The female reproductive tract

THE FEMALE REPRODUCTIVE TRACT (Figs 6.19-6.26)

The vagina (Fig. 6.19; see Fig. 6.21) The vagina is an extraperitoneal structure. This muscular canal extends from the uterus to the vestibule, opening between the labia minora behind the urethra and clitoris. It has a rectangular shape, being flattened from front to back. Superiorly the cervix of the uterus projects into its anterior wall at an acute angle. The cervix invaginates the upper vagina and arbitrarily divides it into a shallow anterior and deep posterior and lateral recesses or fornices. The ureters pass medially above the lateral fornices. In front of the vagina is the base of the bladder and the urethra. Behind the upper vagina is the pouch of Douglas, containing loops of bowel. Below and behind this peritoneal reflection is the rectum. On either side are the levator ani muscles and the pelvic fascia, which are slung around the vagina and rectum, supporting these structures. The vagina traverses levator ani and the urogenital diaphragm, which have a sphincter-like action. The levator muscles insert into a fibromuscular node called the perineal body, which lies between the lower vagina and the anal canal.

Blood supply Vaginal branches of the internal iliac and the uterine arteries supply the vagina. Venous drainage is via a plexus on its lateral walls to the internal iliac vein. Lymph drainage The upper two-thirds drain to internal and external iliac nodes. The lower third drains to superficial inguinal nodes.

The uterus (Figs 6.19-6.21) The uterus is extraperitoneal. It is a pear-shaped muscular organ lying between the bladder and rectum. It has a fundus, a body and a cervix. It lies on the posterosuperior surface of the bladder w i th its cervix projecting into the anterior wall of the upper vagina. The cavity of the uterus is triangular in coronal section, but its anterior and posterior walls are apposed, giving it a slit-like appearance in the sagittal plane. The uterine tubes open into the cornua of the uterus superolaterally. The uterus leads to the vagina via the cervical canal. Just above the cervical canal the uterine cavity narrows to an isthmus. The internal os is at the upper end of the cervical canal and the external os at its lower end.

The inner lining of the uterus is the endometrium, which undergoes cyclical changes of proliferation and desquamation in the premenopausal female.

Peritoneum covers the fundus, body, cervix and upper part of the vagina posteriorly. From here it is reflected on to the anterior surface of the rectum, forming the pouch of Douglas. Anteriorly, the peritoneum is reflected from the upper part of the body to the superior surface of the bladder.

On either side of the uterus the peritoneum is reflected to the lateral pelvic walls covering the fallopian tubes. The

Fig. 6.19 Female pelvis: sagittal section showing pelvic floor.

Fig. 6.20 Support ligaments of female pelvis.

Fig. 6.21 Uterus and fallopian tubes: coronal section to show blood supply and ureter relative to uterine artery, cervix and vaginal fornices.

fold of peritoneum so formed is called the broad ligament. Several important structures run in this fold.

Ligamentous support of the uterus (Figs 6.19 and 6.20) The parametrial 'ligaments' anchor the cervix to the walls of the pelvis. These are condensations of endopelvic fascia which surround the cervix and comprise: • Pubocervical ligaments or fascia which run anteriorly from the cervix, around the base of the bladder to the pubic bone; • Transverse cervical (cardinal) ligaments which run laterally from cervix and lateral aspect of vaginal fornix to pelvic side wall; • Uterosacral ligaments which run posterosuperiorly to the midsacrum on the upper surface of the levator ani muscle.

The round ligament is a fibromuscular band that passes from the upper lateral part of the uterus to the inguinal canal, ending in the labium majoris.

Other uterine supports The bladder supports the uterus on its upper surface in the normal anatomic arrangement. This arrangement and the transverse cervical ligaments are the main passive support of the uterus. It is also supported below by the muscles of the pelvic floor, which provide active support by contracting when intra-abdominal pressure is raised. Of the levator ani muscles, the puborectalis and iliococcygeus are the most important uterine supports. At rest, the levator ani muscles are in contraction, keeping rectum, vagina and urethra elevated and closed.

Normal variants of the uterus The uterus may be retroverted, that is, lie in a posterior plane w i th the axis of the cervix directed upwards and backwards. It may also be retroflexed. Here the cervix bears the usual relationship w i th the vagina but the uterus is bent backwards on the cervix. These positions are not of clinical importance but make visualization of the uterus by ultrasound difficult. The uterus is relatively mobile and changes w i th the degree of fullness of the bladder.

In children the cervix is twice the size of the uterus. The uterus grows disproportionately until they are of equal size at puberty. In adulthood the uterus is twice the size of the cervix.

The uterine tubes

The uterine (fallopian) tubes lie in the 'free edge' of the broad ligament and convey ova from the ovaries to the uterus. They open into the uterine cornua. They are described as having four parts as follows: • The uterine part - this is the part within the wall of the uterus and opens into it; • The isthmus - this is long and narrow and leads to the ampulla; • The ampulla - this is a wide, dilated tortuous part at the outer end where fertilization of the ovum usually occurs; and • The infundibulum - this is the outer extremity of the tube. It is funnel shaped and its rim is fimbriated.

It extends out beyond the broad ligament through the abdominal ostium and opens into the peritoneal cavity.

The fimbriae spread over the upper surface of the ovary.

One of the fimbriae is longer than the rest. This is called the ovarian fimbria as it is attached to the ovary.

Blood supply of the uterus and uterine tubes (Fig. 6.21) The uterine artery, a branch of the internal iliac, runs medially in the base of the broad ligament to reach the lower lateral wall of the uterus. It ascends tortuously w i t h in the broad ligament to supply the uterus and tubes and anastomoses w i th the ovarian artery.

Venous drainage is via a venous plexus in the base of the broad ligament to the internal iliac vein.

Lymph drainage The fundus drains along ovarian vessels to para-aortic nodes.

The body drains via the broad ligament to nodes around the external iliac vessels and occasionally via the round ligament to inguinal nodes. The cervix drains to external and internal iliac nodes and posteriorly to sacral nodes.

The broad ligament (Figs 6.22 and 6.23) This double layer of peritoneum is a mesentery that encloses the fallopian tubes in its upper part and extends from the sides of the uterus to the pelvic side walls and floor. The two layers are continuous w i th each other laterally at a free edge that surrounds the uterine tube where this opens into the abdominal cavity. The uterine artery runs medially in its base. The ureter loops under the uterine artery w i t h in the ligament, passing just lateral to the cervix above the lateral vaginal fornix to enter the bladder. The uterine plexus of veins in the base of the broad ligament communicates w i th the veins of the vagina and bladder via the pelvic plexus of veins. The ligament of the ovary (from ovary to uterus) lies posterosuperiorly w i t h in the broad ligament and the round ligament lies anteroinferiorly w i t h in the layers. Both of these structures attach to the uterus close to the attachment of the uterine tubes.

The ovaries

These are paired oval organs measuring approximately 3 cm x 2 cm x 2 cm. They are usually orientated somewhat vertically and thus can be described as having upper and lower poles. They lie on the posterior surface of the broad ligament in close contact w i th the infundibulum of the fallopian tube and attached to its ovarian fimbria. The fimbriae of the uterine tubes lie superior and lateral. The surface of the ovary is not covered by peritoneum, but by a layer of germinal epithelium that becomes continuous w i th the peritoneum at the hilum of the ovary. The ovary has a tough outer layer of tunica albuginea beneath the germinal layer. The anterior surface of the ovary is attached to the posterior surface of the broad ligament by a short meso-ovarium that fuses w i th the surface of the ovary. The lower pole of the ovary is attached to the uterus by the ovarian ligament. A superolateral extension of the broad ligament, the suspensory ligament of the ovary, runs from the upper pole of the ovary to the pelvic side wall. The ovarian vessels and nerves run in this, crossing over the external iliac vessels. Despite all its attachments

Fig. 6.23 Transverse ultrasound of uterus and broad ligament. The broad ligament is outlined by fluid, therefore can be seen. The right ovary lies in its anterior surface. The broad ligament is attached at the internal iliac vessels. The uterine artery runs in the base of the ligament, within its leaves.

1. Free fluid 2. Uterus - body 3. Uterus - endometrium 4. Broad ligament 5. Ovary 6. Internal iliac vessels the ovary is very mobile, especially in women who have had children. It is frequently found behind the uterus in the pouch of Douglas.

Blood supply Arterial supply is via the ovarian artery, which arises directly from the aorta at the level of the renal arteries.

Venous drainage is via the right ovarian vein into the inferior vena cava, and via the left ovarian vein into the left renal vein.

Lymph drainage Along the ovarian vessels to para-aortic nodes.

Radiology of the female pelvis

Ultrasound (Fig. 6.24) Ultrasound is probably the most common radiological method of imaging the female reproductive tract. Transabdominal ultrasound is performed through a moderately f u ll bladder. The urine-filled bladder lifts gassy loops of bowel out of the way and provides an acoustic window through which the pelvic organs may be visualized.

The cervix usually lies in the midline, but the uterus may lie obliquely to either side. The uterine myometrial wall yields uniform low-level echoes. Sagittal images are

Fig. 6.24 Ultrasound of uterus and vagina,

(a) Longitudinal image.

1. Bladder 2. Uterine fundus 3. Body of uterus 4. Cervix 5. Endometrial stripe 6. Vaginal stripe 7. Air in rectum

(b) Transverse image.

1. Bladder 2. Myometrium - anterior 3. Myometrium - posterior 4. Endometrium 5. Right ovary 6. Left ovary 7. Gas in bowel 8. Skin surface

(c) Endovaginal sagittal image. The ultrasound probe is in the posterior fornix of the vagina, directed anteriorly so that the transducer is parallel to the long axis of the uterus and cervix.

1. Uterine fundus 2. Endometrium 3. Opposing endometrial surfaces 4. Cervix 5. Nabothian cyst

obtained by scanning in the same plane as the uterus, parallel to its long axis. Transverse images of the uterus are obtained by scanning at a right-angle to the sagittal plane. The fundus of a retroflexed or retroverted uterus may be difficult to visualize transabdominally because of its distance from the transducer. The normal ovaries are usually identified lateral or posterolateral to the anteflexed uterus. They commonly lie anterior to the internal iliac vessels, in a somewhat vertical orientation, w i th their long axis parallel to those vessels. When the uterus is tilted to one side, the ovary on that side is frequently located superior to the fundus. However, the ovaries may lie high in the pelvis or may be in the pouch of Douglas. In premenopausal women the normal ovary may contain small anechoic follicles. In the late phase of the menstrual cycle a small amount of f l u id may be seen in the pouch of Douglas.

The myometrium is relatively hypoechoic. The endometrium yields a thin high-level echo that is seen as a long white stripe on longitudinal images and a central echo on transverse images. This is thicker and more obvious perimenstrually. A central sharp echo is caused by the opposing endometrial surfaces. Immediately adjacent to the central echo is the functional layer of endometrium. This is relatively hypoechoic during the proliferative phase and becomes echogenic during the secretory phase, when the entire stripe is thicker and more homogeneous. A narrow hypoechoic layer of myometrium has been noted deep to the endometrium - the subendometrial halo. This represents a layer of compact, relatively avascular myometrium and is analagous to the junctional zone on MRI. In the postmenopausal woman the endometrium atrophies and measures 2-3 mm. In postmenopausal women who take hormone replacement therapy the endometrium may measure up to 10 mm.

The vagina is also seen as a white stripe of increased echogenicity on longitudinal images. The vaginal stripe

makes an acute angle w i th the body of the uterus. Owing to the rectangular shape of the vagina, it is seen as a transverse line on transverse images.

Endovaginal ultrasound (Fig. 6.24c) is performed w i th an ultrasound transducer in the vagina. The basic ultrasound features are the same, but the resolution is better. There is improved visualization of the adnexal area and the internal architecture of the ovary, as well as the uterus and uterine cavity. The cervix is also readily imaged and its canal and os may be demonstrated. Small dilations of cervical glands are frequently seen as small cysts related to the endocervical canal - these are termed nabothian cysts.

Sonohysterography involves transvaginal imaging of the uterus and adnexae, w i th saline being introduced into the uterine cavity via a small catheter through the cervix. This distends the cavity and allows evaluation both of the endometrial surface and for tubal patency. Magnetic resonance imaging (Fig. 6.25)

MRI is an excellent method to image the uterus because it has superior soft-tissue contrast and has the advantage of being able to image all planes. Sagittal and coronal images are obtained along the long axis of the uterus and axial images perpendicular to the coronal plane. The cervix generally lies in the same sagittal plane as the uterus, but often forms an angle to the body of the uterus, w i th the uterus lying in a relatively transverse plane and the cervix directed inferiorly towards the vagina. The cervix may be imaged separately in its own coronal plane, w i th its own transverse images perpendicular to this. It is especially important to image in the correct plane when evaluating for spread of disease beyond the serosa.

On T2-weighted images the endometrium, endocervical canal and vaginal canal are all of high signal intensity. The

Fig. 6.25 (a) MR of female pelvis: transverse T2-weighted image showing lower body of uterus and both ovaries. Note the high signal of endometrium and ovarian follicles.

1. Subcutaneous fat 2. Rectus muscle 3. Bladder 4. External iliac vein 5. External iliac artery 6. Round ligament 7. Transversus abdominis muscle 8. Small bowel loop 9. Uterus - myometrium 10. Uterus - endometrium 11. Ovary containing follicles 12. Rectum 13. Free fluid in cul-de-sac

(b) MR of female pelvis: sagittal section demonstrating uterus and cervix.

1. Serosa (of uterus) 2. Myometrium 3. Junctional zone of uterus 4. Endometrium 5. Anterior lip of cervix 6. Cervical canal 7. Posterior vaginal fornix 8. Vagina 9. Bladder 10. Small bowel loops 11. Sigmoid 12. Rectum 13. Pubic bone 14. Rectus muscle