Written by Dr.M.D.Mazumdar, MD
The changes in the ovary during the menstrual cycle revolves around the growth of a follicle with ovulation as the focal point.
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.
The Ovarian Cycle is subdivided into two phases:
The follicular phase starts with the development of about 20 Graafian follicles in one of the ovaries 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 estrogen 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 estrogenic 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.
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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 a gland known as the 'corpus luteum'.
The cells of the corpus luteum are capable of producing the hormone 'progesterone' which, like estrogen, 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.
If pregnancy occurs, the corpus luteum continues to increase in size and the level of progesterone continues to rise and support the early embryo. The corpus lutuem maintains the pregnancy until the placenta is mature enough to take over its function - this occurs at around the 12 week of pregnancy. The corpus luteum becomes much smaller at this time and eventually atrophies.
If pregnancy does not occur, the corpus luteum decreases in size during the luteal phase and eventually atrophies at the time of menstruation..
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 estrogen 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.
The estrogen and progesterone secreted by the changes in the ovary during the ovarian cycle acts on the endometrium to produce the uterine cycle. and on the cervix to produce the cervical cycle.
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