Graafian follicle

n.
Any of the fluid-filled vesicles in the mammalian ovary containing a maturing ovum.

[After Regnier de Graaf (1641-1673), Dutch physician and anatomist.]

or graaf·i·an follicle (grä'fē-ən, grăf'ē-)
n.

A mature ovarian follicle in which the oocyte attains its full size and the surrounding follicular cells are permeated by one or more fluid-filled cavities. Also called secondary follicle, vesicular ovarian follicle.

A small sac, embedded in the ovary, that encloses an ovum. At sexual maturity each ovary has a large number of immature follicles, each of which contains an undeveloped egg cell. These structures are called primordial, or primitive, follicles. At varying intervals in the different animal species, one of these follicles develops to maturity, or ripens; as it does so the animal shows the signs of sexual receptivity known as estrus.
As the follicle ripens, it increases in size. The ovum within becomes larger, the follicular wall becomes thicker, and fluid collects in the follicle and surrounds the egg. At this point, it is also known as a vesicular ovarian follicle. The follicle also secretes estradiol, the hormone that prepares the endometrium to receive a fertilized egg. As the follicle matures, it moves to the surface of the ovary and forms a projection. When fully mature, the graafian follicle breaks open and releases the ovum, which passes into the uterine tubes. This release of the ovum is called ovulation.
The released ovum travels down the tube to the uterus. Meanwhile, the empty graafian follicle in the ovary becomes transformed into the corpus luteum, or yellow body, by becoming filled with cells containing a yellow substance. The corpus luteum secretes progesterone, a hormone that causes further change in the endometrium, allowing it to provide a good milieu in which a fertilized ovum can grow through the stages of gestation to become a fetus.

• atretic g. f. — a follicle which enlarges and then regresses without proceeding to ovulation; occurs normally in animals as waves of follicles developing and regressing during estrous cycles and sometimes during pregnancy; occurs also in seasonally anestrous females. Follicular atresia can be a disease when it occurs at a time when the female should be coming into estrus but is on an inadequate diet, or suffering from a debilitating primary disease. The effect is a failure of the animal to come into estrus and to be fertile.
• cystic g. f. — see cystic follicle.

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The noun has one meaning:

Meaning #1: a vascular body in a mammalian ovary enclosing a developing egg

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