The Uterus - Obstetrics & Gynecology

Anatomy

The uterus, fallopian tubes, and ovaries - Berek & Novak's Gynecology

Regions of the Uterus

• Uterine cavity is triangular in shape, lined by columnar gland-forming endometrium
• Endocervical canal is approximately 2-3 cm long

Layers of the Uterine Wall

1. Perimetrium (Serosa)

2. Myometrium

• Thick middle layer of smooth muscle fibers, 1.5-2.5 cm thick
• Three interwoven layers of smooth muscle - inner and outer fibers run parallel to the long axis, middle layer is circular
• Some outer fibers are continuous with the round ligament and fallopian tubes
• During pregnancy: undergoes hyperplasia early, then hypertrophy driven by estrogen and mechanical distension; grows from ~70 g to ~1100 g at term
• After delivery: smooth muscle cells shrink and many undergo apoptosis

3. Endometrium

• Inner mucosal layer; simple columnar epithelium (ciliated + secretory cells)
• Contains tubular uterine glands that penetrate the full thickness
• Two concentric zones:
  • Basal layer: adjacent to myometrium; not shed during menstruation; contains basal ends of glands and straight arteries; regenerates the functional layer
  • Functional layer: superficial; shed during each menstrual cycle; contains the bulk of glands, spiral arteries, and surface epithelium

Blood Supply to the Endometrium

Arterial supply to the endometrium - Junqueira's Basic Histology

• Arcuate arteries (in myometrium) → radial arteries → two terminal branches:
  • Straight arteries: supply only the basal layer; NOT sensitive to progesterone
  • Spiral arteries: supply the functional layer; uniquely sensitive to progesterone; coil and elongate during the secretory phase; vasoconstrict at menstruation causing ischemia and shedding of the functional layer
• Uterine arteries branch from the internal iliac arteries; anastomose with the ovarian and vaginal arteries
• In pregnancy: uterine blood flow increases 10-fold (from 2% to 17% of cardiac output at term); 80-90% of uterine blood flow goes to the placenta

Ligamentous Support

Important surgical note: The ureter crosses under the uterine artery within the cardinal ligament, approximately 2-3 cm lateral to the cervix - the classic "water under the bridge" relationship. This is the most common site of iatrogenic ureteral injury during gynecologic surgery.

Innervation

• Sympathetic: via hypogastric and ovarian plexus
• Parasympathetic: lumbosacral plexus (S2-S4)
• Afferent (pain) fibers: travel with sympathetic fibers in the lumbosacral plexus (T11-T12)
• The uterovaginal plexus is the primary nerve supply to the uterus

Cervix

• Exocervix (portio vaginalis): covered by stratified squamous epithelium
• Endocervical canal: covered by mucus-secreting columnar epithelium
• Squamocolumnar junction (SCJ): the transformation zone where the two epithelia meet - most vulnerable area for development of squamous neoplasia (CIN/cervical cancer)
• Eversion/ectopy: columnar epithelium extending onto the exocervix; normal in childhood, pregnancy, and with OCP use
• After menopause: transformation zone recedes into the endocervical canal
• Cervical mucus: clear and thin around ovulation (estrogen effect); scant and thick in post-ovulatory phase (progesterone effect)
• Cervix is primarily fibrous connective tissue (collagen I, collagen III, elastin, glycosaminoglycans) with only a small amount of smooth muscle

Menstrual/Uterine Cycle

Correlation of ovarian cycle, hormone levels, and uterine changes - Junqueira's Basic Histology

Uterine Changes in Pregnancy

• Weight increases from 70 g to 1100-1200 g at term
• Volume increases from 10 mL to 5 L
• Myometrial smooth muscle transitions from quiescent to contractile state at term
• Lower uterine segment: the isthmus does NOT undergo hypertrophy; thins progressively as pregnancy advances - important in caesarean surgery
• Cervical remodeling occurs in 4 phases: softening → ripening → dilation → postpartum repair

Positions of the Uterus

• Version: angle of the uterus relative to the vagina
  • Anteversion (normal) vs. Retroversion
• Flexion: angle between the uterine corpus and the cervical long axis
  • Anteflexion (normal) vs. Retroflexion
• Abnormal positioning may result from endometriosis, adhesions, or fibroids