Primordial follicles first appear in the ovaries during the third month of fetal development. Early growth of the primordial follicles is independent of gonadotropin stimulation. In the mature ovary, primordial follicles are found in the stroma of the cortex just beneath the tunica albuginea. A single layer of squamous follicle cells surrounds the oocyte (Fig. 23.3 and Plate 92, page 872). The outer surface of the follicle cells is bounded by a basal lamina. At this stage, the oocyte and the
FIGURE 23.3 • 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. 640.
surrounding follicle cells are closely apposed to one another. The oocyte in the follicle measures about 30 m in 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. 23.3a). 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 that are morphologically identical to nuclear pores.
The primary follicle is the first stage in the development of the growing follicle.