Graaf·i·an follicle or graaf·i·an follicle (grä f - n, gr f-)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. |
graafian follicle
[grä′fē·ən, -grā′-]
Etymology: Reijnier de Graaf, Dutch physician, 1641-1673; L, folliculus, small bag
a mature ovarian vesicle, measuring about 10 to 12 mm in diameter, that ruptures during ovulation to release the ovum. Many primary ovarian follicles, each containing an immature ovum about 35 μm in diameter, are embedded near the surface of the ovary, just below the tunica albuginea. Under the influence of the follicle-stimulating hormone from the adenohypophysis, one ovarian follicle ripens into a graafian follicle during the proliferative phase of each menstrual cycle. The cells that form the graafian follicle are arranged in a layer three to four cells thick around a relatively large volume of follicular fluid. Within the follicle the ovum grows to about 100 μm in diameter, ruptures, and is swept into the fimbriated opening of the uterine tube. The cavity of the follicle collapses when the ovum is released, and the remaining follicular cells greatly enlarge to become the corpus luteum. If the ovum is fertilized, the corpus luteum grows and becomes the corpus luteum of pregnancy, which degenerates by the end of 9 months and has a diameter of about 30 mm. As the ovarian follicle ripens into the graafian follicle, it produces estrogen, which stimulates the proliferation of the endometrium and the enlargement of the uterine glands. The growing corpus luteum produces progesterone, which triggers endometrial gland secretion and prepares the uterus to receive the fertilized ovum. If the ovum is not fertilized, the graafian follicle forms the corpus luteum of menstruation, which degenerates before the next menstrual cycle, leaving the small scarred corpus albicans.
graafian follicle [graf´e-an]
a small sac, embedded in the ovary, that encloses an ovum. At puberty each ovary has a large number of immature follicles (primordial follicles), each of which contains an undeveloped egg cell.
About every 28 days between puberty and the onset of menopause, one of the follicles develops to maturity, or ripens, into a graafian follicle (or vesicular ovarian follicle). As it ripens, it increases in size. The ovum within becomes larger, the follicular wall becomes thicker, and fluid collects in the follicle and surrounds the ovum. 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 fallopian tubes. This release of the ovum is called ovulation; it occurs midway in the menstrual cycle, generally about 14 days after the commencement of the menstrual flow.
The released ovum travels down the tube to the uterus, a process that takes about 3 days. Meanwhile, the empty graafian follicle in the ovary becomes filled with cells containing a yellow substance, the corpus luteum. The corpus luteum secretes progesterone, a hormone that causes further change in the endometrium, allowing it to provide a good milieu in which a zygote (fertilized ovum) can grow through the stages of gestation to become a fetus.
About every 28 days between puberty and the onset of menopause, one of the follicles develops to maturity, or ripens, into a graafian follicle (or vesicular ovarian follicle). As it ripens, it increases in size. The ovum within becomes larger, the follicular wall becomes thicker, and fluid collects in the follicle and surrounds the ovum. 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 fallopian tubes. This release of the ovum is called ovulation; it occurs midway in the menstrual cycle, generally about 14 days after the commencement of the menstrual flow.
The released ovum travels down the tube to the uterus, a process that takes about 3 days. Meanwhile, the empty graafian follicle in the ovary becomes filled with cells containing a yellow substance, the corpus luteum. The corpus luteum secretes progesterone, a hormone that causes further change in the endometrium, allowing it to provide a good milieu in which a zygote (fertilized ovum) can grow through the stages of gestation to become a fetus.
graafian 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 graafian follicle
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 graafian follicle
see cystic follicle.
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