2.3 Menstrual Cycle

Luteal Phase

After ovulation, LH causes the ruptured follicle to form the corpus luteum, which secretes progesterone. Remember that estrogen helps regenerate the uterine lining, but it is progesterone that maintains it for implantation. Progesterone levels begin to rise, while estrogen levels remain high. The high levels of progesterone again cause negative feedback on GnRH, FSH, and LH, preventing the ovulation of multiple eggs.

Fertilization

After the egg has been fertilized, the zygote will develop into a blastocyst that will implant into the uterine lining and secrete human chorionic gonadotropin.

Super High Levels of Estrogen

Around day 12 to 13 right before ovulation, the massive numbers of granulosa cells are creating a very high amount of estrogen, which causes a paradoxical positive feedback on the APG to begin secreting more FSH and LH.

Menopause

As a woman ages, her ovaries become less sensitive to FSH and LH, resulting in ovarian atrophy and the loss of the ability to remain in their menstrual cycle. Negative feedback of FSH and LH is lost so these two hormones become concentrated in the blood. This causes many physiological changes.

hCG during the First Trimester

Critical for continuing the secretion of estrogen and progesterone from the corpus luteum, which keep the uterine lining in place.

Induction of Ovulation

Late in the follicular phase, the developing follicles secrete higher concentrations of estrogen. This high concentration of estrogen paradoxically causes a positive feedback by causing the secretion of GnRH, FSH, and LH. The surge in LH then causes the release of the mature ovum from the ovary into the abdominal cavity.

hCG during the Second Trimester

Levels decline because the placenta has grown to a sufficient size to secrete estrogen and progesterone by itself. The high levels of estrogen and progesterone exert negative feedback to prevent the continued secretion of GnRH.

Corpus Luteum if Fertilization Occurs

Lives on to continue producing estrogen, progesterone and inhibin. Estrogen and progesterone prepare the endometrium for implantation of the embryo, and inhibin prevents the cycle from restarting.

Progesterone during the Luteal Phase

Maintains the uterine lining in preparation for implantation. Causes negative feedback of GnRH, FSH, and LH. This negative feedback prevents the ovulation of multiple eggs.

Proliferative

Phase characterized by the increasing estrogen levels causing the endometrial lining to proliferate.

Menses

Phase characterized by the shedding of the previous cycle's endometrium during menstruation.

Secretory

Phase in which progesterone stimulates the preparation of the endometrium in a few ways: It increases blood flow to the endometrium through spiral arteries; increases uterine secretions through glands in the endometrium; and it reduces contractility of the uterine muscles to prepare for expansion during gestation.

High Levels of Progesterone

Primary characteristic of the luteal phase, in which the corpus luteum is secreting progesterone.

Implantation of a Blastocyst

Progesterone is a pro-gestational hormone. It prepares the endometrium for this event. Progesterone supresses FSH and LH which would cause the corpus luteum to atrophy, but this event causes the release of human chorionic gonadotropin, which prevents the corpus luteum from withering away.

Follicular Phase

This preovulatory phase begins when the menstrual flow, which sheds the uterine lining of the previous cycle, begins.

Pregnancy Tests

Use of monoclonal antibodies to detect hCG.

Decidua

In the follicular phase, estrogen works to regrow the endometrial lining and stimulates this organ to be glandularized and vascularized.

Progesterone during the Secretory Phase

Increases blood flow to the endometrium through the creation of spiral arteries; also increases uterine secretion through special glands in the endometrium; it reduces the contractility of the uterine muscles to prepare for expansion during gestation.

Human Chorionic Gonadotropin

This hormone is an analogue of LH and can trigger the same receptors as LH. This causes the maintenance of the corpus luteum.

LH Surge

A dramatic increase in LH secretion right before ovulation, caused by the paradoxical positive feedback of super high estrogen concentrations caused by the high number of granulosa cells. Important because it pushes the follicle to ovulation.

Estrogen during the Follicular Phase

Both of the gonadotropins cause this hormone to be secreted from the follicles, which has a negative feedback and causes GnRH to fall off, and thus, FSH and LH concentrations to fall off.

LH after Ovulation

Causes the ruptured follicle to form the corpus luteum, which secretes progesterone.

Estrogen and Progesterone during Menstruation

Estrogen and progesterone levels fall off and the block on GnRH is lost, thus beginning the next cycle.

FSH during the Follicular Phase

Gonadotropin from the APG is stimulating growth of the follicle and increases the number of granulosa cells which secrete estrogen.

Inhibin

Hormone secreted by the granulosa cells of the corpus luteum that decreases the amount of FSH from the APG.

Menstruation

If implantation doesn't occur, the corpus luteum loses its stimulation from LH, progesterone falls off, and the uterine lining falls off.

Corpus Luteum if Fertilization does not Occur

Reaches maximum size ~10 days after ovulation (~25 days) and begins to undergo apoptosis to remove the FSH and LH block and prepare for another cycle.

Corpus Luteum

Remnant follicle after discharge of the egg and antral fluid. The granulosa cells enlarge and begin secreting more estrogen, progesterone, and inhibin.

Progesterone during Pregnancy

Secreted by the corpus luteum, but the corpus luteum dies off when hCG stops being released. At this point, the placenta takes over the role of producing estrogen and progesterone.

GnRH during the Follicular Phase

Secretion from the hypothalamus increases in response to the decreased concentrations of estrogen and progesterone, which fall off at the end of each cycle. This increased concentration triggers the release of FSH and LH.

Missed Period

Sign of pregnancy, because the corpus luteum lives on producing hormones such as inhibin, which prevented another cycle from occurring.

Corpus Luteum Hormones during the Secretory Phase

Suppress FSH and LH production through negative feedback. The inhibin levels peak when the corpus luteum is at maximum size to actively supress FSH secretions from the APG.

LH during the Follicular Phase

The APG releases this hormone which causes the thecal cells to release androstenedione, which the granulosa cells then take hold of and convert to estrogen.

Follicular Phase, Ovulation, Luteal Phase, and Menses

The four stages of the menstrual cycle.

Corpus Luteum Atrophy

The large corpus luteum is secreting hormones that actively suppress FSH and LH production, but the corpus luteum needs those hormones to survive and keep producing estrogen and progesterone. So, near the end of the luteal phase, the estrogen and progesterone levels drop and this structure atrophies in preparation for menstruation.

Paradoxical Release of FSH and LH

The paradoxical release of FSH and LH in response to a ton of estrogen results only in very high levels of LH, with a lower peak of FSH. The reason is because inhibin is still being released from the granulosa cells, which curtails the amount of FSH that would otherwise get released.

Ovulation

The release of a mature ovum from the ovary.