Follicles are structures associated with eggs in a woman's ovary.
A follicle contains:
- Oocyte (the egg which contains the genetic material and gets fertilised)
- Zona pellucida - relatively thick and strong zone of extracellular material protecting the oocyte
- Granulosa cells (the cells surrounding the egg in a circumferential formation, produce estrogen
- Antrum (fluid filled cavity in developed follicles which contains liquor follicular)
- Theca cells (technically not part of follicle, are the cells of the ovary surrounding the follicle which produce androgen)
There are also defined regions of a follicle:
- Corona radiata (cells between the antrum and oocyte in a developed follicle)
- Cumulus oophorus (cells between the arms of the antrum)
Types of Follices
- Primordial follice (egg with one or several layers of granulosa cells)
- Primary follicle (egg with many layers of granulosa cells and zona pellucida)
- Secondary follicle (egg with many more layers of granulosa cells and developing antrum; grows to 2mm in size)
- Graafian follice (fully developed antrum and many layers of granulosa cells; grows to 2cm)
Preantral follicles are those in the primordial and primary stages, antral follicles are those in the seconday and Graafian stages.
Timing of Follicles
The development of follicles is controlled by hormones released from the brain (specifically follicle stimulating hormone - FSH - and luteneizing hormone - LH - released from the anterior pituitary gland). The timing is as follows:
- Primordial to primary follicle - 165 days
- Primary to secondary follicle - 120 days
- Growth stage (secondary to Graafian) - 65 days
- Selection stage - 10 days
- Maturation stage (including final exponential growth) - 10 days
Around 20 follicles will undergo the stages up to selection, however only one follicle will be selected for ovulation and the rest will be discarded.
Only the corona radiata and oocyte are released during ovulation, the remainder of the follicle (including ~80% of the granulosa cells) remains as the corpus luteum. This releases hormones (estrogen and progesterone) which sustain the endometrial wall for a possible implantation. Eventually it degrades into a fibrous scar known as the corpus albicans.
Location and Number
Follicles are located in the outer cortex of a female's ovary. Initially 6-7 million primordial follicles are developed during oogenesis, however one 2 million are present at birth, 500,000 at puberty and around 1000 by menopause. One of the causes of menopause is the low number of follicles being able to produce estrogen to drive the menstrual cycle.