Oogenesis and histology of the ovary

This is the ovary of an adult. Examine it under low power first and notice that much of the ovary consists of a connective tissue matrix. Toward the center of the ovary you should notice some blood vessels. Within the matrix, mostly near the outer edge, are the various cells involved in oogenesis. Oogenesis is the continual process of events involved in the maturation of the stem cells to form the mature egg cell in the ovary. As in the development of the male gamete, oocytes also arise from stem cells (oogonia). In the human embryo, oogonia proliferate mitotically and a small percentage differentiate into primary oocytes. It is likely unlikely you will identify oogonia in the adult ovary.

In the high magnification image, you should be able to see a number of primordial follicles, which consist of a primary oocyte and its investing layer of follicle cells. Some of these will be very small. The follicle is a single layer of cuboidal to columnar shaped follicle cells. Notice the primary oocyte within the circle of cuboidal follicle cells. The primary oocytes of the newborn female are arrested in prophase of meiotic division I.

Starting before birth, a small proportion of primordial follicles sequentially begin to grow and are now called primary follicles. After a period of continued growth, the follicle cells secrete a fluid (liquor folliculi) to form an antral cavity or antrum. We will see secondary follicles in the next section.  Secondary follicles can be identified based on the presence of a clear space (which once contained fluid) that surrounds the oocyte. The primary oocyte within the developing follicle is surrounded by an epithelium of follicle that is multiple cell-layers thick.

You may use the diagram as a guide to the different structures described for images of the follicle in ovary, but remember that it is a diagrammatic drawing. Follicles do not develop in such an orderly pattern in real life! The various stages of follicle development may be found anywhere in the ovary.