DEVELOPMENTAL BIOLOGY 8
SEXUAL CYCLES
ESTROUS CYCLE
The estrous cycle (also oestrous cycle ) comprises the recurring physiologic changes that
are induced by reproductive hormones in most mammalian placental females. Humans
undergo a menstrual cycle instead. Estrous cycles start after puberty in sexually mature
females and are interrupted by anestrous phases or pregnancies. Typically estrous cycles
continue until death. Some animals may display bloody vaginal discharge, often mistaken for
menstruation, also called a "period".
The estrous cycle is the cycle in the female reproductive system that prepares it for
reproduction. The cycle is conveniently divided into 4 phases:
Proestrus
One or several follicles of the ovary 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 estrogen the lining in the uterus( endometrium ) 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. Ovulation may occur
spontaneously in some species (e.g. cow), while in others it is induced by copulation (e.g.
Cat). 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 corpus luteum starts to
form. The uterine lining begins to secrete small amounts of progesterone. 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 pregnancy 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 mammals 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 apes, 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 chimpanzees. In human the average menstrual cycle takes
about 28 days and occurs in phases: the follicular phase, the ovulatory phase (ovulation), and
the luteal phase.
There are four major hormones (chemicals that stimulate or regulate the activity of cells or
organs) involved in the menstrual cycle: follicle-stimulating hormone, luteinizing hormone,
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 estradiol) 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:
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Two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), are
released from the brain and travel in the blood to the ovaries.
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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 vagina,
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 oogenesis
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:
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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:
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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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