The inner structure of the ovary consists of an outer area called the medulla and an inner part, the cortex. When a filly is born, the cortex contains all the eggs she will ever have. The cortex supports the growth and development of the structures on the ovary which help the eggs mature and also secretes hormones that help control the reproductive process. One of the cortical structures is called the follicle. Each ovum is encased by a single layer of follicular epithelial cells, the primordial follicle.
Most follicles degenerate or undergo partial development and then disappear.
This follicular growth process involves enlargement, development of cellular layers around the follicular wall, accumulation of fluid within the central follicular cavity, and production of the hormone estrogen. As the enlarged preovulatory follicle nears maturity, it bulges from the ovary s surface. This bulge can be felt through the rectal wall when the mare s ovaries are manually palpated. Ultrasonic images of the preovulatory follicle appear as a black area within the grayish tones of surrounding ovarian tissues. The follicle s diameter indicates its maturity.