secondary ovarian follicle

A secondary ovarian follicle has two or more layers of granulosa cells (stratified granulosa epithelium) around the primary oocyte, and has begun to develop a surrounding theca cell layer, but has not yet formed a fluid-filled antrum. Once antrum formation begins, it becomes a tertiary (antral) follicle.

The theca cell layer develops from stromal cells that are recruited by the growing follicle at the secondary stage. The theca interna cells develop LH receptors and will produce androgens (androstenedione), which are converted to estrogens by granulosa cells — the two-cell model of steroidogenesis.

At any given time, both ovaries together contain thousands of follicles at various stages of development. Secondary follicles represent an intermediate stage that most follicles pass through before either advancing to the antral stage or undergoing atresia (degeneration), which eliminates the vast majority of developing follicles.