Medicine

Anatomy of the Uterus.

 

Embryology

The coelomic epithelium ( lateral part of the mesonephros) invaginates and forms the paramesonephric ducts (Mullerian) and grow caudally initially but cross eventually and fuse to make the uterus and upper vagina. Cranial ends persist as the uterine tubes. Incomplete fusion form a bicornuate uterus.

Uterus

The uterus is a muscular organ whose purpose is to provide a nidus for the developing embryo. Measures 8x5x3cm. Contains a fundus, body and cervix.

Receives the uterine tubes and opens into the vaginal vault.
The fundus is superior to the fallopian tube insertion. It is convex and covered by peritoneum superiorly.

The body is below the fundus and is flattened. It is also enclosed by peritoneum which laterally becomes the broad ligament.

The vesical surface lies above the bladder forming the vesicouterine pouch.
Below the body is the cerivix whose lower end is within the vaginal vault. The sulcus around the lower pertruding cervix is the fornix (deepest part is posterior).
The posterior cervix is covered by peritoneum posteriorly forming the pouch of douglas. The anterior surface is deep to the vesicouterine pouch and has no peritoneal covering, it is instead attached to the bladder above the trigone by very dense connective tissue.
The ureters pass 2cm lateral to the cervix at this level, and further down it comes anterior to it.

The canal of the cervix that is continuous with the body is called the internal os. The cerix within the vagina opens as the external os and is usually circular in the nulliparous, but transverse after birth. It is also usually at the level of the ishial spines.

Structure

The uteral muscle, myometrium, occur in 3 layers but poorly defined. The outer layers are longitudinal and have expulsive functions while the inner layers are circular and act as sphincters for the blood vessels, uterine tubes and internal os.
It is lined by columnar endometrium which dips down to form glands. Its thickness varies with the position in the menstrual cycle. The base of the glands is maintained even during menstruation. The cervical epithelium do not shed during menstruation and contains mucous glands which secrete mucus. Just before the external os, the epithelium changes to stratified squamous continuous with the vagina.

Uterine supports

Normal uterus is anteverted and antiflexed (leans forward). The external os opens into the anterior wall of the vagina. 20% of nulliparous women have a retroverted uterus. The most fixed part of the uterus is the cevix, due to its attachment to the posterior surface of the bladder, the vaginal fornix, pelvic diaphragm, pelvic ligaments (thickening of visceal pelvic fascia) and (weak) peritoneal attachments.
The pubovaginalis component of the levator ani group of muscles support the vagina and therefore support the cervix. Damage to these muscles during childbirth can lead to vaginal prolapse or retroversion of uterus (uterus moves posterior-inferiorly)
The broad ligament is a double fold of peritoneum laying lateral to the uterus and offers little support. The ureters pass under the posterior fold of this ligament to reach the anterior bladder.
The lateral attachment is to the pelvic wall. The ureters lie under the posterior layer of this ligament. The upper border is free forming the mesosalpinx.

The upper lateral part of the broad ligament continues over the iliac vessels as the suspensory ligament of the ovary. Just below the uterine tube is a bulge in the anterior layer called the round ligament. Between the two layers of broad I garment is Areolar (cloud like) tissue called the parametrium. The round ligament extends from the function of the uterine tubes to the deep inguinal ring. It is continuous with the ovarian ligament (together called the gubernaculum). It passes through the inguinal canal and inserts into the fibrofatty tissue of labia majora. Supplied by ovaian ligament while in the broad ligament and then the inferior epigastric artery when in the inguinal canal. Helps to keep the uterus in the normal forward position when forces (distended bladder, gravity (when supine) push it back.

The transeverse cervical ligaments ( also known as lateral cervical, cardinal or Mackenrodt's) is a thickening at the base of the broad ligament. It extends from the lateral cervical wall (and vaginal fornix) to the medial side of the lateral pelvis. The uterine artery, inferior hypogastric plexus and ureters traverse it. Offers important lateral stability to cervix and indirectly the uterus.

The uterosacral ligaments extends from the cervix, backwards below the peritoneum and hugs the rectouterine pouch and attaches to the front of the sacrum.Palpable only on rectal examination and keep the cervix backwards facing despite the forward pull of the round ligaments (maintains anteversion).

Ligaments

Uterine Tubes (Fallopian)

Each tube is 10cm long with 1cm embeded within the uterus itself (intramural). Where the cavity becomes continuous with the lumen of the Fallopian tubes, is termed the cornu (horn). Emerging from the cornu are therefore, the fallopian tubes and lie on the upper surface of the broad ligament. The peritoneal fold embracing it is the mesosalpinx (between tube and ovary). Adjacent to the intramural part of the tube is the isthmus. Lateral to that is the ampulla forming half of the tube. At th end of the ampulla is the infundibulum with finger-like fimbriae and lies behind the broad ligament.

The tube itself is made up of two smooth muscle layers. The inner layer is circular and the outer is longitudinal (like the gut). The lumen is lined by folded mucosa lined by a misture of ciliate and non-ciliated columnar cells. They are most abundant in the fimbriated end (least muscular) and beat towards the uterus.

Blood supply of the Uterus and Uterine tubes

Supplied by the uterine artery which is a branch of the Internal Iliac. It passes medially at the base of the broad ligament, above the ureter to reach the supra-vaginal part of the cervix. It gives of a brach to the vagina and then to the cervix as it ascends in a tortuous pattern between the layers of broad ligament and reaches as high as the cornu. Its branches penetrate the uterine wall and and anastamose in the midline with the contralateral uterine artery. At the level of the uterine tube, it turns laterally and anatamoses with the ovarian artery which supplies the uterine tube.
The veins of the uterus course the below the artery at the base of the broad ligament where they form a wide plexus accross the pelvic floor. This communicates with the vesical and rectal plexuses and drains to the iliac veins. The tube veins join the ovarian veins.

Lymph drainage

The cervix drains to the internal and external iliac nodes. and sacral nodes along the uterosacral ligament.

Uterosacral Ligament

The lower body drains to the external iliac nodes and the upper part along with the uterine tube drains to the para-aortic nodes.

Nerve supply

Innervated by the inferior hypogastric plexus. Sympathetic supply is vagoconstrictive but high spinal transection does not affect uterine contractility even in labour as uterine smooth muscle also responds to hormones. The lower cervix carries pain with the pelvic splanchnic nerves but in the upper cervix and body of the uterus, pain is carried by the sympathetic nerves (T10-T11, therefore referred pain can be 'felt' by these dermatomes)

Surgical Access to Uterus

Abdominal vs Vaginal. Broad and ovarian ligaments are divided bilaterally. Ureters are safeguarded during dividion of the uterine arteries. Total hysterectomy divides the vaginal wall anteriorly and posteriorly below the vervix while in subtotal, the cervix is cut above the level of the cardinal ligaments.