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What is the Oestrous Cycle? Differences between the Menstrual Cycle and Oestrus Cycle
Men and women become sexually mature at puberty. At this time, certain sequential changes occur in the sex organs of the women at regular intervals. This occurs throughout the reproductive life except during pregnancy and lactating period. These changes are termed as a menstrual cycle in women and an oestrous cycle in lower vertebrates.
The cyclical discharge of blood containing broken endometrium of the uterus, unfertilized ovum, leucocytes, and mucous through the vaginal orifice of the women during reproductive life except during pregnancy and lactation is called menstruation. It occurs on an average interval of 28 days and continues for 4-6 days.
The cyclical and sequential changes in the sex organs from the beginning of one menstrual phase to that of the next in the reproductive life of the women are called the menstrual cycle. Menstruation starts generally at 12 years of female life. But it may occur slightly at earlier or later age depending on the health of the lady. Menstruation does not occur in female life at the following time/period:
- In childhood when the reproductive organs remain immature.
- At pregnancy due to the influence of progesterone hormone.
- During the lactating period.
- At menopause when the reproductive organs become functionless around 40 years or more of age. The first onset of menstruation is called Menarche.
Phase-wise Changes During Menstrual Cycle
1. Menstrual Phase (Day 1 to 5):
In the ovary, the corpus luteum degenerates in case of absence of pregnancy and the secretion of progesterone stops. This phase occurs on the 28th day and continues for 4-5 days. In the uterus, this phase starts on the 28th day of the menstrual cycle due to abrupt less secretion of progesterone hormone. In this phase, the endometrium of the uterus with blood vessels ruptures and hemorrhage occurs through the vagina. This phase continues for 4-5 days. During this phase, the superficial layer of the endometrium is discharged and this is called, menstruation.
2. Repairing and Proliferative Phase (Days 6 to 13):
In the ovary, the primordial follicle grows into the Graafian follicle. After that under the influence of LH of the anterior pituitary gland accumulation of fluid in the liquor folliculin of the Graafian follicle occurs. The duration of this phase is 6-13 days of menstrual cycle. FSH controls this phase. In the uterus, this phase starts immediately after the stoppage of menstruation. In this phase, the broken endometrium of the uterus is reconstructed and the epithelial membrane becomes 1 mm thick. This phase lasts for (1st – 6th) days of the menstrual cycle.
3. Ovulatory Phase (Day 14 to 15):
In the ovary, the quantity of fluid in the liquor folliculin increases and the Graafian follicle ruptures to discharge the ovum. Ovulation also takes place under the influence of oestrogen hormone. From the ruptured Graafian follicle corpus luteum is formed. In the uterus, under the influence of FSH and oestrogen hormone, the endometrium of the uterus grows due to deposition of basement material. It becomes 3 mm thick. The endometrium receives a rich supply of blood vessels which swell up with branches and sub-branches and become coiled. The duration of this phase is 8 days or from the 7th to the 14th day of the menstrual cycle.
4. Premenstrual Phase (Day 16 to 28):
In the ovary, the corpus luteum grows more and the secretion of progesterone hormone is increased. The duration of this phase is 16-28 days of menstrual cycle. In the uterus, the endometrium receives more supply of blood vessels which become more coiled and increase in length. The stroma cells of the uterus grow more. The endometrium becomes 6 mm thick and blood-like fluid accumulates in the cavity of the uterus. The duration of this phase is 13-14 days or from the 16th to the 28th day of the menstrual cycle.
Different Phases and Changes in the Ovary and Uterus During Menstrual Cycle:
Schematic Representation of Hormonal Control of Menstrual Cycle
With advancing age, certain irregularities in the sexual life of a woman occur which cause permanent disappearance or cessation of the menstrual cycle. This condition is termed menopause.
Period: 45 – 55 years.
Cause and Symptoms:
One theory suggests that menopause is a result of changes in the pituitary glands and the nearby area of the hypothalamus, which means changes in pituitary gonadotrophin levels. With these irregularities in gonadal functions, especially in gametogenesis, the cyclical changes in the female reproductive life are not seen. Thus ovarian failure results in low oestrogen and hence raised levels of FSH and LIT. Finally, menstruation fails to occur and the woman can not conceive children anymore.
Atrophy of the breast, uterus, vagina, and ovaries occurs. Flushing of skin, feeling of warmth, temporary depression, and other physiological (such as osteoporosis, obesity, etc.) and psychological problems occur during the menopausal period.
In puberty, the sex organs of lower vertebrate animals become active at a particular season of the year. At this season, the female is physically and mentally prepared to accept the male. This is called a breeding season. In the breeding season, cyclical changes occur in the sex organ of the female. This is called oestrous cycle.
The animals are classified into Monoestrous (Dog) and Polyoestrous (e.g., Rats, mice, guinea pigs, etc.) according to the number of oestrous cycles in a breeding season.
1. Monoestrous Animals:
In these animals, one oestrous cycle occurs in one breeding season, e.g., Dog. The monoestrous cycle has three phases:
(a) Prooestrous: The changes in this phase are
- The external sex organs are swelled up.
- The uterus receives a supply of more blood vessels.
- Matured Graafian follicle is formed
- Vaginal epithelium proliferates.
(b) Oestrous: The changes in this phase are
- The desire for sex increases
- Female accepts the male
- Ovulation occurs
- If fertilization takes place placenta forms, otherwise it goes to the next phase.
(c) Pregnancy: The changes in this phase are
- The endometrium of the uterus grows
- The secretory glands of the uterus grow in volume and their secretion is increased
- The flow of blood in the uterus becomes more
- Mammary glands grow.
2. Polyoestrous Animals:
In these animals, two or more oestrous cycles occur in one breeding season, e.g., Rat, Guineapig, Cat, Horse.
The polyoestrous cycle is of short duration and has four phases-
(a) Prooestrous: The changes of this phase are-
- The immature follicle gradually becomes a mature follicle
- Fluid accumulates in the uterus
- In vaginal smear the nucleated epithelial cells become dominant.
(b) Oestrous: The changes in this phase are-
- The desire for sex increases and lasts for 9-15 hours.
- Only at this time, the female accepts the male.
- Under the influence of GTH, the intrauterine layers grow rapidly and swell up.
- New cells are formed by multiple fission of the mucous layer of the vagina. The superficial layers are keratinized.
- Ovulation takes place. If fertilization occurs, the placenta is formed and pregnancy begins. If fertilization does not occur it passes on to the next phase.
(c) Metoestrous: This phase lasts for 10-14 hours. The changes in this phase are-
- The Corpus luteum is formed in the ovary
- Contractility of the uterus and plenty of supply of blood vessels in it are reduced
- In vaginal smear white blood cells with some cornified cells are seen in abundance.
(d) Dioestrous or Anoestrous: This phase lasts for 60-70 hours. In rats, it lasts for 4-6 days. The changes in this phase are-
- Corpus luteum disappears
- The uterus becomes small and bloodless
- The mucous layer of the vagina becomes thin
- In a vaginal smear, nothing but white blood cells are seen.
Differences between the Menstrual Cycle and Oestrous Cycle:
|Characteristic||Menstrual Cycle||Oestrous Cycle|
|Animals in which it is seen||Generally in some primates including humans.||Generally seen in non-primates.|
|Time||At about 12 years of age.||Occurs during the breeding season.|
|Bleeding||Bleeding occurs.||Bleeding does not occur.|
|Eliminated material (if secreted)||Endometrial wall, unfertilized ovum along with blood and mucus oozes out.||The endometrial wall is absorbed and is not eliminated from the body.|