Folliculogenesis

In biology, folliculogenesis is the maturation of the ovarian follicle, a densely-packed shell of somatic cells that contains an immature oocyte. Folliculogenesis describes the progression of a number of small primordial follicles into large preovulatory follicles that enter the menstrual cycle.

Contrary to male spermatogenesis which can last indefinitely, folliculogenesis ends when the limited pool of follicles in the ovaries run out. This depletion in follicle supply signals the beginning of the menopause.

Note: Although the process is similar in many animals, this article will deal exclusively with human folliculogenesis.

Overview
The primary role of the follicle is oocyte support. From birth, the ovaries of the human female contain a number of immature, primordial follicles. These follicles contain a similarly immature primary oocyte. A clutch of follicles begins folliculogenesis, entering a growth pattern that will end in death or in ovulation (the process where the oocyte leaves the follicle).

Over the course of roughly a year, the primordial follicle undergoes a series of critical changes in character, both histologically and hormonally. Two-thirds of the way through, the follicles have transitioned to tertiary, or antral, follicles. They become dependent on hormones emanating from the host body, causing a substantial increase in growth rate.

With a little more than ten days until the end, most of the original group of follicles have died (a process known as atresia). The remaining cohort of follicles enter the menstrual cycle, competing with each other until only one follicle is left. This remaining follicle, the preovulatory follicle, ruptures and discharges the oocyte (that has since grown into a secondary oocyte), ending folliculogenesis.



Phases of development
Folliculogenesis lasts for approximately 375 days. It coincides with thirteen menstrual cycles. The process begins continuously, meaning that at any time the ovary contains follicles in all stages of development, and ends when a mature oocyte departs from the preovulatory follicle in a process called ovulation.

The growing follicle passes through the following distinct stages that are defined by certain structural characteristics (Unfamiliar terms will be defined in their respective sections):

In a a larger perspective, the whole folliculogenesis, from primordial to preovulatory follicle, belongs to the stage of ootidogenesis of oogenesis.

Up until the preovulatory stage, the follicle contains a primary oocyte that is arrested in prophase of meiosis I. During the late preovulatory stage, the oocyte continues meiosis and becomes a secondary oocyte, arrested in metaphase II.

Primordial


Before birth, the cortex of the female ovary contains its peak number of follicles (about seven million). These primordial follicles contain immature oocytes surrounded by flat, squamous granulosa cells (the support cells) that are segregated from the oocyte's environment by the basal lamina. They are quiescent, showing little to no biological activity. Because primordial follicles can be dormant for up to 50 years in the human, the length of the ovarian cycle does not include this time.

The supply of follicles decreases to two million by birth and 300,000 by puberty. By virtue of the "inefficient" nature of folliculogenesis (discussed later), only 400 of these follicles will ever reach the preovulatory stage.

The process by which primordial cells wake up is known as initial recruitment. Research has shown that initial recruitment is mediated by the counterbalance of various stimulatory and inhibitory hormones and locally produced growth factors.

Primary
The granulosa cells of these primordial follicles change from a flat to a cuboidal structure, marking the beginning of the primary follicle. The oocyte genome is activated and genes become transcribed. Rudimentary paracrine signalling pathways that are vital for communication between the follicle and oocyte are formed. Both the oocyte and the follicle grow dramatically, increasing to almost 0.1 mm in diameter.

Primary follicles develop receptors to follicle stimulating hormone (FSH) at this time, but they are gonadotropin-independent up until the antral stage. Research has shown, however, that the presence of FSH accelerates follicle growth in vitro.

A glycoprotein polymer capsule called the zona pellucida forms around the oocyte, separating it from the surrounding granulosa cells. The zona pellucida, which remains with the oocyte after ovulation, contains enzymes that catalyze with sperm to allow penetration.

Secondary
The acquisition of a second layer of granulosa cells marks the graduation of the primary follicle to the secondary follicle. By this point, follicle mitotic activity is high and it isn't long before more and more layers of granulosa cells are formed.

Stroma-like theca cells are recruited by oocyte-secreted signals. They surround the follicle's outermost layer, the basal lamina, and undergo cytodifferentiation to become the theca externa and theca interna. An intricate network of capillary vessels forms between these two thecal layers and begins to circulate blood to and from the follicle.

The late-term secondary follicle is also known as the preantral follicle. Histologically, the preantral follicle is marked by a fully grown oocyte surrounded by a zona pellucida, approximately nine layers of granulosa cells, a basal lamina, a theca interna, a capillary net, and a theca externa.

290 days have lapsed since recruitment.

Early tertiary
The tertiary follicle, also known as a Graafian follicle or antral follicle, is marked by the formation of a fluid-filled cavity adjacent to the oocyte called the antrum. The basic structure of the mature follicle has formed and no novel cells are detectable. Granulosa and theca cells continue to undergo mitotis concomitant with an increase in antrum volume. Tertiary follicles can attain a tremendous size that is hampered only by the availability of FSH, which it is now dependent on.

By command of an oocyte-secreted morphogenic gradient, the tertiary follicle's granulosa cells begin to differentiate themselves into four distinct subtypes: corona radiata that surrounds the zona pellucida, membrana that's interior to the basal lamina, periantral that's adjacent to the antrum, and cumulus oophorous that connects the membrana and corona radiata granulosa cells together. Each type of cell behaves differently in response to FSH.

Theca cells express receptors for luteinizing hormone (LH). LH kicks off the production of androgens by the theca cells, most notably androstendione, which are aromatized by granulosa cells to produce estrogens, primarily estradiol. Consequently, estrogen levels begin to rise.

Late tertiary and preovulatory (the follicular phase of the menstrual cycle)
At this point, the majority of the group of follicles that started growth 360 days ago have already died. This process of follicle death is known as atresia, and it is characterized by radical apoptosis of all constituent cells and the oocyte. Although it is not known what causes atresia, the presence of high concentrations of FSH has been shown to prevent it.

A rise in pituitary FSH caused by the disintegration of the corpus luteum at the conclusion of the twelfth menstrual cycle precipitates the selection of five to seven class 5 follicles to participate in the thirteenth. These follicles enter the end of the twelfth menstrual cycle and transition into the follicular phase of the thirteenth cycle. The selected follicles compete with each other for growth-inducing FSH.

Estradiol and later inhibin secreted by these follicles begin to suppress FSH. Follicles that have lower amounts of FSH-receptors will not be able to weather the drought; they will show retardation of their growth rate and become atretic. Eventually, only one follicle will be viable. This remaining follicle, called the dominant follicle, will grow quickly and dramatically--up to 20 mm in diameter--to become the preovulatory follicle.

''Note: Many sources misrepresent the pace of follicle growth, some even suggesting that it takes only fourteen days for a primordial follicle to become preovulatory. In all cases, the follicular phase of the menstrual cycle means the time between selection of a tertiary follicle and its subsequent growth into a preovulatory follicle.''

Ovulation and the corpus luteum
By the end of the follicular phase of the thirteenth menstrual cycle, the preovulatory follicle will develop an opening called a stigma and excrete the oocyte with a complement of cumulus cells in a process called ovulation. The oocyte is now competent to undergo fertilization and will travel down the fallopian tubes to eventually become implanted in the uterus. The fully developed oocyte (gamete) is now at the behest of the menstrual cycle.

The ruptured follicle will undergo a dramatic transformation into the corpus luteum, a steroidiogenic cluster of cells that maintains the endometrium of the uterus by the secretion of large amounts of progesterone.

These two steps, while not part of folliculogenesis, are included for completeness. They are discussed in their entirety by their respective articles, and placed into perspective by the menstrual cycle article. It is recommended that these three topics be reviewed.

Hormone function
As with most things related to the reproductive system, folliculogenesis is controlled by the endocrine system. Five hormones participate in an intricate process of positive and negative feedback to regulate folliculogenesis. They are:
 * hypothalamus secreted gonadotropin-releasing hormone (GnRH),
 * two gonadotropins:
 * follicle stimulating hormone (FSH)
 * luteinizing hormone (LH)
 * estrogen
 * progesterone

GnRH stimulates the release of FSH and LH from the anterior pituitary gland that will later have a stimulatory effect on follicle growth (not immediately, however, because only antral follicles are dependent on FSH and LH). When theca cells form in the tertiary follicle the amount of estrogen increases sharply (theca-derived androgen is aromatized into estrogen by the granulosa cells).

A high amount of estrogen, interestingly, has an opposite stimulatory effect on the gonadotropins. LH and FSH begin to increase in high fashion. As more estrogen is secreted, more LH receptors are made by the theca cells, inciting theca cells to create more androgen that will become estrogen downstream. This positive feedback loop causes LH to spike sharply, and it is this spike that causes ovulation.

Following ovulation, LH stimulates the formation of the corpus luteum. Estrogen has since dropped to negative stimulatory levels after ovulation and therefore serves to maintain the concentration of FSH and LH. Inhibin, which is also secreted by the corpus luteum, contributes to FSH inhibition.

The endocrine system coincides with the menstrual cycle and goes through thirteen cycles (and thus thirteen LH spikes) during the course of normal folliculogenesis. However, coordinated enzyme signalling and the time-specific expression of hormonal receptors ensures that follicle growth does not become disregulated during these premature spikes.

Number of follicles
Recently, two publications have challenged the ovarian biology dogma that a finite number of follicles are set around the time of birth. Renewal of ovarian follicles from germline stem cells (originating from bone marrow and peripheral blood) was reported in the postnatal mouse ovary. Due to the revolutionary nature of these claims, further experiments are required to examine the dynamics of small follicle formation.