PPT Slide
The wall of a secondary (and later mature) follicle has three layers of cells: 1) an avascular epithelioid granulosal layer adjacent to the antrum, 2) a vascularized theca interna layer separated from the granulosa layer by a thin basement membrane (not seen) and 3) an outer theca externa layer composed of smaller flattened stromal cells. The granulosa and theca interna cells cooperate for the production of ovarian steroids, especially estrogen during the follicular phase (days 6-14) of the menstrual cycle.