Ovarian Cycle Phases and Ovulation
The ovarian cycle is a series of events in the ovaries that occur during and after the maturation of the oocyte (egg or ovum). During their reproductive years, nonpregnant females usually experience a cyclical sequence of changes in their ovaries and uterus. Each cycle takes about one month and involves both oogenesis, the process of formation and development of oocyte, and preparation of the uterus to receive a fertilized ovum. Hormones secreted by the hypothalamus, anterior pituitary gland, and ovaries control the principal events.
Monthly series of events occurring in ovaries associated with the maturation of an egg is called an ovarian cycle. It consists of follicular phase and luteal phase
Follicular phase –
- period of follicle growth (days 1–14).
- Through the influence of a rise in follicle stimulating hormone (FSH), five to seven tertiary-stage ovarian follicles are recruited for entry into the next menstrual cycle.
- These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. Under the influence of several hormones, all but one of these follicles will undergo atresia, while one (or occasionally two) dominant follicles will continue to maturity. As they mature, the follicles secrete increasing amounts of estradiol, an estrogen.
- The estrogens that follicles secrete, initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium.
- During the follicular phase the lining of the uterus thickens, stimulated by gradually increasing amounts of estrogen.
- Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two follicles become dominant (non-dominant follicles atrophy and die). The dominant follicle releases an ovum or egg in an event called ovulation.
- occurs at the mid of the cycle
- An egg that is fertilized by a spermatozoon will become a zygote, taking one to two weeks to travel down the fallopian tubes to the uterus. If the egg is not fertilized within about a day of ovulation, it will die and be absorbed by the woman’s body.
- When the egg has matured, it secretes enough estradiol to trigger the acute release of luteinizing hormone (LH).
- In the average cycle this LH surge starts around cycle day 12 and may last 48 hours. The release of LH matures the egg and weakens the wall of the follicle in the ovary.
- The egg is swept into the fallopian tube by the fimbria – a fringe of tissue at the end of each fallopian tube. If fertilization occurs, it will happen in the fallopian tube.
- In some women, ovulation features a characteristic pain called mittelschmerz
- The sudden change in hormones at the time of ovulation also causes light mid-cycle bleeding for some women. An unfertilized egg will eventually disintegrate or dissolve.
Luteal phase –
- A period of corpus luteum activity (days 14–28).
- The corpus luteum is the solid body formed in the ovaries after the egg has been released into the fallopian tube which continues to grow and divide for a while.
- After ovulation, the residual follicle transforms into the corpus luteum under the support of the pituitary hormones. This corpus luteum will produce progesterone in addition to estrogens for approximately the next 2 weeks.
- Progesterone plays a vital role in converting the proliferative endometrium into a secretory lining receptive for implantation and supportive of the early pregnancy.
- It raises the body temperature by one-half to one degree Fahrenheit
- If fertilization of an egg has occurred, it will travel as an early blastocyst through the fallopian tube to the uterine cavity and implant itself 6 to 12 days after ovulation
- One very early signal consists of human chorionic gonadotropin (hCG), a hormone that pregnancy tests can measure. This signal has an important role in maintaining the corpus luteum and enabling it to continue to produce progesterone.
- In the absence of a pregnancy and without hCG, the corpus luteum demises and inhibin and progesterone levels fall. This will set the stage for the next cycle. Progesterone withdrawal leads to menstrual shedding (progesterone withdrawal bleeding), and falling inhibin levels allow FSH levels to rise to raise a new crop of follicles.