Editing Menstrual cycle (section)

==== Follicular phase ====
{{main|Follicular phase}}
The ovaries contain a finite number of [[Oogonium|egg stem cells]], [[granulosa cells]] and [[Theca of follicle|theca cells]], which together form primordial follicles.{{sfn|Tortora|2017|p=929}} At around 20 weeks into [[gestation]] some 7 million immature eggs have already formed in an ovary. This decreases to around 2&nbsp;million by the time a girl is born, and 300,000 by the time she has her first period. On average, one egg matures and is released during ovulation each month after menarche.{{sfn | Ugwumadu | 2014 | p=115}} Beginning at puberty, these mature to primary follicles independently of the menstrual cycle.{{sfn | Watchman | 2020 | p=8}}  The development of the egg is called [[oogenesis]] and only one cell survives the [[Prophase#Meiotic prophase|divisions]] to await fertilization. The other cells are discarded as [[polar body|polar bodies]], which cannot be fertilized.<ref name="pmid21268179">{{cite journal |vauthors=Schmerler S, Wessel GM |title=Polar bodies – more a lack of understanding than a lack of respect |journal=Molecular Reproduction and Development |volume=78 |issue=1 |pages=3–8 |date=January 2011 |pmid=21268179 |pmc=3164815 |doi=10.1002/mrd.21266 |type= Review}}</ref> The follicular phase is the first part of the ovarian cycle and it ends with the completion of the [[antral follicles]].{{sfn | Sherwood | 2016 | p=741}} [[Meiosis]] (cell division) remains incomplete in the egg cells until the antral follicle is formed. During this phase usually only one ovarian follicle fully matures and gets ready to release an egg.{{sfn | Tortora | 2017 | p=945}} The follicular phase shortens significantly with age, lasting around 14 days in women aged 18–24 compared with 10 days in women aged 40–44.{{sfn | Tortora | 2017 | pp=942–946}}

Through the influence of a rise in [[follicle stimulating hormone]] (FSH) during the first days of the cycle, a few ovarian follicles are stimulated. These follicles, which have been developing for the better part of a year in a process known as [[folliculogenesis]], compete with each other for dominance. All but one of these follicles will stop growing, while one dominant follicle – the one that has the most FSH receptors –  will continue to maturity. The remaining follicles die in a process called [[follicular atresia]].{{sfn|Johnson|2007|page=86}} [[Luteinizing hormone]] (LH) stimulates further development of the ovarian follicle. The follicle that reaches maturity is called an antral follicle, and it contains the [[ovum]] (egg cell).{{sfn|Tortora|2017|p=942}}

The theca cells develop receptors that bind LH, and in response secrete large amounts of [[androstenedione]]. At the same time the granulosa cells surrounding the maturing follicle develop receptors that bind FSH, and in response start secreting androstenedione, which is converted to estrogen by the enzyme [[aromatase]]. The estrogen inhibits further production of FSH and LH by the pituitary gland. This [[negative feedback]] regulates levels of FSH and LH. The dominant follicle continues to secrete estrogen, and the rising estrogen levels make the pituitary more responsive to GnRH from the hypothalamus. As estrogen increases this becomes a [[positive feedback]] signal, which makes the pituitary secrete more FSH and LH. This surge of FSH and LH usually occurs one to two days before ovulation and is responsible for stimulating the rupture of the antral follicle and release of the oocyte.{{sfn | Watchman | 2020 | p=8}}{{sfn | Sherwood | 2016 | p=745}}