicular bodies and the amount of rough endoplasmic reticulum (rER) increase. Occasional lipid droplets and masses of lipochrome pigment may also be seen. The oocytes of many species, including mammals, exhibit specialized secretory vesicles known as cortical granules (see Fig. 22.5). They are located just beneath the plasma membrane (oolemma). The granules contain proteases that are released by exocytosis when the ovum is activated by the sperm (discussed in the section on fertilization).

Numerous irregular microvilli project from the oocyte into the perivitelline space between the oocyte and the sur-

blood vessel theca externa antrum theca interna basal lamina granulosa cells

antrum oocyte basal lamina granulosa cells blood vessels zona pellucida blood vessel antrum theca externa theca interna basal lamina granulosa cells

Call-Exner body antrum blood vessels zona pellucida oocyte basal lamina granulosa cells

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