Ovarian follicle

An ovarian follicle (also known as a Graafian follicle in its mature state) is the basic unit of female reproductive biology and is composed of roughly spherical aggregations of cells. It contains a single oocyte. 

An ovarian follicle can be initiated to grow and develop, culminating in ovulation of usually a single competent oocyte in humans. Approximately 10 ovarian follicles begin to mature during a normal menstrual cycle and out of these usually one will turn into a dominant ovarian follicle.

During ovulation, the primary follicle forms the secondary follicle and then the mature vesicular follicle. After rupture, the follicle turns into a corpus luteum and eventually involutes to the corpus albicans. 

Radiographic features

Ultrasound

Following maturations under the development of gonadotropins, small follicles can be seen as small sonolucent rounded structures around the ovary. In the normal physiological state one of the follicles enlarges to ~1 cm to become a dominant follicle which grows further to ~2.5 cm ¹. This is then called an ovarian follicular cyst.

See the 1-2-3 rule.

Pelvic MRI

Ovarian follicles as well as follicular cysts may be seen as rounded structures around the ovary. 

Signal characteristics within the follicle include:

• T2: high signal (as with many fluid filled entities)

History and etymology

It is named after Reinier de Graaf: Dutch physician and anatomist (1641-1673).