undergoing these early changes is called a primary follicle. Thus, an early primary follicle may still be unilaminar, but it is surrounded by cuboidal cells, and this distinguishes it from the more numerous unilaminar primordial follicles that are surrounded by squamous cells.
large that the nucleus is often not included in the plane of section, as in the oocyte marked X. The group of epithelioid-appearing cells (arrowhead) are follicular cells of a primordial follicle that has been sectioned in a plane that just grazes the follicular surface. In this case, the follicular cells are seen en face.
Surrounding the follicles are elongate cells of the highly cellular connective tissue, referred to as stromal cells. The stromal cells surrounding a secondary follicle become disposed into two layers designated the theca interna and the theca externa. As seen in figure above stromal cells become epithelioid in the cell-rich theca interna (TI).
A, antrum AF, atretic follicle F, follicle cells, primordial FC, follicle cells GEp, germinal epithelium N, nucleus of oocyte PF, primordial follicles SF, growing follicles TA, tunica albuginea TI, theca interna X, oocyte showing only cytoplasm ZP, zona pellucida arrowhead, follicle cells seen en face KEY
Atresia of follicles is a regular event in the ovary, beginning in embryonic life. In any section through the postpubertal ovary, follicles of various stages can be seen undergoing atresia. In atresia, the initial changes involve pyknosis of the nuclei of the follicular cells and dissolution of their cytoplasm. The follicle is then invaded by macrophages and other connective tissue cells. The oocyte degenerates, leaving behind the prominent zona pellucida. This may fold inward or collapse, but it usually retains its thickness and staining characteristics. When included in the plane of section, a distorted zona pellucida serves as a reliable diagnostic feature of an atretic follicle. In atresia of large, nearly mature follicles, cells of the theca interna remain to form clusters of epithelioid cells in the ovarian cortex. These are referred to collectively as interstitial glands and continue to secrete steroid hormones.
Secondary follicles, ovary, monkey, H&E ×120.
Two follicles growing under the influence of FSH are shown in figure on left. The more advanced follicle is a secondary follicle. The oocyte in this follicle is surrounded by several layers of follicular cells (FC) that, at this stage, are identified as granulosa cells. At a slightly earlier time, small lakes of fluid formed 874 between the follicular cells, and these lakes have now fused into a well-defined larger cavity called the follicular antrum (FA), which is evident in the figure. The antrum is also filled with fluid and stains with the periodic acid-Schiff (PAS) reaction, although only lightly. The substance that stains with the PAS reaction has been retained as an eosinophilic precipitate in the antra of the secondary follicles shown here and in figure on right. Immediately above the obvious secondary follicle is a slightly smaller follicle. Because no antral spaces are evident between the follicular cells, it is appropriate to classify it as a primary follicle. In both follicles, but particularly in the larger follicle with the antrum, the surrounding stromal cells have become altered to form two distinctive layers designated theca interna (TI) and theca externa (TE). The theca interna is a more cellular layer, and the cells are epithelioid. When seen with the electron microscope, they display the characteristics of endocrine cells, particularly steroid-secreting cells. In contrast, the theca externa is a connective tissue layer. Its cells are more or less spindle shaped.