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The Menstrual Cycle
The menstrual cycle involves the release of numerous hormones by the
hypothalamic-pituitary-gonadotropin axis that are tightly regulated through negative
feedback mechanisms are is closely linked to normal female physiology and sexual
function.
Hormone secretion first begins in the hypothalamus where the neurosecretory
cells release Gonadotropin-releasing hormone onto the anterior pituitary gland.
The release of this hormone occurs in elevated pulsatile fashion upon the onset
of puberty.
The release of GnRH onto the anterior pituitary activates its G-protein coupled
receptors in the endocrine cells of this gland, inducing the production and
release of Follicle-stimulating hormone (FSH) and Luteinizing hormone. These
hormones are released into the blood and primarily target the cells harbored in
the ovaries.
In the ovaries, there are two major cell types that produce hormones. These are
the theca cells and the granulosa cells (these cells types are both associated
with the support and nurturing of developing pre-ovulatory follicles)
The Theca cells respond to the LH produced by the pituitary glands by producing
progesterone and androstenedione by the activation of the enzyme cholesterol
The Menstrual Cycle 1
, desmolase. LH is also needed to produce these steroidal hormones as it
activates cholesterol esterase to release free cholesterol in the cell.
Androstenedione is an androgen precursor needed for the production of male
androgens, particularly testosterone. The pathway for the production of estrogen
revolves around the aromatization of these male androgens. FSH activates
granulosa cells in the ovarian follicle by activating the enzyme aromatase - this
enzyme converts the androstenedione produced from the nearby theca cells
testosterone and then 17-bete-estradiol.
As the levels of progesterone and estradiol rise during the menstrual cycle, they
exert a negative feedback mechanism onto the anterior pituitary as they inhibit
the production of further FSH and LH, thus limiting further production of
progesterone and estradiol. There is one important exception to this rule that
occurs upon ovulation - during the ovulatory phase of the cycle, estrogen
exhibits a positive feedback mechanism with the anterior pituitary. This
mechanism allows for a sharp rise in the production of FSH and LH by the
anterior pituitary, with the surge in LH inducing ovulation.
In this feedback mechanism there are also the roles of the hormones inhibin and
activin, released by the granulosa cells. These hormones can inhibit or promote
the release of FSH from the anterior pituitary respectively. They function by
either reducing or elevating the expression of GnRH receptors on the anterior
pituitary cells.
Phases of the Menstrual Cycle
The Menstrual cycle may be divided into the uterine and ovarian cycles to describe
the chain of events that occur within these structures per cycle. Each of these may
be further divided into phases that occur simultaneously with one another:
The Uterine cycle is divided into the proliferative and secretory phases
The Ovarian cycle is divided into the follicular and luteal phases
The proliferative and follicular phases occur together, as do the secretory and
luteal phases.
These phases are separated by ovulation.
The Proliferative and Follicular phase
The Menstrual Cycle 2
The Menstrual Cycle
The menstrual cycle involves the release of numerous hormones by the
hypothalamic-pituitary-gonadotropin axis that are tightly regulated through negative
feedback mechanisms are is closely linked to normal female physiology and sexual
function.
Hormone secretion first begins in the hypothalamus where the neurosecretory
cells release Gonadotropin-releasing hormone onto the anterior pituitary gland.
The release of this hormone occurs in elevated pulsatile fashion upon the onset
of puberty.
The release of GnRH onto the anterior pituitary activates its G-protein coupled
receptors in the endocrine cells of this gland, inducing the production and
release of Follicle-stimulating hormone (FSH) and Luteinizing hormone. These
hormones are released into the blood and primarily target the cells harbored in
the ovaries.
In the ovaries, there are two major cell types that produce hormones. These are
the theca cells and the granulosa cells (these cells types are both associated
with the support and nurturing of developing pre-ovulatory follicles)
The Theca cells respond to the LH produced by the pituitary glands by producing
progesterone and androstenedione by the activation of the enzyme cholesterol
The Menstrual Cycle 1
, desmolase. LH is also needed to produce these steroidal hormones as it
activates cholesterol esterase to release free cholesterol in the cell.
Androstenedione is an androgen precursor needed for the production of male
androgens, particularly testosterone. The pathway for the production of estrogen
revolves around the aromatization of these male androgens. FSH activates
granulosa cells in the ovarian follicle by activating the enzyme aromatase - this
enzyme converts the androstenedione produced from the nearby theca cells
testosterone and then 17-bete-estradiol.
As the levels of progesterone and estradiol rise during the menstrual cycle, they
exert a negative feedback mechanism onto the anterior pituitary as they inhibit
the production of further FSH and LH, thus limiting further production of
progesterone and estradiol. There is one important exception to this rule that
occurs upon ovulation - during the ovulatory phase of the cycle, estrogen
exhibits a positive feedback mechanism with the anterior pituitary. This
mechanism allows for a sharp rise in the production of FSH and LH by the
anterior pituitary, with the surge in LH inducing ovulation.
In this feedback mechanism there are also the roles of the hormones inhibin and
activin, released by the granulosa cells. These hormones can inhibit or promote
the release of FSH from the anterior pituitary respectively. They function by
either reducing or elevating the expression of GnRH receptors on the anterior
pituitary cells.
Phases of the Menstrual Cycle
The Menstrual cycle may be divided into the uterine and ovarian cycles to describe
the chain of events that occur within these structures per cycle. Each of these may
be further divided into phases that occur simultaneously with one another:
The Uterine cycle is divided into the proliferative and secretory phases
The Ovarian cycle is divided into the follicular and luteal phases
The proliferative and follicular phases occur together, as do the secretory and
luteal phases.
These phases are separated by ovulation.
The Proliferative and Follicular phase
The Menstrual Cycle 2
