Menstrual Cycle Phases

• The average menstrual cycle is 28 days, but it normally varies in length by several days.
• 90% of women have cycles ranging between 25 and 35 days.
• Variations in cycle length result from alterations in the proliferative phase.

Phases of Menstrual Cycle

• Estrogen and progesterone level changes cause cyclic changes in the endometrium.
• The menstrual cycle is a continuous process; each phase gradually passes into the next one.

Menstrual Phase

• The functional layer of the uterine wall is sloughed off and discarded along with menstrual flow (menses).
• Menses usually lasts 4 to 5 days.
• Blood discharged from the vagina is combined with small pieces of endometrial tissue.
• After menstruation, the eroded endometrium is thin.

Proliferative Phase

• Lasts approximately 9 days, coinciding with the growth of ovarian follicles.
• Controlled by estrogen secreted by the follicles.
• Results in a two- to three-fold increase in the thickness of the endometrium and in its water content.
• The surface epithelium reforms and covers the endometrium early in this phase.
• Glands increase in number and length, and spiral arteries elongate.

Luteal Phase

• Lasts approximately 13 days, coinciding with the formation, functioning, and growth of the corpus luteum.
• Progesterone produced by the corpus luteum stimulates the glandular epithelium to secrete a glycogen-rich material.
• Glands become wide, tortuous, and saccular.
• The endometrium thickens due to progesterone and estrogen and increased fluid in the connective tissue.
• Spiral arteries grow into the superficial compact layer and become increasingly coiled.
• A venous network becomes complex, and large lacunae develop.
• Arteriovenous anastomoses are prominent features of this stage.

If Fertilization Does Not Occur

• The corpora lutea degenerate.
• Estrogen and progesterone levels fall.
• The secretory endometrium enters an ischemic phase.
• Menstruation occurs.

Ischemia

• Occurs when the oocyte is not fertilized.
• Spiral arteries constrict, giving the endometrium a pale appearance.
• Results from decreasing secretion of hormones, primarily progesterone, by the degenerating corpora lutea.
• Hormone withdrawal results in the stoppage of glandular secretion, a loss of interstitial fluid, and shrinkage of the endometrium.
• Spiral arteries constrict for longer periods toward the end of the ischemic phase.
• This leads to venous stasis and patchy ischemic necrosis in the superficial tissues.
• Rupture of vessel walls follows, and blood seeps into the surrounding connective tissue.
• Small pools of blood form and break through the endometrial surface, causing bleeding into the uterine cavity and through the vagina.
• Pieces of the endometrium detach and pass into the uterine cavity.
• Torn ends of the arteries bleed into the cavity, resulting in a blood loss of 20 to 80 ml.
• Entire compact layer and most of the spongy layer of the endometrium are discarded in the menses over 3 to 5 days.
• Remnants of the spongy and basal layers undergo regeneration during the subsequent proliferative phase.
• Cyclic hormonal activity of the ovary is linked with cyclic histologic changes in the endometrium.

If Fertilization Occurs

• Cleavage of the zygote and blastogenesis begin.
• The blastocyst begins to implant in the endometrium on approximately the sixth day of the luteal phase.
• Human chorionic gonadotropin keeps the corpora lutea secreting estrogens and progesterone.
• The luteal phase continues, and menstruation does not occur.

Pregnancy

• Menstrual cycles cease, and the endometrium passes into a pregnancy phase if pregnancy occurs.
• Ovarian and menstrual cycles resume after a variable period after pregnancy termination.
• Reproductive cycles normally continue until menopause, except during pregnancy.

Oocyte Transport

• The secondary oocyte is expelled at ovulation from the ovarian follicle with follicular fluid.
• The fimbriated end of the uterine tube becomes closely applied to the ovary during ovulation.
• Fimbriae move back and forth over the ovary.
• Fimbriae motion and fluid currents produced by cilia sweep the secondary oocyte into the infundibulum of the uterine tube.
• The oocyte passes into the ampulla of the tube as a result of peristalsis.
• Peristalsis causes the oocyte to pass toward the uterus.

Sperm Transport

• Reflex ejaculation of semen is divided into two phases: emission and ejaculation.

Emission

• Semen passes to the prostatic part of the urethra through the ejaculatory ducts by peristalsis.
• Emission is a sympathetic response.

Ejaculation

• Semen is expelled from the urethra through the external urethral orifice.
• Results from closure of the vesical sphincter, contraction of the urethral muscle, and contraction of the bulbospongiosus muscles.
• Sperms are rapidly transported from the epididymis to the urethra by peristaltic contractions of the ductus deferens.
• Accessory sex glands produce secretions added to the sperm-containing fluid.
• Sperm is deposited in the fornix of the vagina during intercourse (200 to 600 million).
• Sperms pass through the cervical canal by movements of their tails.
• Vesiculase assists with reducing the viscosity of the seminal fluid coagulum.
• Viscosity promotes sperm transport.
• Passage of sperms through the uterus into the uterine tubes results from muscular contractions of the walls.
• Prostaglandins thought to stimulate uterine motility and assist sperm movement to the fertilization site.
• Fructose is an energy source for the sperms in the semen.
• The volume of ejaculate averages 3.5 ml, with a range of 2 to 6 ml.
• Sperms move 2 to 3 mm per minute, but the speed varies with the pH of the environment.
• They move slowly in the acid environment of the vagina but more rapidly in the alkaline environment of the uterus.
• Motile sperms have been recovered from the ampulla 5 minutes after their deposition near the external uterine os, with some taking 45 minutes.
• Approximately 200 sperms reach the fertilization site; however, most sperms degenerate and are absorbed in the female genital tract.

Maturation of Sperms

• Freshly ejaculated sperms are unable to fertilize an oocyte.
• Sperms must undergo capacitation lasting approximately 7 hours.
• A glycoprotein coat and seminal proteins are removed from the surface of the sperm acrosome during this period.
• The membrane components of sperms are extensively altered.
• Capacitated sperms show no morphologic changes.