An ovarian follicle is a roughly spheroid cellular aggregation set found in the

ovaries The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are endocr ...

. It secretes hormones that influence stages of the

menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible. The ovarian cycle controls the production and release of eg ...

. In humans, women have approximately 200,000 to 300,000 follicles at the time of

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a female, the testicles i ...

, each with the potential to release an

egg cell The egg cell or ovum (: ova) is the female Reproduction, reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female game ...

(ovum) at

ovulation Ovulation is an important part of the menstrual cycle in female vertebrates where the egg cells are released from the ovaries as part of the ovarian cycle. In female humans ovulation typically occurs near the midpoint in the menstrual cycle and ...

for

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

. These eggs are developed once every

with around 300-400 being ovulated during a woman's reproductive lifetime.

Structure

Ovarian follicles are the basic units of female reproductive biology. Each of them contains a single

oocyte An oocyte (, oöcyte, or ovocyte) is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ger ...

(immature

ovum The egg cell or ovum (: ova) is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is not capa ...

or egg cell). These structures are periodically initiated to grow and develop, culminating in

of usually a single competent oocyte in humans. They also consist of granulosa cells and

theca of follicle The theca folliculi comprise a layer of the ovarian follicles. They appear as the follicles become secondary follicles. The theca are divided into two layers, the theca interna and the theca externa. Theca cells are a group of endocrine cell ...

Oocyte

Once a month, one of the ovaries releases a mature egg (ovum), known as an oocyte. The nucleus of such an oocyte is called a '' germinal vesicle (see picture).''

Cumulus oophorus

Cumulus oophorus is a cluster of cells (called cumulus cells) that surround the oocyte both in the ovarian follicle and after ovulation.

Membrana granulosa

It contains numerous granulosa cells.

Granulosa cell

Granulosa cells or follicular cells are cells that surround the

within the follicle; their numbers increase directly in response to heightened levels of circulating gonadotropins or decrease in response to

testosterone Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...

. They also produce peptides involved in ovarian hormone synthesis regulation.

Follicle-stimulating hormone Follicle-stimulating hormone (FSH) is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the gonadotropic cells of the anterior pituitary gland and regulates the development, growth, puberty, pubertal maturat ...

(FSH) induces granulosa cells to express

luteinizing hormone Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (G ...

(LH) receptors on their surfaces; when circulating LH binds to these receptors, proliferation stops.

Theca of follicle

The granulosa cells, in turn, are enclosed in a thin layer of

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix (ICM), is a network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide structural and bio ...

– the follicular basement membrane or basal lamina (''fibro-vascular coat'' in picture). Outside the

basal lamina The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often incorrectly referred to as the basement membrane, though it does constitute a portion of the basement membrane. The b ...

, the layers theca interna and theca externa are found.

Development

Primordial follicles are indiscernible to the naked eye. However, these eventually develop into primary, secondary and tertiary vesicular follicles. Tertiary vesicular follicles (also called "mature vesicular follicles" or "ripe vesicular follicles") are sometimes called Graafian follicles (after

Regnier de Graaf Regnier de Graaf (English spelling), original Dutch spelling Reinier de Graaf, or Latinized Reijnerus de Graeff (30 July 164117 August 1673), was a Dutch physician, physiologist and anatomy, anatomist who made key discoveries in reproductive bio ...

). In humans, oocytes are established in the ovary before birth and may lie dormant awaiting initiation for up to 50 years. After rupturing, the follicle is turned into a

corpus luteum The corpus luteum (Latin for "yellow body"; : corpora lutea) is a temporary endocrine structure in female ovaries involved in the production of relatively high levels of progesterone, and moderate levels of estradiol, and inhibin A. It is the ...

Development of oocytes in ovarian follicles

In a larger perspective, the whole folliculogenesis from primordial to preovulatory follicle is located in the stage of meiosis I of

ootidogenesis Oogenesis () or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic devel ...

oogenesis Oogenesis () or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic devel ...

Embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

nic development in males and females follows a common pathway before gametogenesis. Once gametogonia enter the gonadal 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 third trimester of pregnancy. 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' or dictyate 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, the dominant 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 a

zygote A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...

Clinical significance

Any ovarian follicle that is larger than about three

centimeters upright=1.35, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is in-between 1 meter to 1 millimeter. A centimetre (International spelling) or centimeter (American ...

is termed an

ovarian cyst An ovarian cyst is a fluid-filled sac within the ovary. They usually cause no symptoms, but occasionally they may produce bloating, lower abdominal pain, or lower back pain. The majority of cysts are harmless. If the cyst either #Cyst rupture, br ...

. Ovarian function may be measured by gynecologic ultrasonography of follicular volume. Presently, ovarian follicle volumes can be measured rapidly and automatically from three-dimensionally reconstructed ultrasound images. Rupture of the follicle can result in abdominal pain (

mittelschmerz ''Mittelschmerz'' () is a term for pain due to ovulation. It occurs mid-cycle (between days 7 and 24) and can last minutes to up to several days. The pain affects one side of the lower abdomen and may be dull or sharp in nature. Other symptoms ma ...

) and is to be considered in the differential diagnosis in people of childbearing age. Cryopreservation and culture tissue after

cryopreservation Cryopreservation or cryoconservation is a process where biological material - cells, tissues, or organs - are frozen to preserve the material for an extended period of time. At low temperatures (typically or using liquid nitrogen) any cell ...

. 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, for example in hematologic malignancies or breast cancer. For ''

in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...

'' culture of follicles, there are various techniques to optimize the growth of follicles, including the use of defined media,

growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for ...

s and three-dimensional extracellular matrix support. Molecular methods and

immunoassay An immunoassay (IA) is a biochemical test that measures the presence or concentration of a macromolecule or a small molecule in a solution through the use of an antibody (usually) or an antigen (sometimes). The molecule detected by the immunoassay ...

can evaluate stage of maturation and guide adequate differentiation. Animal studies have generally shown correct imprinted

DNA methylation DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter (genetics), promoter, DNA methylati ...

establishment in oocytes resulting from follicle culture.

Additional images

File:Human Egg (2722043681).jpg, Primordial ovarian follicle. The oocyte is surrounded by a single layer of flat granulosa cells. File:Human ovarian follicle.jpg, A histological slide of a human primary ovarian follicle in greater magnification

References

External links

* - "The Female Pelvis: The Ovary" *
Images at okstate.edu

{{DEFAULTSORT:Ovarian Follicle Mammal female reproductive system