Figure 223

Primordial follicle, a. Schematic drawing of a primordial follicle shows the oocyte arrested in prophase of the first meiotic division. The oocyte is closely surrounded by a single layer of squamous follicle cells. The outer surface of these cells is separated from the connective tissue by a basal lamina. The ooplasm contains characteristic organelles, as seen with the electron microscope, including a Balbiani body, annulate lamellae, and small spherical mitochondria.

b. This photomicrograph of primordial follicles shows the oocytes surrounded by a single layer of flattened follicle cells (FC). Usually, the nucleus (N) of the oocyte is in an eccentric position. Two oocytes in which the nucleus is not included in the plane of section are indicated (X). Similarly, there are two follicles (arrows) in which the follicle cells are revealed in face or tangential view and the enclosed oocytes are not included in the section. X640.

diameter and has a large, eccentric nucleus that contains finely dispersed chromatin and one or more large nucleoli. The cytoplasm of the oocyte, referred to as ooplasm, contains a Balbiani body (Fig. 22.3a). At the ultrastructural level, the Balbiani body is revealed as a localized accumulation of Golgi membranes and vesicles, endoplasmic reticulum, numerous mitochondria, and lysosomes. In addition, human oocytes contain annulate lamellae, and numerous small vesicles are scattered throughout the cytoplasm along with small, spherical mitochondria. Annulate lamellae resemble a stack of nuclear envelope profiles. Each layer of the stack includes pore structures morphologically identical to nuclear pores.

The primary follicle is the first stage in the development of the growing follicle

As a primordial follicle develops into a growing follicle, changes occur in the oocyte, in the follicle cells, and in the adjacent stroma. Initially, the oocyte enlarges and the surrounding flattened follicle cells proliferate and become cuboidal. At this stage, i.e., when the follicle cells become cuboidal, the follicle is identified as a primary follicle. As the oocyte grows, a homogeneous, deeply staining, acidophilic refractile layer called the zona pellucida appears between the oocyte and the adjacent follicle cells (Fig. 22.4). The zona pellucida is first apparent in the light microscope when the oocyte, surrounded by a single layer of cuboidal or columnar follicle cells, has grown to a diameter of 50 to 80 ¡jlm. The growing oocyte secretes the gel like zona pellucida, which is rich in glycosaminogiycans and glycoproteins and stains with the periodic acid-Schiff (PAS) reaction.

Follicle cells undergo stratification to form the granulosa layer of the primary follicle

Through rapid mitotic proliferation, the single layer of follicle cells gives rise to a stratified epithelium, the membrana granulosa (stratum granulosum), surrounding the oocyte. The follicle cells are now identified as granidosa cells. The basal lamina retains its position between the outermost layer of the follicle cells, which become columnar, and the connective tissue stroma.

During follicular growth, extensive gap junctions develop between granulosa cells. Unlike Sertoli cells in the testis, however, the basal layer of the granulosa cells does not possess elaborate tight junctions (zonulae occlu-dentes), indicating the absence of a blood-follicle barrier. Movement of nutrients and small informational macromolecules from the blood into the follicular fluid is essential for normal development of the ovum and follicle.

Connective tissue cells form the theca layers of the primary follicle

As the granulosa cells proliferate, stromal cells immediately surrounding the follicle form a sheath of connective tissue cells, known as the theca folliculi, just external to follicle cells follicle cells

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