This image shows three
secondary follicles (antral follicles) in various stages of formation. The
distinguishing characteristic of secondary follicles is the presence of a
fluid filled space, called the antrum. The wall
of the antrum is formed by multiple layers of granulosa
cells. The primary oocyte surrounded by its zona
pellucida, lies in the center of a mound of granulosa cells, called
the cumulus oophorus, which extends into the
antrum. The follicle is surrounded by a thick theca,
composed of the the theca interna and the theca
externa.
