Theca externa

The theca folliculi comprise a layer of the ovarian follicles. They appear as the follicles become tertiary follicles.

The theca are divided into two layers: theca interna and theca externa.

Theca interna
Theca interna cells express receptors for luteinizing hormone to produce androstenedione, which via a few steps, gives the granulosa the precursor for estrogen manufacturing.

After rupture of the mature ovarian follicle, the theca interna cells differentiate into the theca lutein cells of the corpus luteum. These cells secrete estradiol.

Theca externa
The theca externa is the outer layers of the theca folliculi. They contain abundant collagen and are mainly supportive.