Uterus
The uterus is a muscular organ whose function is to provide a nidus for the developing embryo. In the virginal state it is the shape of a flattened pear. Its size is about 8 × 5 × 3cm. It possesses a fundus, body and cervix. It receives the uterine tubes, and the cervix protrudes into the vault of the vagina where it opens.
The fundus is the part above the entrance of the tubes (Fig. 5.63). It is convex and possesses a serous coat of pelvic peritoneum which continues downwards over the front and back of the body (Fig. 5.64).
The body of the uterus tapers downwards from the fundus and is flattened anteroposteriorly. Each upper angle (cornu), at the junction of fundus and body, receives the uterine tubes. The body is enclosed by peritoneum which laterally becomes the broad ligament. The intestinal surface of the body faces upwards with coils of intestine lying upon it, while the vesical surface faces downwards resting on the bladder with the peritoneum of the vesicouterine pouch intervening (Fig. 5.64). The cavity of the uterus occupies the body. A narrow slit in the virgin, it enlarges during pregnancy by growth of the uterine walls to accommodate the fetus.
The cervix of the uterus tapers below the body and its lower end is clasped by the vault of the vagina, into which it protrudes (Fig. 5.64). It thus has vaginal (lower) and supravaginal (upper) parts. The deep sulcus which surrounds the protruding cervix is the fornix of the vagina, and is deepest posteriorly. The posterior surface of the cervix is covered by peritoneum that continues from the body on to the upper part of the fornix, forming the anterior wall of the rectouterine pouch (of Douglas). The anterior surface has no peritoneal covering, being deep to the vesicouterine pouch and attached to the bladder above the trigone by rather dense connective tissue. The ureter is about 2cm from the cervix as it passes first lateral to and then in front of the fornix (Fig. 5.63). The body of the uterus is rarely exactly in the midline; when deviated to one side the cervix becomes deflected to the opposite side, so one ureter may be closer to the cervix than the other.
The canal of the cervix is continuous with the cavity of the body at what is commonly called the internal os. The lower opening into the vagina is the external os; this is circular in the nulliparous but usually a transverse slit after childbirth, with anterior and posterior lips, the anterior lying at a lower level than the posterior. The external os is normally on a level with the ischial spines.
Uterine tubes
Each tube is 10cm long. The medial 1cm (intramural part) is embedded within the uterine wall. Emerging from the cornu, the tube then lies in the upper edge of the broad ligament (Fig. 5.63), the peritoneal fold embracing it being the mesosalpinx. The part adjacent to the uterus (the isthmus of the tube) is straight and narrow. Next to it is the wider ampulla, forming more than half the length of the tube. The lateral end of the tube has a trumpet-shaped expansion, the infundibulum or fimbriated end, with a number of finger-like processes, the fimbriae, one of which is longer and typically applied to the ovary. This open end lies behind the broad ligament adjacent to the lateral pelvic wall.
The tube, formed of two layers of smooth muscle (inner circular and outer longitudinal, like the gut), is lined by a mucous membrane thrown into folds. The surface epithelium is a mixture of ciliated and non-ciliated columnar cells. The cilia are most abundant at the fimbriated end, which is least muscular. The cilia beat towards the uterus.
Blood supply of uterus and uterine tubes
The uterus is supplied by the uterine artery, a branch of the internal iliac. It passes medially across the pelvic floor in the base of the broad ligament, above the ureter (Fig. 5.63), to reach the side of the supravaginal part of the cervix. Giving a branch to the cervix and vagina, the vessel turns upwards between the layers of the broad ligament to run in a tortuous manner alongside the uterus as far as the cornu, giving off branches which penetrate the uterine walls and anastomose across the midline with corresponding branches of the opposite uterine artery. At the junction of uterus and uterine tube the artery turns laterally and ends by anastomosing with the tubal branch of the ovarian artery, which supplies the uterine tube.
The veins of the uterus course below the artery at the lower edge of the broad ligament where they form a wide plexus across the pelvic floor. This communicates with the vesical and rectal plexuses and drains to the internal iliac veins. The tubal veins join the ovarian veins (see p. 305).
Lymph drainage
Lymph from the cervix drains to external and internal iliac nodes, and also to sacral nodes along the uterosacral ligaments. The lower part of the uterine body drains to external iliac nodes. Lymphatics from the upper part of the body, the fundus and the uterine tube accompany those from the ovaries to para-aortic nodes; a few pass to external iliac nodes, and a few from the region of the uterine cornua accompany the round ligaments to reach the superficial inguinal nodes.
Nerve supply
The nerves of the uterus are branches from the inferior hypogastric plexus (see p. 311). The smooth muscle of the uterus is sensitive to hormonal influences. The sympathetic supply is vasoconstrictor, and also has a facilitating function in relation to uterine muscle, but division of all uterine nerves or high transection of the spinal cord does not affect uterine contractility, even in labour. Pain from the cervix is usually considered to be carried by the pelvic splanchnic nerves, although from the upper cervix it appears to run with sympathetic nerves as does pain from the body of the uterus (including labour pains). The cord segments concerned are T10–L1, and pain can be referred to the corresponding dermatomes. However, presacral neurectomy (cutting the hypogastric nerves from the superior hypogastric plexus) does not abolish labour pain, although it may improve dysmenorrhoea. The abolition of uterine sensation requires the division of all nerves, or transection of the cord, above T10 level. As with most hollow viscera, distension causes pain, but both the cervix and body are relatively insensitive to cutting and burning; in contrast, the uterine tube is sensitive to touching and cutting.
Structure
The bulk of the uterus is smooth muscle, the myometrium, whose fibres are often described as being in three layers, but these are ill-defined. The outer muscle fibres tend to be longitudinal and expulsive in function, while many of those more deeply placed are circular and act as sphincters round the larger blood vessels, the openings of the uterine tubes and the internal os. The mucous membrane or endometrium has a lining of columnar epithelium which dips down into the endometrial stroma to form the endometrial glands. The thickness of course varies with the different stages of the menstrual cycle; at menstruation the bases of the glands remain to provide the source for the new epithelial covering. The mucosa of the cervix does not take part in the cyclical changes and is not shed at menstruation; the surface cells are mucus-secreting and there are also mucous glands. Just inside the external os the epithelium changes to the stratified squamous variety of the vagina. The outer or serous covering of the uterus is the peritoneum.
Supports
The normal position of the uterus is one of anteflexion and anteversion i.e. the fundus and upper part of the body are bent forward in relation to the long axis of the cervix (angle of anteflexion), while the organ thus flexed leans forward as a whole from the vagina (angle of anteversion); the external os thus opens through the anterior wall of the vagina. As many as 20% of nulliparous females may have a retroverted uterus, without any ill effects. The most fixed part of the uterus is the cervix, because of its attachment to the back of the bladder and to the vaginal fornix, and a number of structures help directly or indirectly to maintain the normal position. These include the pelvic diaphragm, condensations of visceral pelvic fascia forming ligaments, and to a lesser extent peritoneal attachments.
The pubovaginalis part of levator ani and the perineal body with its inserted muscles (see p. 317) support the vagina and so assist indirectly in holding the cervix up. If these muscles are unduly stretched or damaged during childbirth the posterior vaginal wall sinks downwards (prolapses), and this is often followed by prolapse or retroversion of the uterus.
The broad ligament (Fig. 5.63) is not strictly speaking a ligament in the usual sense, as it consists of no more than a lax double fold of peritoneum lying lateral to the uterus, and it plays little part in uterine support. Its medial edge is attached to the side wall of the uterus and flows over its intestinal and vesical surfaces as its serous coat. The lateral edge is attached to the side wall of the pelvis. The two layers of its inferior edge, or base, pass forwards and backwards to line the pelvic cavity; as the posterior layer does so, it has the ureter adhering underneath it. The line of lateral attachment crosses the obturator nerve, superior vesical or obliterated umbilical vessels, and the obturator artery and vein. The upper border of the broad ligament is free, forming the mesosalpinx and containing the uterine tube. The upper lateral part of the broad ligament contains the ovarian vessels and lymphatics and is extended over the external iliac vessels as a fold, the suspensory ligament of the ovary.
The anterior layer of the broad ligament is bulged forwards by the round ligament of the uterus just below the uterine tube. The posterior layer has a fold projecting backwards suspending the ovary, the mesovarium. Between the two layers of the broad ligament is a mass of areolar tissue, the parametrium, in which lie the uterine vessels and lymphatics, the round ligament of the uterus, the ligament of the ovary (see p. 305), and vestigial remnants of mesonephric tubules (the epoöphoron and paroöphoron, see p. 306).
The round ligament of the uterus extends from the junction of the uterus and tube to the deep inguinal ring. It lies in the broad ligament below the uterine tube and bulges the anterior layer of the ligament forwards. Through its uterine attachment it is continuous with the ligament of the ovary, the two ligaments together representing the gubernaculum, the counterpart of the gubernaculum of the testis. The round ligament passes through the inguinal canal and is attached at its distal extremity to the fibrofatty tissue of the labium majus of the vulva. It is supplied by a branch of the ovarian artery in the broad ligament and by a branch from the inferior epigastric artery in the inguinal canal. It consists of smooth muscle and fibrous tissue, and it acts to hold the uterus forwards in anteflexion and anteversion, especially when forces tend to push the uterus backwards (e.g. distension of the bladder, gravity during recumbency).
The transverse cervical ligament (also known as the lateral cervical, cardinal or Mackenrodt's ligament) consists of thickenings of connective tissue in the base of each broad ligament, extending from the cervix and vaginal fornix laterally to the side wall of the pelvis. The ureter, uterine artery and inferior hypogastric plexus traverse the connective tissue of the ligament. It imparts lateral stability to the cervix and is an important support of the uterus. The uterosacral ligaments, comprising fibrous tissue and smooth muscle, extend backwards from the cervix below the peritoneum, embracing the rectouterine pouch and rectum and becoming attached to the front of the sacrum. They are palpable on rectal (not vaginal) examination. They keep the cervix braced backwards against the forward pull of the round ligaments on the fundus and so maintain the body of the uterus in anteversion.
Development
The paramesonephric (Müllerian) ducts develop as a linear invagination of the coelomic epithelium on the lateral aspect of the mesonephros (see pp. 23 and 232). They grow caudally lateral to the mesonephric ducts, but then cross ventral to them, fuse at their caudal ends to make the uterus and continue to reach the dorsal wall of the urogenital sinus (see p. 29), thereby forming the upper part of the vagina. Their cranial ends persist as the uterine tubes. Incomplete fusion results in a median septum in the uterus or in a bicornuate uterus.
Surgical approach
An abdominal or vaginal approach may be used for total hysterectomy (removal of body and cervix). The broad, round and ovarian ligaments and the uterine tubes are divided on each side near the uterus. The lower ends of the ureters need to be safeguarded particularly when the uterine arteries are divided. The anterior and posterior vaginal walls are cut across below the cervix. For subtotal hysterectomy by the abdominal route, the cervix is cut across at the level of the lateral ligaments without opening into the vagina.
Ovary
The ovary is ovoid in shape, smaller than the testis. It is about 3cm long, 2cm wide and 1cm thick, being smaller before menarche and postmenopausally. In the erect position, the ovary lies almost vertically. Its upper pole, the tubal extremity, is tilted laterally and is overlapped by the fimbriated end of the uterine tube. Its lower pole is tilted towards the uterus to which it is attached by a fibromuscular band, the ligament of the ovary. This is continuous with the round ligament; both are attached to the cornu of the uterus, and are the remnants of the gubernaculum. The anterior border of the ovary is attached to the posterior leaf of the broad ligament by a double fold of peritoneum, the mesovarium. The peritoneum does not invest the rest of the surface of the ovary, which is covered with cuboidal epithelium and faces the peritoneal cavity.
The lateral surface of the ovary lies in the angle between the internal and external iliac vessels, against the parietal peritoneum which separates it from the obturator nerve laterally and the ureter posteriorly. A diseased ovary may therefore cause referred pain along the cutaneous distribution of this nerve on the inner side of the thigh. The medial surface is mainly related to the uterine tube.
The location and line of the ovary change during pregnancy and usually never return to their original state. The ovary in its normal position can just be reached through the vagina by the tip of the examining finger. It is overlaid by the coils of sigmoid colon and ileum that occupy the rectouterine pouch of Douglas.
Blood supply
The ovary is supplied by the ovarian artery, a branch of the abdominal aorta from just below the renal artery. The vessel runs down behind the peritoneum of the infracolic compartment and the colic vessels, crossing the ureter obliquely, on the psoas muscle. It crosses the brim of the pelvis and enters the suspensory ligament at the lateral extremity of the broad ligament. It gives off a branch to the uterine tube which runs medially between the layers of the broad ligament and anastomoses with the uterine artery, and it ends by entering the ovary (Fig. 5.63).
The ovarian veins form a plexus in the mesovarium and the suspensory ligament (the pampiniform plexus, as in the testis). The plexus drains into a pair of ovarian veins which accompany the ovarian artery. They usually combine as a single trunk before their termination. That on the right joins the inferior vena cava, that on the left the left renal vein.
Lymph drainage
The lymphatics of the ovary drain to para-aortic nodes alongside the origin of the ovarian artery (L2), just above the level of the umbilicus. Clinical observation shows that it is also possible for lymph to reach inguinal nodes via the round ligament and the inguinal canal, and to reach the opposite ovary by passing across the fundus of the uterus.
Nerve supply
Sympathetic (vasoconstrictor) fibres reach the ovary from the aortic plexus along its blood vessels; the preganglionic cell bodies are in T10 and 11 segments of the cord. Some parasympathetic fibres may reach the ovary from the inferior hypogastric plexus along the uterine artery and are presumably vasodilator. Autonomic fibres do not reach the ovarian follicles; an intact nerve supply is not required for ovulation. Sensory fibres accompany the sympathetic nerves, so that ovarian pain may be periumbilical, like appendicular pain.
Structure
The ovary consists of an inner vascular medulla and an outer cortex (containing the ovarian follicles) encapsulated by a fibrous connective tissue layer, the tunica albuginea, covered by a layer of cubical cells, the superficial epithelium. During early fetal development primitive germ cells (oogonia) derived from endodermal cells of the yolk sac migrate into the developing ovarian cortex, where they multiply and grow to become primary oocytes, which are surrounded by a single layer of follicular cells to form primordial follicles. There are about 1 million primordial follicles at birth but this number is reduced to about 40 000 by puberty. After puberty, during each ovarian cycle a relatively small number of primordial follicles undergo a series of development changes and of these usually only one from either ovary comes to full maturity and releases its oocyte (ovulation) into the peritoneal cavity for transport into the uterine tube, potentially for fertilization. This development involves oocyte enlargement, follicle (granulosa) cell proliferation and fluid (liquor folliculi) accumulation as the primordial follicle is transformed successively into a primary, a secondary and a tertiary (Graafian) follicle, the surrounding stromal cells forming the theca of these follicles. Before ovulation the primary oocyte undergoes meiosis (cell division whereby the DNA amount and the chromosome number are halved), forming a secondary oocyte. At ovulation this is discharged, the liquor folliculi escapes and haemorrhage occurs into the collapsed follicle. The granulosa cells and some of the thecal cells now develop into a corpus luteum. This persists for 1 week if pregnancy does not occur, or for 9 months if it does. At the end of either time it atrophies and becomes replaced by a fibrous scar, the corpus albicans.
Since only about 400 ova can be shed in the course of reproductive life, most oocytes and follicles are destined never to reach maturity, and they can undergo degeneration at any stage of their development, becoming known as atretic follicles.
Development
The ovary develops from the paramesonephric gonadal ridge of the intermediate cell mass (see p. 23) in the same way as the testis. Its site of origin lies in the peritoneum of the posterior abdominal wall. It descends, preceded by the gubernaculum. The gubernaculum proceeds through the inguinal canal, as in the male, and becomes attached to the labium majus. The ovary does not follow its gubernaculum so far, and its descent is arrested in the pelvis as the gubernaculum becomes attached to the uterus and persists as the ligament of the ovary and the round ligament of the uterus.
The mesonephric tubules and mesonephric duct normally disappear in the female. Should they persist their remnants are to be found between the layers of the broad ligament. The epoöphoron consists of a number of tubules joining at right angles a persistent part of the mesonephric duct. It lies in the mesosalpinx between ovary and tube. The mesonephric duct may persist as a tube (duct of Gartner) opening into the lateral fornix of the vagina or even at the vestibule of the vulva alongside the vaginal orifice. The paroöphoron lies nearer the base of the broad ligament. It consists of a number of minute tubules, blind at each end. Distension of such a tubule produces a parovarian cyst.
Vagina
The vagina is a highly expandable fibromuscular tube, about 10cm in length, that is directed upwards and backwards from its lower end, the vaginal orifice, or introitus. For much of its total length the anterior and posterior walls are in opposition and the lumen is an H-shaped slit, but the introitus is an anteroposterior cleft. It lies in front of the rectum, anal canal and perineal body, and behind the bladder and urethra (Fig. 5.64). Below the floor of the rectouterine pouch the vagina is separated from the rectum by the thin rectovaginal septum.
The upper end is slightly expanded and receives the uterine cervix which projects into it, forming round the margin of the cervix a circular groove or vaginal fornix, which for descriptive convenience is subdivided into anterior, posterior and lateral fornices. The posterior wall of the vagina is longer than the anterior wall and the posterior fornix is deeper than the other fornices. The posterior fornix is covered by peritoneum of the front of the rectouterine pouch (of Douglas); this is the only part of the vagina to have a peritoneal covering. The ureter is first adjacent to the lateral fornix and then passes across the front of the anterior fornix to enter the bladder.
Below the cervix the anterior wall of the vagina is in contact with the base (posterior surface) of the bladder, and below the bladder the urethra is embedded in the vaginal wall.
The vagina passes down between the pubovaginalis parts of levator ani, through the urogenital diaphragm and perineal membrane (i.e. through the deep perineal space) into the superficial perineal space where the vaginal orifice lies in the vestibule, the space between the labia minora (see p. 322). Here it may show internally the remains of the hymen, and the duct of the greater vestibular (Bartholin's) gland opens on each side just below the hymen in the posterolateral wall. The urethra opens immediately in front of the vaginal orifice, and the minute openings of the lesser vestibular glands are between the two orifices.
Blood supply
The vaginal branch of the internal iliac artery is supplemented by the uterine, inferior vesical and middle rectal vessels, whose branches all make good anastomotic connexions on the vaginal wall. Veins join the plexuses on the pelvic floor to drain into the internal iliac vein.
Lymph drainage
The lymphatics of the vagina, like those of the cervix, drain to external and internal iliac nodes, but the lowest part (below the hymen level) drains like other perineal structures to superficial inguinal nodes.
Nerve supply
The lower end of the vagina receives sensory fibres from the perineal and posterior labial branches of the pudendal nerve, and (with the anterior part of the vulva) from the ilioinguinal nerve. Autonomic nerve fibres from the inferior hypogastric plexuses supply blood vessels, the smooth muscle of the vaginal wall and the vestibular glands. The upper vagina is said to be sensitive only to stretch, the afferent fibres running with sympathetic nerves.
Structure
The vagina has a muscular layer of smooth muscle lined internally by mucous membrane and covered externally by fibrous tissue continuous with the pelvic fascia, except at the posterior fornix which has a peritoneal covering. The smooth muscle fibres consist of outer longitudinal and an inner circular layer which interlace. The mucous membrane has stratified squamous non-keratinizing epithelium overlying a connective tissue lamina propria in which there are large thin-walled veins as in erectile tissue. There are no muscularis mucosae and no glands; the mucous membrane is kept moist by mucus secreted by the uterine cervix. Before parturition, the mucous membrane of the anterior and posterior walls have median longitudinal ridges from which several transverse rugae extend bilaterally.
Vaginal examination
Using the index and middle fingers, the uterine cervix can be felt in the upper vagina, and the bladder, urethra and pubic symphysis via its anterior wall. Posteriorly, the contents of the rectouterine pouch are palpable. With pressure applied on the lower abdominal wall, the body of the uterus, ovaries and uterine tubes can be felt.
Development
Most of the vagina is formed (like the uterus, see p. 304) from the distal part of the fused paramesonephric (Müllerian) ducts, but the lower part is derived from the urogenital sinus (see p. 29), whose epithelium appears to replace that derived from the ducts. The labia minora that bound the vaginal orifice are formed from the urogenital folds (the labia majora are from the more laterally placed labioscrotal swellings).
Female urethra
The female urethra is about 4cm long, passing from the neck of the bladder at the lower angle of the trigone to the external urethral meatus (Fig. 5.64), which is in front of the vaginal orifice and 2.5cm behind the clitoris. Except its uppermost end, the urethra is embedded within the anterior vaginal wall. As it leaves the bladder, fibres of the pubovaginalis part of the levator ani lie adjacent to it, and they play some part in compressing it.
With the urethra being such a short straight tube, catheterization in the female is simple compared with the male, but it must be remembered that in the later stages of pregnancy the urethra may be considerably stretched so that the catheter may have to be passed for more than twice the normal distance. Vaginal stretching during birth can increase the urethral length to 10cm. The pubic symphysis lies in front and the full-term fetal head can compress the urethra against it.
Blood supply
The upper part of the urethra is supplied by the inferior vesical and vaginal arteries, with the lower end receiving contributions from the internal pudendal artery. Veins drain to the vesical plexus and the internal pudendal vein.
Lymph drainage
Lymph vessels pass mainly to internal iliac nodes but some reach the external iliac group.
Nerve supply
Fibres reach the urethra from the inferior hypogastric plexuses and from the perineal branch of the pudendal nerve.
Structure
The mucous membrane is lined proximally by urothelium and distally by non-keratinized stratified squamous epithelium. There are a few mucous glands in the wall. The largest of these, the paraurethral glands (of Skene) open by a single duct on each side just inside the external meatus, and are the female homologue of the prostate. Superficial trigonal muscle fibres of the bladder extend into the upper urethra. The urethral smooth muscle is orientated mainly longitudinally; its contraction during micturition shortens the urethra and widens its lumen. Outside the smooth muscle is the striated circular muscle of the sphincter urethrae (external urethral sphincter). The sphincter is thickest near the middle of the urethra, and thicker in front than at the sides or back. It consists of small fibres of the slow twitch variety and is supplied by the pudendal nerve.
Development
The female urethra is developed from the urogenital sinus (see p. 29), and corresponds to the part of the male prostatic urethra that is proximal to the openings of the prostatic utricle and ejaculatory ducts (see p. 299).