Uterus: Anatomy and Menstruation

• The topic is Menstruation

Uterus

• Function: site of menstruation, and the development of the fetus
• Shape: 7.5 by 5 by 2.5 cm; inverted pear; on a slight anterior tilt
• Location: superior to bladder, anterior to rectum
• Fundus: upper region
• Body: middle region
• Cervix: lower region
• Isthmus: where uterine cavity narrows into the cervix
• Histology: very thick walls
• Endometrium: inner lining comprised of epithelium and lamina propria with compound tubular glands
• Epithelium: simple columnar epithelium with occasional cilia and some secretory cells
• Stroma: areolar connective tissue
• Endometrial glands: layers of epithelium dipping down into myometrium.
• Stratum basalis: close to muscle layer; helps rebuild after menses; arterioles go into this layer at all times
• Stratum functionalis: close to lumen; thin after menses, but thickens afterward, spiral arterioles are built into this region
• Myometrium: thick muscular wall with inner and outer longitudinal or oblique layers, middle layer is circular and thick
• Perimetrium: visceral peritoneum
• Peritoneum: continuous with perimetrium
• Broad ligament: Folds of the perimetrium past the uterus, which attach it to the pelvic cavity
• Peritoneal fold (mesosalpinx): upper part of the broad ligament, which covers the uterine tubes
• Ovarian ligament: holds the ovary onto the uterus
• Suspensory ligament: houses an ovarian artery, vein, and nerves

Vagina

• Function: provides a passageway for birth, menstrual flow, and intercourse
• Location: 10 cm fibromuscular canal ending at the cervix, lying between the urethra and rectum
• Histology: has rugae
• Mucosa: stratified squamous epithelium with rugae to accommodate stretch, produces glycogen (which is converted to lactic acid, creating an acidic environment)
• Muscularis: inner circular and outer longitudinal layers allowing stretch
• Adventitia: no serous membranes; areolar connective tissue binds it to other organs
• Hymen: thin mucous membrane that partially closes the vaginal opening

Vulva

• Also known as the pudendum
• Contains a series of structures of the external anatomy
• Mons pubis: on the anterior side; fatty pad over pubic symphysis, made of adipose tissue and coarse pubic hair
• Labia: folds of skin encircling the vestibule
• Majora: outer; looks like skin, has pubic hair, sebaceous glands, and sweat glands
• Minora: more internal, more mucous membranes, less skin-like
• Vestibule: region surrounding urethra and vagina
• Clitoris: small mass of erectile tissue on the anterior side
• Prepuce: a portion of labia minora that covers the external portion of the clit
• Bulb of vestibule: masses of erectile tissue just deep to the labia on either side of the vaginal orifice
• External urethral orifice: surrounded by clitoris
• Stimulation: fills with blood, becomes engorged, shrinking the vaginal orifice

Menstrual Hormones

• Puberty initiates the cycle at ages 10-14
• Monthly hormones: secretions from anterior pituitary glands, hypothalamus, and ovaries control the reproductive cycles
• Gonadotropin-releasing hormone (GnRH): released from the hypothalamus; targets the anterior pituitary gland to stimulate the release of FSH and LH
• Follicle stimulating hormone (FSH): initiates growth of follicles (which in turn secrete estrogen from the theca interna)
• Luteinizing hormone (LH): stimulates ovulation and promotes formation of the corpus luteum (which secretes estrogens, progesterone, relaxin, and inhibin)
• Ovarian hormones: estrogen and progesterone prepare the uterus for implantation
• Relaxin: facilitates implantation by stopping smooth muscle from contracting, allowing the embryo easier access to the functionalis
• Inhibin: inhibits further secretions of FSH to stop growth of further follicles
• Ovarian cycle: changes in the ovary (follicles) during and after the maturation of oocytes
• Regulation: by follicle stimulating hormone (FSH) and luteinizing hormones (LH)
• Phases: follicular phase > ovulation > luteal phase
• Uterine cycle: preparation of uterus to potentially receive a fertilized ovum
• Regulation: by ovarian hormones (estrogen, progesterone)
• Phases: menstrual phase > proliferative phase > ovulation > secretory phase

Menstrual Cycle

• Length: 24-36 days, averaging at 28
• Menstrual phase: days 1-5; functionalis is sloughed off
• Ovarian (follicular phase): FSH stimulates primordial follicles to form primary follicles; primary oocytes are arrested in prophase i of meiosis i
• Uterine (menstrual phase): declining progesterone and estrogens cause constriction of spiral arteries, leading to hypoxic cell death of the functionalis layer (50-150 mL blood, fluid, and cells expelled)
• Preovulatory phase: days 6-13; length of this phase is the most variable; follicular development
• Ovarian (follicular phase): a few primary follicles form secondary follicles, which secrete estrogen and inhibin (granulosa), inhibiting GnRH and slowing release of FSH so other follicles stop growing; only 1 dominant follicle survives to day 6
• Uterine (proliferative phase): increasing estrogen levels from secondary and mature follicles signal the stratum functionalis to repair to 4-10 mm thick
• Ovulation: day 14; rupture of follicle and release of secondary oocyte
• 16 hours before: FSH and LH surge to cause final maturation of follicle, the primary oocyte completes meiosis i to form the haploid secondary oocyte
• Hormones: mature follicle releases estrogen > stimulates GnRH > stimulates LH and FSH > stimulates ovary to rupture follicle, creating corpus hemorrhagicum
• Post-ovulatory phase: days 15-28; time between ovulation and next menses
• Ovarian (luteal phase): corpus luteum forms (basement membrane disappears and cells mix), causing upregulation in relaxin and progesterone
• Corpus albicans: formed by day 14 if no fertilization, or by 18-24 if fertilization (hCG maintains health of corpus luteum for a time, until the placenta functionally replaces it)
• Uterine (secretory phase): hormones from corpus luteum promote thickening of endometrium to 12-18 mm
• Endometrium: if no fertilization, low estrogens and progesterone trigger menses; if fertilization, the endometrial wall thickens
• No fertilization: corpus albicans forms by day 14, and low estrogens and progesterone trigger menses
• Fertilization: embryo releases hCG which maintains the corpus luteum until day 18-24, when the placenta functionally replaces it; menses is not triggered and the endometrial wall remains thick