hypothalamus-pituitary-gonad axis (in menstrual cycle)
Last reviewed 01/2018
In the fifth month of intrauterine life the primordial follicles in the female fetus reach their maximum number (about 6 to 7 million per ovary). From this point in development follicles grow and become atretic - a process that is independent of stimulation by gonadotrophins and occurs during all physiological circumstances including ovulation, pregnancy and periods of anovulation. After initiation of follicular growth then the follicle progresses to the preantral stage of development (enlarged oocyte surrounded by zona pellucida).
Further development of the follicle is dependent on the onset of puberty. Puberty results in changes in sensitivity in the hypothalamus-pituitary-gonad axis that result in pulsed release of gonadotrophin releasing hormone. This in turn results in release of luteninizing hormone (LH) and follicle stimulating hormone (FSH) by the anterior pituitary. After puberty there is pulsed release of FSH and LH every 70-220 minutes dependent on the particular phase of the menstrual cycle. The control of the release of FSH and LH is modulated by the feedback of steroids (on the hypothalamus and pituitary).
The hormonal changes that occur during the menstrual cycle can be divided into:
- follicular phase
- ovulation
- luteal phase