Uterus anatomy

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Here is a thorough overview of uterus anatomy drawn from Gray's Anatomy for Students, Sabiston's Surgery, Junqueira's Histology, and the Color Atlas of Human Anatomy.

Uterus Anatomy

Anterior view of the female internal genitalia showing the uterus, fallopian tubes, ovaries, and surrounding pelvic structures

Internal female genitalia - anterior view (Sabiston Textbook of Surgery)

Overview

The uterus is a thick-walled, pear-shaped muscular organ that sits in the midline of the pelvis, posterior to the bladder and anterior to the rectum. It is an intraperitoneal organ. In the nonpregnant adult, it typically weighs 40-80 g. It is smaller before puberty and after menopause.

Anatomical Divisions

The uterus is divided into four regions:

Region	Description
Fundus	The rounded superior part, above the openings of the uterine tubes
Body (Corpus)	The main bulk of the organ; flattened anteroposteriorly; cavity is triangular when viewed anteriorly
Isthmus	A narrow constriction between the body and cervix
Cervix	The lower cylindrical portion projecting into the vagina; ~3 cm long; mostly fibrous tissue

Uterus divisions - fundus, body, cervix, and vagina with uterine tube openings

Uterus - anterior view showing the fundus, body, cervix, and vagina (Gray's Anatomy for Students, Fig. 5.54)

The uterine cavity is a narrow slit when viewed from the side, and triangular when viewed anteriorly. Its superior corners communicate with the lumen of each uterine tube; the inferior corner continues into the cervical canal.

The cervix has:

An internal os - opens into the isthmus
An external os - opens into the vagina
The transformation zone - where the ectocervix squamous epithelium transitions to endocervical columnar epithelium; the site where cervical intraepithelial neoplasia (CIN) develops

Position

The normal adult uterus is:

Anteverted with respect to the vagina (tilted forward)
Anteflexed with respect to the cervix (body bent forward on the cervix)

Its exact position changes with bladder and rectal distension.

Wall Layers

The uterine wall has three layers:

1. Perimetrium (Serosa)

The outermost layer - a thin connective tissue layer covered by mesothelium (peritoneum)
Continuous with the broad ligaments and pelvic peritoneum
Absent over the anterior cervix (where the bladder is adjacent)

2. Myometrium

The thickest layer, composed of smooth muscle, connective tissue, and vessels
Organized into three sublayers:
- Outer (supravascular) - thin
- Middle - thickest; very richly vascular with a sponge-like appearance; its 3D smooth muscle meshwork is the main layer driving fetal expulsion during labor
- Inner (subvascular) - thin
During pregnancy, smooth muscle cells hypertrophy, allowing the uterus to expand to ~7-10x its original size
Benign smooth muscle tumors of the myometrium = leiomyomas (fibroids)

3. Endometrium

The innermost mucosal lining; simple columnar epithelium
Contains uterine glands (tubular invaginations of the epithelium) and a cellular connective tissue stroma
Divided into two functional layers:
- Basalis (basal layer) - permanent; not shed during menstruation; regenerates the functionalis
- Functionalis (functional layer) - undergoes cyclic changes under ovarian hormones; shed during menstruation

Histology of uterine myometrium and endometrium showing basal layer, glands, arteries, and surface epithelium

Uterine wall histology: (a) Basal endometrium bordering myometrium (M), showing glands (G) and arteries (A). (b) Functional layer with spiral arteries. (c) Surface epithelium (SE) with underlying stroma (S). (Junqueira's Basic Histology)

Endometrial Cycle

During reproductive years, the functionalis undergoes monthly changes:

Phase	Days	Hormone	Changes
Desquamation (menstruation)	1-4	Hormone withdrawal	Functional layer shed
Proliferative	5-14	Estradiol	Glands and stroma regenerate and grow
Secretory	15-28	Progesterone + estrogen	Glands produce viscous secretion; spiral arteries multiply; compact and spongy zones form

Curettage removes only the functionalis; the basalis remains intact
Endometriosis = endometrial tissue migrating outside the uterus to the ovary or pelvic peritoneum

Ligaments (Supports)

The uterus is held in place by several ligaments:

Ligament	Origin	Insertion	Function
Broad ligament	Lateral borders of uterus	Continuous with pelvic peritoneum	Drapes over uterus/tubes/ovaries
Round ligament	Uterine cornua	Through inguinal ring to labia majora	Maintains anteversion
Cardinal (transverse cervical) ligaments	Lateral pelvic diaphragm	Cervix and upper vagina	Main structural support at the cervical level
Uterosacral ligaments	Upper posterior cervix	Sacrum (S3)	Support; arch frames the rectum

Blood Supply

Uterine arteries - branches of the internal iliac arteries; travel within the cardinal ligament then run superiorly along the lateral uterus
Ovarian arteries - arise directly from the abdominal aorta; anastomose with uterine arteries
Venous drainage mirrors the arterial supply via the uterine venous plexus

Surgical note: During hysterectomy, the uterine arteries must be ligated with careful identification of the ureters (which cross directly beneath the uterine arteries - "water under the bridge").

Innervation

Type	Pathway
Sympathetic	Hypogastric and ovarian plexus
Parasympathetic	Lumbosacral plexus (S2-S4)
Afferent (pain)	With sympathetic fibers via lumbosacral plexus (T11-T12)

Cervix Histology

The cervical mucosa differs from the uterine body:

Ectocervix - stratified squamous epithelium (like vagina)
Endocervix - simple columnar epithelium with branching mucus-secreting glands
No functional/basal division - the cervical mucosa does not shed cyclically
The transformation zone (squamocolumnar junction) is the clinically important region for cervical cancer screening (Pap smear)

Key Clinical Points

Blastocyst implantation normally occurs in the body of the uterus
During pregnancy, the uterus expands dramatically into the abdominal cavity
Fibroids (leiomyomas) are the most common benign uterine tumors
Hysterectomy (removal of uterus) can be total (body + cervix) or subtotal (cervix spared)

Sources: Gray's Anatomy for Students; Sabiston Textbook of Surgery; Color Atlas of Human Anatomy Vol. 2 (Internal Organs); Junqueira's Basic Histology, 17e

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