Ovarian cycle

From puberty onwards, follicles mature in cycles. A cycle lasts about 28-30 days with ovulation occurring in the middle of the cycle. This subdivides the cycle into a follicular phase (1st half of cycle) and a luteal phase (2nd half of cycle). The cycle is regulated by the interaction of at least three endocrine organs. The hypothalamus releases the regulating hormone LH-RH (luteinizing hormone releasing hormone), which causes the adenohypophysis to secrete the gonadotrophins FSH (follicle-stimulating hormone) and LH (luteinizing hormone). In the follicular phase, gonadotrophins stimulate the growth of tertiary follicles and the production of oestrogen in the follicular cells and the theca interna. In the luteal phase, the corpus luteum secretes progesterone. Oestrogen and progesterone affect the target organs and lead to a feed-back reaction involving the hypophysis and the hypothalamus.
At the beginning of the follicular phase, several secondary follicles mature into tertiary follicles under the influence of FSH. About 6-8 days before ovulation, one of the tertiary follicles becomes dominant and takes over steroid production. Simultaneously, the growth of other tertiary follicles is inhibited. These tertiary follicles become atretic and are replaced by connective tissue (corpus atreticum). The dominant tertiary follicle reaches a diameter of 20-24 mm at ovulation and so can be seen by ultrasound. Around 16-24 hours before ovulation, the LH in the plasma becomes stronger (LH-peak). The prolongations of the corona radiata cells which have reached the surface of the oocyte through the zona pellucida retract; due to this, meiosis is no longer inhibited. The primary oocyte terminates the 1st maturation division. The 2nd maturation division begins, ending when it reaches the metaphase.