| Ovarian follicle | |
|---|---|
.jpg) Histology section of a mature ovarian follicle. Theoocyte is the large, round,pink-staining cell at top center of the image. | |
| Details | |
| Precursor | Cortical cords |
| Identifiers | |
| Latin | folliculus ovaricus |
| MeSH | D006080 |
| TA98 | A09.1.01.013 |
| TA2 | 3482 |
| FMA | 18640 |
| Anatomical terminology | |
Anovarian follicle is a roughly spheroid cellular aggregation set found in theovaries. It secretes hormones that influence stages of themenstrual cycle. In humans, women have approximately 200,000 to 300,000 follicles at the time ofpuberty,[1][2] each with the potential to release anegg cell (ovum) atovulation forfertilization.[3] These eggs are developed once everymenstrual cycle with around 450–500 being ovulated during a woman's reproductive lifetime.[4]
Ovarian follicles are the basic units of female reproductive biology. Each of them contains a singleoocyte (immatureovum or egg cell). These structures are periodically initiated to grow and develop, culminating inovulation of usually a single competent oocyte in humans.[5] They also consist ofgranulosa cells andtheca of follicle.
Once a month, one of the ovaries releases a mature egg (ovum), known as an oocyte. The nucleus of such an oocyte is called agerminal vesicle[6] (see picture).
Cumulus oophorus is a cluster of cells (called cumulus cells) that surround the oocyte both in the ovarian follicle and after ovulation.
It contains numerous granulosa cells.
Granulosa cells or follicular cells are cells that surround theoocyte within the follicle; their numbers increase directly in response to heightened levels of circulatinggonadotropins or decrease in response totestosterone. They also produce peptides involved in ovarian hormone synthesis regulation.Follicle-stimulating hormone (FSH) induces granulosa cells to expressluteinizing hormone (LH) receptors on their surfaces; when circulating LH binds to these receptors, proliferation stops.[7]
The granulosa cells, in turn, are enclosed in a thin layer ofextracellular matrix – the follicular basement membrane or basal lamina (fibro-vascular coat in picture). Outside thebasal lamina, the layerstheca interna andtheca externa are found.
Primordial follicles are indiscernible to the naked eye. However, these eventually develop intoprimary, secondary and tertiary vesicular follicles. Tertiary vesicular follicles (also called "mature vesicular follicles" or "ripe vesicular follicles") are sometimes called Graafian follicles (afterRegnier de Graaf).
In humans, oocytes are established in the ovary before birth and may lie dormant awaiting initiation for up to 50 years.[8]
After rupturing, the follicle is turned into acorpus luteum.
In a larger perspective, the wholefolliculogenesis from primordial to preovulatory follicle is located in the stage of meiosis I ofootidogenesis inoogenesis.
Embryonic development in males and females follows a common pathway beforegametogenesis. Once gametogonia enter thegonadal ridge, however, they attempt to associate with these somatic cells. Development proceeds and the gametogonia turn into oogonia, which become fully surrounded by a layer of cells (pre-granulosa cells).
The oogonia multiply by dividing mitotically; this proliferation ends when the oogonia enter meiosis. The amount of time that oogonia multiply by mitosis is not species specific. In the human fetus, cells undergoing mitosis are seen until the second and thirdtrimester of pregnancy.[9][10] After beginning the meiotic process, the oogonia (now called primary oocytes) can no longer replicate. Therefore, the total number of gametes is established at this time. Once the primary oocytes stop dividing the cells enter a prolonged 'resting phase'. This 'resting phase' ordictyate stage can last anywhere up to fifty years in the human.
For several primary oocytes that complete meiosis I each month, only one or a few functional oocyte, thedominant follicles, completes maturation and undergoes ovulation. The other follicles that begin to mature will regress and become atretic follicles, eventually deteriorating.
The primary oocyte turns into a secondary oocyte in mature ovarian follicles. Unlike the sperm, the egg is arrested in the secondary stage of meiosis until fertilization.
Upon fertilization by sperm, the secondary oocyte continues the second part of meiosis and becomes azygote.
Any ovarian follicle that is larger than about threecentimeters is termed anovarian cyst.
Ovarian function may be measured bygynecologic ultrasonography of follicular volume. Presently, ovarian follicle volumes can be measured rapidly and automatically from three-dimensionally reconstructed ultrasound images.[11]
Rupture of the follicle can result in abdominal pain (mittelschmerz) and is to be considered in the differential diagnosis in people of childbearing age.[12]
Cryopreservation and culture tissue aftercryopreservation. Cryopreservation of ovarian tissue is of interest to people who want to preserve their reproductive function beyond the natural limit, or whose reproductive potential is threatened by cancer therapy,[13] for example in hematologic malignancies or breast cancer.[14]
Forin vitro culture of follicles, there are various techniques to optimize the growth of follicles, including the use ofdefined media,growth factors and three-dimensional extracellular matrix support.[15] Molecular methods andimmunoassay can evaluate stage of maturation and guide adequate differentiation.[15] Animal studies have generally shown correct imprintedDNA methylation establishment in oocytes resulting from follicle culture.[16]
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