LATE PRIMARY FOLLICLE granulosa cells stratum granulosum cortical granules oocyte cytoplasm granulosa cells microvilli of oocyte gap junction basal lamina zona pellucida process of granulosa cell theca folliculi a bbbbb GCGCGCGCGC ZPZPZPZPZP FIGURE 23.5 • Late primary follicle. a. Schematic drawing of a late primary follicle shows a multilayered mass of granulosa cells (differentiated from follicle cells) surrounding the oocyte. Note that the innermost layer of granulosa cells is adjacent to the zona pellucida, and the outermost layer of these cells rests on the basal lamina, which is adjacent to the stromal cells now called the theca folliculi. The Balbiani body at this stage reorganizes into multiple Golgi units, and cortical granules appear in the cytoplasm. The wedge-shaped enlargement depicts the ultrastructure of an oocyte and adjacent follicle cells. Numerous microvilli from the oocyte and slender processes from the granulosa cells extend into the zona pellucida that surrounds the oocyte. Processes of the granulosa cells contact the plasma membrane of the oocyte. b. Photomicrograph of a late primary follicle (monkey). Multiple layers of granulosa cells (GC) can be seen surrounding the primary oocyte. The zona pellucida (ZP) is present between the oocyte and follicle cells. 160.
lamina (Fig. 23.5). The theca folliculi further differentiates into two layers:  The theca interna is the inner, highly vascularized layer of cuboidal secretory cells (Plate 93, page 874). 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 (LH) receptors. In response to LH 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-surfaced 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. 23.5a). They are located just beneath the oolemma (oocyte’s plasma membrane). The granules contain proteases that are released by exocytosis when the ovum is activated by the sperm (discussed in the section on fertilization).