Ovarian Cycle

The ovarian cycle is a monthly cycle which involves the development and release of an oocyte. A female has acquired the full number of eggs in her ovaries at birth, but they are all arrested in the prophase stage of meiosis I until puberty when the cycles begin. Out of the 400,000 oocytes present at birth, only approximately 480 will be released with the potential to create a new life.

HPG-Axis
The cycle begins with the synthesis of a hormone called gonadotrophin-releasing hormone (GnRH) by neurosecretory cells in the hypothalamus. This hormone stimulates the release of two other hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), from the anterior pituitary. These gonadotrophins act on the ovaries in different ways.
FSH levels slowly rise and stimulate several follicles (immature oocytes) to enlarge and proliferate at the start of each cycle. They compete with each other to become the biggest and most mature, with only one being selected to fully develop each month. This dominant follicle goes on to become a primary oocyte attached to the edge of the follicular cells surrounding it, whilst the other follicles degenerate and die – a process called atresia. This is why so few oocytes are released despite the massive numbers at birth. The dominant follicle develops a zona pellucida and an outer connective tissue capsule which differentiates into the theca interna (produces androgens) and theca externa (remaining capsule). The androgens produced by the theca interna are converted to oestrogens by granulosa cells, the blood levels of which slowly increase as the follicles grow. Oestrogen exerts negative feedback on the thalamus, suppressing FSH release until ovulation.

The Hypothalamic-Pituitary-Gonadal Axis


Before ovulation occurs, a fluid-filled cavity called an antrum appears between the primary oocyte and the surrounding follicular cells. The ovarian follicle is now called a secondary follicle with a secondary oocyte contained within. The Ovaryfollicle continues to grow and prepares for ovulation by edging closer to the surface of the ovary, and the oocyte detaches itself from the follicular wall.
The rising levels of oestrogen reach a critical point just before ovulation. Upon reaching this peak a positive feedback loop is initiated which stimulates the anterior pituitary to produce a surge of luteinizing hormone, which coincides with the peak of FSH. This is the trigger for ovulation, which occurs on roughly the 14th day of the cycle (timing varies with each individual). The mature oocyte is expelled into the abdominal cavity, wafted towards the fallopian tube by fimbriae on the end of the tube. After this dramatic peak the levels of LH, and FSH, return to minimal for the remainder of the cycle.


The rest of the ovarian follicle remains in the ovary and collapses in on itself. This structure develops into a body known as the corpus luteum which secretes progesterone, and some oestrogen, to maintain the lining of the endometrium in case the oocyte is fertilised. If fertilisation doesn’t occur then the corpus luteum soon degenerates and leaves a fibrous scar in the ovary. If fertilisation does occur then the corpus luteum persists and continues to produce progesterone, kept functioning by human chorionic gonadotrophin (hCG) produced by the embryo.

Once the corpus luteum has degenerated and production of progesterone has ceased, the body resets itself to prepare for the next cycle, approximately 28 days after it started the last one.

Hormones

The levels of various hormones throughout the ovarian cycle