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Home , Estrous cycle, Menstruation (mammal)

DEVELOPMENTAL BIOLOGY 8 SEXUAL CYCLES

The estrous cycle (also oestrous cycle ) comprises the recurring physiologic changes that are induced by reproductive in most mammalian placental . Humans undergo a instead. Estrous cycles start after puberty in sexually mature females and are interrupted by anestrous phases or . Typically estrous cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for , also called a "period".

The estrous cycle is the cycle in the reproductive system that prepares it for reproduction. The cycle is conveniently divided into 4 phases:

Proestrus One or several follicles of the are starting to grow. Their number is specific for the species. Typically this phase can last as little as one day or as long as 3 weeks, depending on the species. Under the influence of the lining in the ( ) starts to develop. Some animals may experience vaginal secretions that could be bloody. The female is not yet sexually receptive.

Estrus Estrus refers to the phase when the female is sexually receptive (" in heat," or" on heat" in British English). Under regulation by gonadotropic hormones, ovarian follicles are maturing and estrogen secretions exert their biggest influence. The animal exhibits a sexually receptive behavior, a situation that may be signaled by visible physiologic changes. A signal trait of estrus is the lordosis reflex , in which the animal spontaneously elevates her hind quarters. In some species, the vulvae are reddened. may occur spontaneously in some species (e.g. cow), while in others it is induced by (e.g. ). If there is no copulation in an induced ovulator, estrus may continue for many days, followed by 'interestrus', and the estrus phase starts again until copulation and ovulation occur.

Metestrus During this phase, the signs of estrogen stimulation subside and the starts to form. The uterine lining begins to secrete small amounts of . This phase typically is brief and may last 1 to 5 days. In some animals bleeding may be noted due to declining estrogen levels.

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Diestrus Diestrus is characterized by the activity of the corpus luteum that produces progesterone. In the absence of the diestrus phase (also termed pseudo-pregnancy) terminates with the regression of the corpus luteum. The lining in the uterus is not shed, but will be reorganized for the next cycle.

MENSTRUAL CYCLE

The term, ‘menstrual cycle’ refers to the hormonal and reproductive tissue changes that occur in adult female during their reproductive years. The term menstruation refers to the periodic shedding of the uterine lining. (Menstru means "monthly.''). With every cycle, a woman's body prepares for a potential pregnancy, whether or not that is the woman's intention.

The menstrual cycle starts, i.e., menarche occurs, at different ages in different species. In Old World monkeys, such as the rhesus monkey, menarche occurs at about 4 years of age. In the great , it occurs at about 8-10 years if the animals are living in nature, at about 6-7 years if they live in captivity where they receive more nutritious food and mature more quickly. In humans, menarche occurs at about 11-14, but this can vary depending upon heredity, diet, and perhaps, climate.

The duration of the menstrual cycle varies with species; about 29 days in orang-utans, about 30 days in gorillas and about 37 days in . In human the average menstrual cycle takes about 28 days and occurs in phases: the , the ovulatory phase (ovulation), and the .

There are four major hormones (chemicals that stimulate or regulate the activity of cells or organs) involved in the menstrual cycle: follicle-stimulating , , estrogen, and progesterone.

Follicular Phase

During the follicular phase (the first 14 days of the menstrual cycle), the granulosa cells and theca interna cells in the developing follicles in the ovary produce increasing amounts of estrogen, but virtually no progesterone. The estrogen (mainly ) which is released prepares the rest of the female reproductive tract for a pregnancy. This phase starts on the first day of Menstrual period. During the follicular phase of the menstrual cycle, the following events occur:

 Two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), are released from the brain and travel in the blood to the .

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 The hormones stimulate the growth of about 15 to 20 eggs in the ovaries, each in its own "shell," called a follicle.  These hormones (FSH and LH) also trigger an increase in the production of the female hormone estrogen.  As estrogen levels rise, like a switch, it turns off the production of follicle-stimulating hormone. This careful balance of hormones allows the body to limit the number of follicles that mature.  As the follicular phase progresses, one follicle in one ovary becomes dominant and continues to mature. This dominant follicle suppresses all of the other follicles in the group. As a result, they stop growing and die. The dominant follicle continues to produce estrogen.

Estrogen (estradiol) has the following effects in the female during the follicular phase:

1. maintenance and development of the secondary (accessory) sexual organs (e.g. the , uterus, Fallopian tubes, fimbriated funnel, the cervix)

2. maintenance and development of the secondary sexual characteristics (e.g. softer skin, breast development and growth, higher voice, fat deposition around the hips)

3. travels to the follicles to help stimulate

4. inhibits the release of FSH from the anterior pituitary (adenohypophysis)

5. stimulates the proliferative phase of the endometrium in the uterus

6. causes the cervical mucus to become copious, watery and hospitable to sperm. This cervical mucus will dry in a characteristic "fern-leaf" pattern on a microscope slide.

- As the follicle matures into a Graafian follicle (the most mature follicle type) estrogen levels increase until an estrogen surge occurs about 48 hours before ovulation. The estrogen surge is a sudden, rapid increase in estrogen secretion over a short period of time (less than 24 hours).

- This estrogen surge from the Graafian follicle in the ovary stimulates ovulation by causing a surge in LH release from the anterior pituitary by positive feedback about 24 hours before ovulation. That is, the more estrogen that is released, then the more LH that is released. The LH released during the LH surge travels to the ovary and stimulates ovulation (ovulation is the release of the oocyte from the ovary and into the fimbriated funnel).

- During most of the follicular phase, the increasing estrogen levels inhibit FSH release from the anterior pituitary by negative feedback. This is how the combination oral contraceptive (which contains both estrogen and progesterone) can prevent pregnancy. A woman who is taking the combined oral contraceptive does not typically ovulate because her follicles do not develop

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(she has low FSH secretion) and her ovaries do not release an estrogen surge (so she does not ovulate).

Ovulatory Phase

The ovulatory phase, or ovulation, starts about 14 days after the follicular phase started. The ovulatory phase is the midpoint of the menstrual cycle, with the next menstrual period starting about two weeks later. During this phase, the following events occur:

 The rise in estrogen from the dominant follicle triggers a surge in the amount of luteinizing hormone that is produced by the brain.  This causes the dominant follicle to release its egg from the ovary.  As the egg is released (a process called ovulation), it is captured by finger-like projections on the end of the fallopian tubes (fimbriae). The fimbriae sweep the egg into the tube.  Also during this phase, there is an increase in the amount and thickness of mucus produced by the cervix (lower part of the uterus). If a woman were to have intercourse during this time, the thick mucus captures the man's sperm, nourishes it, and helps it to move towards the egg for fertilization.

Luteal Phase

The luteal phase of the menstrual cycle begins right after ovulation and involves the following processes:

 Once it releases its egg, the empty follicle develops into a new structure called the corpus luteum. The corpus luteum is made of two cell types, the luteal cells (which were the granulosa cells in the follicles) and the paraluteal cells (which were the theca interna cells in the follicles).  The corpus luteum secretes the hormone progesterone. Progesterone prepares the uterus for a fertilized egg to implant.  If intercourse has taken place and a man's sperm has fertilized the egg (a process called conception), the fertilized egg (embryo) will travel through the fallopian tube to implant in the uterus. The woman is now considered pregnant.  If the egg is not fertilized, it passes through the uterus. Not needed to support a pregnancy, the lining of the uterus breaks down and sheds, and the next menstrual period begins

Progesterone has the following effects in the female during the luteal phase:

1. increases basal body temperature by about 1/2 to 1 degree centigrade

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2. inhibits the release of LH from the anterior pituitary (adenohypophysis). Prevents ovulation in women who are taking the pill. Women on the pill do not experience an LH surge, so they do not ovulate.

3. stimulates the secretory phase of the endometrium in the uterus

4. causes the cervical mucus to become thick, viscous and inhospitable to sperm. This cervical mucus will not dry in a characteristic "fern-leaf" pattern on a microscope slide.

Remember that estrogen is also secreted during the luteal phase, but since progesterone levels are higher during the luteal phase the effects of progesterone are more pronounced than those of estrogen during the second half of the menstrual cycle. The estrogen that is secreted helps to maintain the secondary sexual organs and characteristics.

Testosterone has the following effects in the female: (mostly from the adrenal cortex)

1. responsible for sex drive (libido) 2. maintains muscle mass

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