Biology · Ch 2 — Human Reproduction
Microscopic Anatomy of the Ovary
Microscopic Anatomy of the Ovary
The cortex of the ovary contains, at any given time, ovarian follicles at every stage of development, from the many thousands of immature follicles already present at birth to, in a given cycle, a single fully mature follicle ready for ovulation, illustrating the whole process of follicular growth (folliculogenesis) in one section.
At birth, the ovary already contains its full lifetime complement of primordial follicles — several hundred thousand to a few million of them — each consisting simply of a primary oocyte (a primary oocyte arrested in prophase I of meiosis, as described in section 2.7) surrounded by a single layer of flattened, squamous follicular cells. From puberty onward, a small batch of primordial follicles begins growing in each cycle: the surrounding follicular cell layer becomes cuboidal and then multiple cell layers thick, at which point the structure is called a primary follicle, and then a secondary follicle once these surrounding granulosa cells begin secreting fluid that collects into a single fluid-filled cavity, the antrum, and once the oocyte itself becomes surrounded by a thick, transparent glycoprotein coat, the zona pellucida, secreted jointly by the oocyte and the surrounding granulosa cells.
Of the batch of secondary follicles that begins growing in a given cycle, ordinarily only one continues all the way to full maturity (the rest degenerate, a process called atresia); this fully mature follicle, called the Graafian follicle (or tertiary/mature follicle), is much larger than earlier stages, with a large, fluid-filled antrum, and bulges out at the surface of the ovary shortly before ovulation. Within the Graafian follicle, the oocyte is held eccentrically, suspended within a mound of granulosa cells called the cumulus oophorus that projects into the antral cavity, and is immediately surrounded by a layer of radially arranged granulosa cells, the corona radiata, which stays attached to the oocyte even after ovulation. Two further cell layers, the theca interna and theca externa, surround the whole follicle on its outside and, together with the granulosa cells, are responsible for oestrogen secretion by the developing follicle. …
What this figure shows. A diagram of a section through the ovary, with an outer covering (germinal epithelium) and, beneath it, an outer cortex region packed with follicles at every stage of development arranged roughly in order of maturity around the periphery, and an inner medulla region containing blood vessels, nerves and connective tissue. The cortex is shown containing, in sequence: numerous small primordial follicles, each a primary oocyte surrounded by a single layer of flattened follicular cells; growing primary follicles in which the surrounding cell layer has become cuboidal; secondary follicles in which a fluid-filled cavity (antrum) has begun to appear and the oocyte has become surrounded by a distinct membrane, the zona pellucida; and finally a large, fluid-filled mature Graafian follicle bulging at the ovarian surface, with the oocyte held eccentrically within a mound of cells called the cumulus oophorus and surrounded immediately by a layer of radiating cells, the corona radiata. A separate part of the same diagram shows the corpus luteum, a solid yellowis …