THE OVARIAN CYCLE OF MENSTRUATION
( Changes in the Ovary, Ovulation )
The changes in the ovary during the menstrual cycle revolves around the growth of a follicle with ovulation as the focus.
The ovarian cycle starts on the first day of the menstrual cycle. The menstrual cycle begins on the first day of the periods - which is the first day of bleeding. At this time, the levels of all the hormones - estrogen, progesterone , FSH and LH - which are primarily responsible for maintaining menstruation, are at the baseline levels.
The low level of estrogen in the blood at this time, stimulates the pituitary gland in the brain to start producing Follicle Stimulating Hormone (FSH). The level of FSH rises and stimulates the Graafian follicles in the ovaries to develop. This indicates the start of the ovarian cycle in menstruation.
Menopause is usually diagnosed by the lack of healthy developing follicles in the ovaries as well as a sustained high level of FSH and LH hormones which do not rise and fall in relation to the phases of the ovarian cycle.
Phases of the Ovarian Cycle
The Ovarian Cycle is subdivided into two phases:
The follicular phase starts with the development of about 20 Graafian follicles in the ovary under the stimulation of the hormone FSH released from the pituitary gland. These follicles grow steadily, releasing the hormone ‘estrogen’ into the blood stream.
The oestrogen released by the follicles, acts on the endometrium of the uterus and stimulates it to proliferate, indicating the start of the uterine cycle. The proliferative phase of the uterine cycle is thus also called the oestrogenic phase.
At around the 10th day of the follicular phase, one of the follicles becomes distinctly larger than the others. It continues to grow, becoming larger and larger (‘dominant follicle’) to become mature, while the growth of the others are arrested. These eventually die out.
The dominant follicle usually ruptures when it is about 18 – 20 mm in size and releases an ovum at about the 14-16th day of the menstrual cycle. This process is called ‘Ovulation’. The follicular phase ends at this stage and the Luteal Phase begins.
In women who are undergoing infertility treatment and taking medicines to stimulate the ovaries, the dominant follicle can grow up to 22 - 25 mm before it ruptures. They can also have more than 1 dominant follicle (usually 2-3 in each ovary). Ovulation from all these follicles and subsequent fertilization can cause pregnancies with twins, triplets etc.
As soon as the Graafian follicle ruptures and releases the ovum (‘ovulation’), the ovary undergoes further changes. The cells of the follicle itself changes. Fat globules get deposited in them, they grow larger, and they assume a yellowish colour. These cells are called luteal cells and the follicle now forms the 'corpus luteum'.
The cells of the corpus luteum are capable of producing the hormone ‘progesterone’ which, like oestrogen, acts on the uterine endometrium. Hence this phase is also called the progestogenic phase. The level of progesterone reaches a peak at around the 26th day of the cycle.
The changes in the uterine endometrium is dependent on the hormones secreted by the ovaries.
Comparison between the Ovarian Cycle and the Uterine Cycle
If pregnancy occurs, the corpus luteum continues to increase in size and the level of progesterone continues to rise and support the early embryo.
If pregnancy does not occur, the corpus luteum decreases in size and eventually atrophies.
The progesterone level also falls in tandem with the decreasing action of the corpus luteum. When it reaches a minimum critical level, the endometrium that had been primed by the oestrogen and the progesterone to receive an embryo, sheds from the inner wall of the uterus. This shedding of the endometrium with the consequent bleeding is termed menstruation.
As soon as the menstruation starts, it denotes the beginning of another menstrual cycle.