Yaniv Larish1 and Elizabeth Kavaler1, 2
Department of Urology, Lenox Hill Hospital, New York, NY, USA
Weil Cornell Medical College, New York, NY, USA
Elizabeth KavalerAssistant Professor of Urology (Corresponding author)
The following chapter will give the reader a fundamental understanding of female pelvic and anterior abdominal wall anatomy, including the relationships between the genital, urinary, colorectal, and musculoskeletal systems. In addition, we will describe the vascular and nervous supply to each organ and explore the interrelationships and function between the pelvic organs and their support mechanisms.
Abdominal Wall Musculature
Embrology (Fig. 1.1)
By the third week of gestation, the epiblastic layer of the bilaminar embryonic disc transforms into three layers. The middle layer is composed of mesoblastic cells that give rise to the embryonic mesoderm. The embryonic mesoderm divides into the paraxial mesoderm, the intermediate mesoderm, and the lateral plate mesoderm. The lateral plate mesoderm forms into the somatic mesoderm and the splanchnic mesoderm. The somatic mesoderm becomes the lateral and ventral body wall.
Embryology of abdominal wall musculature
The first layer encountered just deep to the skin and subcutaneous tissues of the anterior abdominal wall is a fascial plane consisting of two layers: a fatty superficial layer which is called Camper’s Fascia and a deeper membranous layer, Scarpa’s Fascia. Scarpa’s Fascia runs inferiorly to attach to the thigh laterally and fuses medially with the perineal membrane as the inferior border of the superficial perineal pouch through which the urethra passes.
Deep to the superficial fascia lies the anterolateral muscles, which run in three separate directions, creating a strong latticework of muscle fibers. The most superficial of these muscle groups is the external oblique muscle, which runs caudal and medially from the lower eight ribs to the pelvis. The inferior border of the external oblique forms the inguinal ligament. A defect in the inguinal ligament, the superficial inguinal ring, affords passage of the round ligament from the abdomen to the pelvis. Deep to the external oblique, the internal oblique muscle fibers run cranially and laterally from the lower six costal cartilages to the pelvis, where they fuse with the muscle layer deep to it, the transversus abdominus, forming the conjoint tendon.
Medially, the rectus abdominus muscle originates at the pubic bone and runs superiorly to insert in the fifth, sixth, and seventh costal cartilages and the xiphoid process. It is encircled by the rectus sheath, which varies in composition depending on its location. Superiorly, the internal oblique splits to cover the rectus muscle both anteriorly and posteriorly. These layers are reinforced by the external oblique anteriorly and the internal oblique posteriorly. Caudally, all three anterolateral muscle groups pass anteriorly to the rectus. Above the coastal margin, the external oblique covers the rectus along its costal cartilage insertion. Together, these muscle fibers provide support for the abdominal organs. The ability to contract these muscles results in a rise in intraabdominal pressure, which aids processes of micturition and defecation.
Running along the posterior aspect of the rectus sheath, the superior epigastric artery comes off of the internal mammary artery and anastomoses with the last five intercostal arteries above, and the inferior epigastric artery below. It supplies the upper abdominal wall. The inferior epigastric artery, a branch of the external iliac artery, comes through the abdominal inguinal ring, where it courses medially toward the superficial inguinal ring between the transversalis fascia and the peritoneum. This vessel passes through the transversalis fascia into the rectus muscle and meets the superior epigastric artery. The inferior epigastric artery supplies the lower abdominal wall.
Branches of the external iliac artery, mostly by way of the deep circumflex iliac artery, supply the lower abdominal wall. Traveling along the anterior superior iliac spine between the transversus and the internal oblique muscles, it anastomoses with the iliolumbar artery. The deep circumflex iliac artery also anastomoses with the inferior epigastric artery.
The lower six thoracic nerves run between the internal oblique and the transversus muscles, penetrare the rectus sheath and muscle, and end as cutaneous branches. The 12th thoracic and first lumbar nerves join to form the iliohypogastric nerve, supplying the psoas and the quadratus lumborum before veering anteriorly to supply the transversus and the internal oblique muscles. The ilioinguinal nerve also comes off of the last thoracic and first lumbar nerve and communicates with the iliohypogastric nerve. The iliohypogastric nerve divides into the iliac branch, which supplies the lower internal and external oblique muscles above the iliac crest, and the hypogastric branch, which supplies the inferior portions of the transversus and the internal oblique muscles, the external oblique aponeurosis, the skin, and the symphysis pubis. This nerve then meets the ilioinguinal nerve near the anterior superior iliac spine.
The embryologic development of the bony and muscular pelvis is similar to that of the abdominal wall musculature. During the third week of gestation, the bilaminar embryonic disc transforms into the trilaminar embryonic disc, composed of ectoderm, mesoderm, and endoderm. A thickening of cells forms at the caudal region (primitive streak) and migrates cephalad, creating a depression on the ventral aspect of the trilaminar disc. This area of cells is called the embryonic mesoderm. Three clusters of mesodermal cells form by the end of the third week of gestation. They are the paraxial mesoderm, the lateral plate mesoderm, and the intermediate mesoderm. The paraxial mesoderm gives rise to the axial skeleton. The lateral plate mesoderm becomes the lateral and ventral body wall as well as the gut. The intermediate mesoderm will evolve into the urogenital ridge, and ultimately the reproductive and urinary tracts.
The pelvis can be separated into two compartments separated by the iliopectineal line: the greater (false) pelvis and the lesser (true) pelvis (Fig. 1.2). The iliac fossa and the sacrum form the boundaries of the greater pelvis, which houses the sigmoid colon and segments of the small intestine. The lesser pelvis lies below the pelvic brim and contains the pelvic organs, including the bladder, reproductive organs, the rectum, and the anus. It is bordered by the sacrum and coccyx posteriorly, the pubic symphysis anteriorly, and the obturator internus laterally. The pelvic brim demarcates the roof of the lesser pelvis while the pelvic musculature constitutes its floor.
The bony pelvis, divided into two compartments divided by the iliopectineal line: the greater (false) pelvis and the lesser (true) pelvis
The bony pelvis is composed of three bones: the ilium, the ischium, and the pubis (Fig. 1.3). The amalgam of these three bones is called the innominate bone. The ilium sits posteriorly, and articulates with the acetabulum, the ischium, and the pubis. The superior border of the ilium is called the crest, extending inferiorly, ending at the iliac spines. The components of the ischium include the body, the superior and inferior rami, and the tuberosity. The posterior border joins with the ilium to form the greater sciatic notch. The inferior ramus meets the pubis to form the ischiopubic arch. The pubis is made up of a body and two rami. The obturator sulcus is located inferior to the superior ramus. The symphysis is a synarthrodial joint within the pubic bone.
The bony pelvis is composed of three bones: the ilium, the ischium, and the pubis
The muscular attachments (Fig. 1.4a, b) to the ilum, ischium, and pubis create the bowl of the pelvic floor. The external oblique, the internal oblique, the transversus abdominus, latissimus dorsi, quadratus lumborum, tensor fascia latae, and Sartorius muscles all attach to the ilium. The rectus femoris muscle connects to the iliac spine. The iliacus muscle sits in the iliac fossa, and the tendon of the psoas inserts between the iliac spine and the iliopectineal eminence. Both the gluteus maximus and minimus muscles attach to the ilium.
The muscular attachments to the bony pelvis
The ischium gives rise to the obturator internus muscle. The coccygeus and levator ani muscles attach to the ischial spines deep in the pelvis. The transversus perinei muscle originates from the lower border of the ischium. Laterally, the adductor magnus muscle connects to the ischial tuberosity. The obturator externus muscles and adductor longus muscles come off of the superior ramus of the pubis, while the obturator internus muscles and levator attach on the anterior surface. The rectus and pyrimidalis muscles attach on the superior border. The anterior surface of the inferior ramus attaches to the abductor brevis, magnus, and the obturator internus. See Fig. 1.5a, b.
The muscularture of the bony pelvis, with an abdominal view
The sacrum is formed by the fusion of 5–6 sacral vertebrae. The body of the sacrum articulates with the fifth lumbar vertebrae through a fibrocartilage disk, where it juts out anteriorly and is labeled the sacral promontory. The lamina and spines of the third, fourth, and fifth sacral vertebrae are absent, leaving a wide space called the sacral hiatus. The coccyx is formed by 3–5 caudal vertebrae that fuse and join the sacrum at the sacrococcygeal joint. Muscular attachments to the sacrum include the piriform muscle, sacrospinalis muscles, and the gluteus maximus muscle. Muscles that connect to the coccyx include the coccygeus and the iliococcygeus musles.
The sacrospinalis ligament connects the lateral border of the sacrum and the coccyx to the ischial spine. The sacrotuberous ligament extends from the sacral vertebrae to the tuberosities of the ilium. This is a deep posterior ligament with fibers that connect to the hamstrings. The iliolumbar ligament joins the iliac crest with L4-5. The iliofemoral ligament attaches the crest of the ilium to the acetabulum. See Fig. 1.6.
The ligaments of the bony pelvis
The blood supply to the innominate bones comes from branches off of the iliolumbar, deep circumflex iliac, and obturator arteries, which all come off of the hypogastric (internal iliac) artery. The lower part of the bone receives nutrients from branches of the gluteal arteries, which also comes off of the hypogastric artery.
The sacrum receives its blood supply from the middle sacral artery, a branch of the aorta, and the lateral sacral arteries, which comes off of the hypogastric artery (internal iliac). These vessels anastomose with the lumbar arteries and the superior gluteal artery.
The innominate bones derive their nerve supply from the superior hypogastric plexus, which sits over the sacral promontory. It receives nerve fibers from inferior mesenteric and lumbar sympathetic ganglia. The superior hypogastric plexus gives rise to the middle hypogastric plexus and then divides into two lateral hypogastric nerves. They travel in the uterosacral ligament into the rectum and the vagina. At this juncture, they are labeled the pelvic plexuses. The fibers that make up the pelvic plexuses come from the S2-4 parasympathetic and sympathetic trunks. As the nerves descend into the pelvis, they innervate the bones and ligaments through which they pass.
Embryology (Fig. 1.7a, b)
The pelvic diaphragm arises out of all three layers of the embryonic tissue: the endoderm, the mesoderm, and the ectoderm. The endoderm develops into the primitive gut, which gives rise to the cloaca. At week 6 of gestation, the cloaca divides into the urogenital sinus and the anorectal canal. The mesoderm gives rise to the urogenital ridge, which becomes the urinary tract, the uterus and upper vagina, and the lateral plate mesoderm. The lateral plate mesoderm divides into the somatic and splanchnic mesoderm. The somatic mesoderm gives rise to the lateral and ventral walls, and the splanchnic mesoderm evolves into the visceral peritoneum. The ectoderm gives rise to the skin of the external genitalia and the nervous system.
(a) Embryology of the pelvis muscles. (b) Embryology of the pelvic floor musculature
Pelvic Floor Musculature
In addition to providing support to the pelvic and abdominal organs and ensuring that they stay in their proper anatomical locations, the pelvic floor apparatus aids in the evacuation of both urine and feces. Additionally, the female pelvic floor has a central role in the acts of conception and parturition.
The pelvic floor is comprised of two main components; a visceral fascial layer, the endo-pelvic fascia (comprised of connective tissue), and a muscular layer, collectively referred to as the levator ani and coccygeus muscles. The endo-pelvic fascia provides a passive support mechanism when the genital hiatus is open, and provides an apical tethering point for the pelvic viscera. Conversely, the musculature provides an active support mechanism by contracting and closing the genital hiatus.
The pelvic diaphragm refers to the bowl of muscle that provides a supportive plate on which the bladder, uterus, and rectum sit. Perforated by the urethra, the vagina, and the rectum, these muscles serve as the support structures of the pelvic floor. The bony supports include the pubic bone anteriorly, the sacrum posteriorly, and the ischium bilaterally. The three main muscle groups that comprise the pelvic support include the levator ani, anteriorly and inferiorly, the obturator muscles laterally, and the coccygeus muscle, posteriorly.
Divided into three parts, the levator ani muscle is comprised of the Iliococcygeus muscle, the pubococcygeus muscle, and the puborectalis muscle. The iliococcygeus muscle arises laterally from the arcus tendineus it extends from the symphysis to the ischial spine, where it meets the obturator internus muscle. Medially it meets the pubococcygeus, and posteriorly, it inserts into the coccyx. The pubococcygeus is a strap of muscle that runs anterior to posterior from the pubis to the coccyx where it meets the sacrococcygeal ligament, forming a strong raphe over the sacrum. The puborectalis muscle creates a band of tissue around the rectum, which supports the posterior pelvis. The levator ani muscles function to both contract around the urethra, vagina, and rectum, and to counteract intraabdominal and intrathoracic pressures on the pelvic organs. See Fig. 1.8.
Pelvic muscles groups and their attachments
Arising from the pubic rami and the ischium, the obturator internus muscle connects the pelvis to the hip, where its fibers insert into the trochanter. The obturator vessels and nerves traverse the notch on the lateral aspect of the obturator foramen. Intrinsic to the functioning of the inner thigh, the obturator muscle is involved in lateral rotation, abduction, and extension of the upper leg.
Not comprising many muscle fibers, the coccygeus appears more as an aponeurosis than as a muscle. It arises from the ischial spine and extends into the fourth and fifth sacral vertebrae, as well as the coccyx. Besides supporting the pelvic organs, it helps with flexing and abducting of the coccyx.
Perineum and Structures from Below
From the perineal perspective, the fascial supports overlying the organs play a crucial role in both the pathophysiology of pelvic organ prolapse and the methods of repairing those defects. An extension of the transversus abdominus muscle, the endopelvic fascia lays over the abdominal portion of the pelvic musculature. The levator ani muscles are covered from the perineal side by the same facial material but it is divided into three sections depending on what structures are being covered.
From the vaginal view, looking at the anterior vaginal wall, the urethropelvic (also called the pubourethral) ligament creates a bridge extending from the inferior aspect of the pubic ramis from side to side, thus supporting the urethra. If there were any weakness or damage to this fascial layer, stress incontinence would be the end result. The pubocervical fascia (sometimes called the vesicocervical fascia) is the component of the sheet of support under the bladder. It is not discrete and cannot be visually separated from the urethropelvic ligament, except by determining the organ under which it sits. This fascial layer can be torn, causing a central defect, which results in a cystocele. Correcting central defects in the pubocervical fascia requires replacing that fascia support from below. See Fig. 1.9.
Fascial layers of the pelvic organs: vaginal view
The urethropelvic fascia and the pubocervical fascia insert into the pelvic sidewall where the levator ani muscles meet the obturator internus. From the view looking into the pelvis from above, the endopelvic fascia can be seen inserting into the same place. This condensation of both fascial layers into the same insertion point is called the arcus tendineus. An important support for the pelvic organs, the arcus tendineus can be damaged in surgery, pregnancy, or childbirth, resulting in the pelvic organs sliding down the sidewall and causing a prolapse. See Fig. 1.10.
Ligaments of vaginal support
The posterior section of the support under the anterior vaginal wall is the cardinal ligament, which supports the uterus and the apex of the vagina. The cardinal ligament extends from the sciatic foramen, encircles the cervix, and fuses with the sacrouterine ligament posteriorly. The sacrouterine ligament is a non-discrete condensation of fibers that originates from the second through fourth sacral vertebrae and merges with the pubocervical fascia at the vaginal apex. If the fibers of the sacrouterine ligament are intact, they can be used to support a prolapse repair. Deep to the sacrouterine ligament is the sacrospinous ligament, which runs from the sacrum to the ischial spine. It is a strong, fibrous true ligament, in that it joins two bony structures.
The posterior vaginal wall support structures include the rectovaginal septum, the pararectal fascia, and the perineal body. At the apex of the vaginal, the rectovaginal septum fuses with the cardinal ligament/sacrouterine ligament complex. Disruption of the cardinal ligaments through surgery, pregnancy, or childbirth can lead to weakness of the septum and posterior apical prolapse. The perineal body is a central point where the deep and superficial transverse perineal muscles, the anal sphincter, the bulbocavernosus muscle, and the levator ani muscles all converge. Flexibility of the perineal body allows for absorption of pressure during valsalva, and contraction increases both vaginal compression and stability. See Fig. 1.11.
Posterior vaginal support structures
Blood Supply (Fig. 1.12)
The common iliac artery branches into the internal (hypogastric) and external iliac arteries at the lumbosacral articulation. The internal iliac artery divides into three parts once it crosses through the psoas and piriformis muscles: the anterior branch, the posterior branch, and the visceral branch. Arising first, the posterior branch gives rise to the iliolumbar, the lateral sacral, and the superior gluteal arteries. The anterior branch of the internal iliac artery (hypogastric artery) divides into three branches: the obturator, inferior gluteal, and the internal pudendal arteries. The visceral branch becomes the superior vesical, middle hemorrhoidal, the uterine, and the vaginal arteries.
Branches of the internal iliac artery (hypogastric artery)
Sometimes coming off of the external iliac artery, the obturator artery, which usually arises from the anterior branch of the internal iliac artery, terminates in iliac, vesical, pubic, anterior, and posterior branches. These vessels supply the muscles and bones of the deep pelvis. These branches communicate with the inferior epigastric artery and other divisions of the external iliac artery. The umbilical artery comes off of the inferior gluteal artery from the anterior branch of the internal iliac artery. It gives off branches to the bladder, including the middle vesical artery.
The visceral portion of the internal iliac artery includes the middle hemorrhoidal artery which anastomosis with the superior mesenteric artery through the superior hemorrhoidal artery. The uterine artery branches off early in the pelvis close to the common iliac artery. The vaginal artery has variable origins. It can arise from the hypogastric artery directly, the uterine artery, or the superior vesical artery.
Nerve Supply (Fig. 1.13)
Innervation of the pelvis arises from the second, third, and fourth sacral nerves that enter the pelvis through the greater sciatic foramen, between the piriformis and coccygeus muscles beneath the ischial spines. It enters the perineum through Alcock’s canal, where it branches into three parts, the inferior hemorrhoidal nerve, the perineal nerve, and the dorsal nerve of the clitoris. The inferior hemorrhoidal nerve supplies the anal sphincter and peri-anal skin. The perineal nerve supplies the levator ani muscles before it enters through the urogenital diaphragm to innervate the ischiocavernosus and bulbocavernosus muscles, and the urinary sphincter. A superficial branch of the perineal nerve supplies the labia major. The dorsal nerve of the clitoris enters through the urogenital diaphragm before heading toward the glans of the clitoris.
Nerve supply to the pelvis
Embryology of the Urinary Tract (Fig. 1.14)
In utero, the kidneys mature through a successive and predictable series of phases prior to reaching maturity. The first two stages, the pronephros and mesonephros, regress. The final stage, the metanephros, persists. The pronephros is nonfunctional, appears late in the third week, and completely degenerates by the start of the fifth week. The mesonephros serves as primitive excretory organ while the metanephros develops. The mesonephros regresses by the fourth month. Meanwhile, the metanephros forms near the sacrum as the ureteric buds and metanphros mesenchyme.
Embryology of the urinary tract
The ureteric buds and mesenchyme exert reciprocal biochemical effects on each other. The metanephric mesenchyme develops into the glomerulus, proximal tubule, loop of Henle, and distal tubule, while the ureteric bud forms the collecting ducts, calyces, pelvis, and ureter. Despite renal maturation continuing postnatally, the functioning elements of the kidney has fully developed by 32–34 weeks of gestation.
The terminal portion of the hindgut, the cloaca, is divided into the dorsal primitive rectum and ventral primitive urogenital sinus by the urorectal septum. The urinary bladder and urethra are derived from the primitive urogenital sinus and the surrounding splanchnic mesenchyme. The urogenital sinus gives rise to the urinary bladder, which is continuous cranially with the allantois, and caudally with the urethra. The allantois is obliterated during fetal development and forms a fibrous cord, the urachus, which following birth becomes the median umbilical ligament. The trigone of the bladder is formed in a complex process of Wollfian duct involution, while the bladder wall is of endodermal origin. The smooth musculature of the bladder develops during the 12th week of gestation and is ultimately coated on the inside by epithelial cells of endodermal origin.
The kidneys are paired retroperitoneal organs approximately 10 cm long and weighing 135–150 g. They lie opposite T12-L3 and the right kidney lies slightly lower than the left. The upper pole of the kidney is nearer to the midline than the lower pole, and the medial border of the kidney faces slightly anterior. The right kidney is bordered superiorly by the adrenal gland, medially by the second portion of the duodenum, laterally by the posterior abdominal wall, and inferiorly by the mesocolon and small intestine. The left kidney is bordered by the spleen and stomach and adrenal superiorly, the descending colon laterally, the aorta medially, and the mesocolon and small intestine inferiorly. The medial portion of the kidney is concave and houses the renal hilum. The renal hilum transmits the renal vein, renal artery, and ureter into the kidney (anteriorly to posteriorly). The kidneys and adrenals are surrounded (except inferiorly) by a perinephric fat capsule, called Gerota’s fascia. The capsule invades the kidney medially to surround the structures of the renal hilum, and runs inferiorly over the course of the ureter.
Blood Supply (Fig. 1.15)
The renal arteries branch from the aorta below the superior mesenteric artery. The right renal artery is longer than the left, and usually courses beneath the inferior vena cava. At the renal hilum, the renal artery branches into several segmental branches, acting as the end-arterial supply to the various segments of the kidney. Small segmental veins coalesce to form the renal vein. The vein of the left is longer and, in addition to draining the kidney, drains the left adrenal and gonadal vein. The renal vein on the right is short, and drains only the kidney, while the adrenal and gonadal veins drain directly into the inferior vena cava.
Blood supply to the kidneys
The innervation of the kidneys includes sympathetic, parasympathetic, and visceral afferent nerve fibers. The sympathetic (originating in the sympathetic chain) and parasympathetics (originating from the vagus) nerves join to form an autonomic plexus which runs with the renal artery. The visceral afferents, conveying sensation of pain, travel along the path of the autonomic plexus to the spinal ganglia and spinal cord.
The ureters start at the level of the ureteral pelvic junction where the renal pelvis tapers into a narrow tube within the renal hilum posterior to the renal artery and vein. The lumen of the ureter is lined by transitional epithelium. A layer of inner longitudinal and outer circular smooth muscle in turn covers the epithelial layer, and is responsible for the peristaltic movements necessary to propel urine toward the bladder. The ureters course inferiorly toward the bladder along the psoas muscle. The gonadal vessels cross the ureters anteriorly. As they enter the pelvis, the ureters cross anterior to the bifurcation of the common iliac artery. Upon entering the bladder, they take an oblique course through the bladder wall to form a tunnel, which aids in preventing reflux of urine back to the kidney.
The blood supply to the ureters varies along their course. The abdominal ureters receive their arterial supply from the renal, gonadal, and common iliac arteries and from the abdominal aorta, all of which run in a medial to lateral direction. After entering the pelvis, the ureters receive their blood supply from branches of the internal iliac artery, including the vesical, uterine, middle rectal, and vaginal arteries, all of which run in a lateral to medial direction. The venous and lymphatic drainage of the ureter parallels the arterial supply.
Ureteral peristalsis originates in smooth muscle pacemaker nodes within the collecting system. The autonomic nervous system modulates this input; however, the mechanism is unclear. Pain fibers are stimulated by distention and/or mucosal irritation within the ureter. The pain fibers run with the sympathetic nerves corresponding to the segment of ureter affected. Therefore, the distribution of pain varies, and can be referred to the flank, groin, or labia.
The bladder is a compliant, distensible, hollow viscos, which allows for the storage of urine. When empty, it remains confined to the lesser pelvis, but can distend beyond the pelvic brim into the greater pelvis when full. The most superior segment of the bladder is covered by a reflection of peritoneum, while the rest of the bladder lies inferior to the peritoneum. At its most superior point, the uterus drapes the bladder. Anterior to the bladder is a fat filled potential space, the space of Retzius, which protects the bladder from the pubic symphysis. The base of the bladder rests on the anterior aspect of the cervix and anterior vaginal wall. See Fig. 1.16.
Relationship of bladder to neighboring organs
The wall of the bladder is composed of three muscle layers that run in different directions. The outer layer of muscles runs longitudinally, the middle layer’s fibers run circularly, and the inner layer’s fibers run longitudinally gain. This configuration aids in effective contraction of the bladder wall and full expulsion of urine. Deep to the muscular layers of the bladder wall lies the mucosal layer, which is composed of transitional epithelium which prevents the reabsorption of urine from the bladder into the body. At the posterior–inferior aspect of the bladder is a raised crescent-shaped ribbon of muscle, the trigone, which is bordered on either side by the ureteral orifices.
The arterial supply to the bladder is derived from the superior and inferior vesicle arteries as well as branches arising from the internal iliac artery. Because the branches are variable, the blood supply is thought of as lateral and posterior pedicles, which run along the cardinal and uterosacral ligaments. The venous drainage of the bladder forms a plexus, which drains into the external iliac vein.
The bladder derives most of its nerve supply from sacral nerves 2–4, with some contribution from the hypogastric plexus. Most of the innervation to the bladder base comes from the parasympathetic nervous system, while the bladder neck and trigone are under sympathetic control from the hypogastric nerve (T10 to L2). Under somatic control from S2, the pudendal nerve innervates the external urinary sphincter.
The Urethra (Fig. 1.17)
The female urethra is 2.5 to 5 cm in length and runs under the pubic bone along the anterior vaginal wall. It is supported by the pubourethral ligaments (also called the urethropelvic ligament), which are contiguous with the pubocervical ligaments (vesicopelvic) that support the bladder. The three walls of the urethra include the mucosa, the submucosa, and the fibromuscular layer, which contains spongy tissue and the cavernous veins. Skene’s glands enter into the urethra cephalad to the external urinary sphincter. The distal third of the urethra is lined by squamous epithelium, while transitional cells line the proximal portion. The fibromuscular layer is divided into a circular layer on the outside and a longitudinal layer on the inside. Near the bladder neck, the muscle fibers are interwoven with those of the bladder neck, creating a contiguous structure. In the middle and lower thirds of the urethra, striated fibers of the bulbocavernosus and ischiocavernosus muscles encircle the urethra, creating the external urinary sphincter.
Anatomy of the urethra
The urethra shares its blood supply with the anterior vaginal wall and the bladder. The uterine artery gives off the vaginal artery and the artery to the cervix. Branches from both arteries supply the urethra. The artery to the clitoris, from the internal pudendal artery, also gives off branches to the urethra as well.
The parasympathetic and sympathetic nerves to the urethra travel through the hypogastric plexus. The pudendal nerve controls the external urinary sphincter.
The Female Genital Tract
Embryology (Fig. 1.18)
Sexual differential of the external genitalia occurs at the eighth week of gestation; however, it isn’t until three months that sexual differentiation can be recognized. In the first 4 weeks, a small outgrowth at the external surface of the cloacal membrane appears in front of the genital tubercle. On either side of the membrane are swellings that will eventually form either labia majora or scrotal sacs. By the end of the seventh week, the urogenital sinus forms a separate opening under the genital tubercle. By the eighth week, the genital tubercle begins to form the clitoris. The caudal portion of the urogenital sinus will form the vaginal vestibule and the urethral folds remain separate, becoming the labia minora. The genital swellings merge in front of the anus and form the labia majora.
Embryology of the female genital tract
Anatomy (Fig. 1.19)
The vulva includes the mons pubis, the labia minor and majora, the clitoris, and the glandular structures, including Batholin’s glands and Cowper’s glands. Pubic hair grows in an inverted triangle pattern, although 25 % of women will have hair that extends upward along the linea alba. The vestibule is the area that is bordered by the labia minora, laterally, the posterior commissure at the perineum, and anteriorly, at the clitoris and urethra. The inferior border is the hymen. Skene’s glands open into the vagina from under the urethra. At the 5 and 7o’clock positions Bartholin’s glands ducts can be seen as small papillary swellings. During sexual activity, they secrete fluid. By about age 30, they generally involute. The skin of the vestibule is made up of squamous cells.
Anatomy of the vulva
Analogous to the scrotum, the labia majora are two thick, hair-bearing mounds of tissue that extend from the mons pubis to the perineum. They extend 7–9 cm in length and 2–4 cm in width. Three layers of the labia majora can be identified. The most superficial layer is called the tunica dartos labialis whose fibers run perpendicular to the folds of the skin. The middle layer contains adipose tissue and areolar tissue as well as sweat glands. The deepest layer contains muscle fibers that are contiguous with the round ligament.
The labia minora measure 5 cm in length and only 1 cm in thickness. Fusing with the prepuce of the clitoris, they originate at the clitoris and fuse posteriorly with the labia majora. The skin is smooth, pigmented, and hairless. The glands of the labia minora are analogous to the glands of Littre in the male urethra.
The clitoris contains two cavernous bodies that join at the glans, which contains erectile tissue. The suspensory ligament of the clitoris inserts into the inferior rami of the pubic symphysis. Inferiorly, the labia minora fuse to form a frenulum.
The anterior branch of the internal iliac artery (hypogastric artery) gives rise to the internal pudendal artery. It courses through the lesser sciatic foramen where it joins the pudendal nerve and dives through Alcock’s canal and then branches into the arteries to the gluteal region, the inferior hemorrhoidal artery, the perineal artery, and the artery to the clitoris. The inferior hemorrhoidal artery supplies the anus, the anal canal, and the perineum. The perineal artery perforates the urogenital diaphragm and supplies the ishiocavernosus, bulbocavernosus, and transverse perinei muscles. The termination of the perineal artery is in the labia.
The artery to the clitoris runs along the inferior pubic rami where it branches into four parts: the artery of the bulb, the urethral artery, the deep artery of the clitoris, and the dorsal artery of the clitoris. The artery of the bulb supplies the vestibule and Bartholin’s gland. The urethra artery supplies the urethra and anastomoses with the artery of the bulb. The deep artery of the clitoris supplies the corpus cavernosum. The superficial artery supplies the glans.
The vulva is highly innervated with nerves, originating from the lumbosacral region of the spinal cord. The main nervous supply to the region comes from the pudendal nerve, which combines branches from S2-4. It divides into three branches after it exits Alcock’s canal, including the inferior hemorrhoidal nerve, the perineal nerve, and the deep nerve to the clitoris. The inferior hemorrohoidal nerve supplies the external anal sphincter and peri-anal skin. The perineal nerve divides into deep and superficial branches. The deep branch supplies the bulbocarvernosus and ischiocavernosus muscles. The superficial branches supply the labia. The deep nerve to the clitoris supplies the glans.
Many nerves supply the skin of the vulva. Arising from T12 to L1, the iliohypogastric nerve runs between the internal iliac and transversus muscles to the iliac crest, where it divides into an anterior and posterior portion. The anterior hypogastric nerve runs superficially along the skin of the symphysis, and supplies the labia majora and the mons pubis. The posterior hypogastric nerve dives down into the gluteal area. The ilioinguinal nerve originates from L1 and supplies the labia majora as well. Also arising from L1-2, the genitofemoral nerve runs along the psoas muscle and innervates the muscle fibers within the labia majora. The femoral cutaneous nerve originates from posterior branches of S1-2 and the anterior branches of S2-3. It supplies the lateral aspect of the thigh and the labia majora.
Embryology (Fig. 1.18)
Early in the embryonic stage, both genders form the Wolffian (mesonephric) ducts and the Mullerian (mesonephric) ducts. The Wolffian ducts eventually develop into the male ducts and the seminal vesicles. The Mullerian ducts become the fallopian tubes, the uterus, and the upper vagina. The muscles and fascia that line these ducts are derived from mesoderm that is adjacent to the ducts. Both sets of ducts terminate in the cloaca, which is later separated by the urogenital ridge.
The upper vagina originates from primordial mesoderm that differentiates into the urogenital ridge, which forms the Mullerian duct. By the eighth week of gestation, the two caudal ends of the Mullerian duct fuse in the midline, and the septum between involutes, creating the uterovaginal canal. The epithelium of the vagina, the vestibular glands, and the hymen, comes from the urogenital sinus, which comes from the cloaca, a derivative of the endoderm.
The vagina is a muscular canal approximately 7–8 cm in length that extends from the vestibule to the uterus. Laying parallel to the sacrum, it meets the cervix at a 45° angle. The anterior wall is slightly shorter than the posterior wall because of the insertion of the cervix. The apex of the vagina includes the anterior and posterior fornices and the two lateral fornices. The distal end of the vagina traverses the urogenital diaphragm and is surrounded by the bulbocaversnosus muscles and bodies, which can be voluntary contracted during Kegel maneuvers.
Anteriorly, the urethra, ureters, and bladder lie on top of the vagina. The posterior aspect is covered by peritoneum, which forms the pouch of Douglas, or the rectovaginal pouch. Proximally, the vaginal sits on top of the rectum but the organs separate more distally as the rectum dives posteriorly. This muscular space creates the perineal body. The lateral apex of the vaginal canal sits about 1 cm from where the uterine vessels cross the ureter. Distally, the vaginal abuts the levator ani muscles. The vaginal supports include the bulbocavernosus muscles at the introitus, the levator ani muscles (puborectalis muscle) in the distal third, and the cardinal ligaments at the apex. Gartner ducts, vestiges of the Wolffian ducts (mesonephric duct), are sometimes palpable on the lateral aspect of the vagina.
The vagina is lined by stratified squamous epithelium, which is devoid of glands. Vaginal secretions are composed of cervical mucus, desquamated epithelium, and direct transudate. The mucosa is arranged in rugae on the anterior and posterior wall that can distend during parturition and intercourse. The submucosa contains a rich plexus of veins, as well as the lymphatics and nerves. The muscular component of the vagina is arranged in three layers, an outer longitudinal layer, and middle and inner circumferential layers.
Although it may arise from the hypogastric artery or the superior vesical artery, the main blood supply to the vagina usually comes from the uterine artery, which is a branch of the internal iliac (hypogastric) artery. Passing behind the ureter at the cephaled end of the canal, the vaginal artery anastomoses with branches of the uterine artery, creating a rich blood supply. The posterior aspect of the vaginal canal receives blood from the inferior and middle hemorrhoidal arteries.
The vaginal receives somatic innervation from the pudendal nerve, which branches into the perineal nerve as it comes through Alcock’s canal. The perineal nerve supplies the bulbocavernosus and ischiocavernosusm muscles that surround the vaginal canal. Both sympathetic and parasympathetic nerves that emanate from the inferior hypogastric plexus supply the vaginal mucosa.
Embryology (Fig. 1.18)
Even in the absence of ovaries, the mullerian ducts differentiate into the uterine tubes, the uterus, and the upper vagina in the female embryo. The intermediate mesoderm evolves into the urogenital ridge, by the fourth week of gestation. The urogenital ridge gives rise to the mesonephric (wolffian) duct and the paramesonephric (mullerian) duct. The paramesonephric ducts become the uterine tubes and the uterovaginal primordia. Between weeks 9 and 16, the uterus and upper part of the vagina form. The uterovaginal primordia join the urogenital sinus as it is separating from the cloaca. The septum between the fused paramesonephric ducts, now the uterovaginal primordia, degenerates, creating a single cavity. The fundus, body, and isthmus of the uterus, including the glands and endometrial epithelium, form from this structure. The endometrial stroma and smooth muscle are derived from splanchnic mesenchyme. Developing over the entire gestation, the form of the uterus evolves in the first trimester, the glands and muscle layers form in the second trimester, and the mucus producing cells of the cervix form during the last trimester.
The uterus is a muscular organ that sits in the pelvis between the bladder and the rectum. Divided into two main portions, the body is connected to the cervix by the isthmus, a transverse constriction. The broad ligament, a fold of peritoneum, covers the body of the uterus. The fundus is the part of the uterus that sits above the axis of the tubes. The communication of the cervix into the body of the uterus is called the internal os and the opening of the cervix into the vagina is called the external os. In nulliparous women the external os is round, whereas it forms a slit in women who have borne children. The uterus changes size with age and parity. In adult women who have not had children, it measures 7 cm in length and 4–5 cm in diameter. It is larger in parous women.
Anteriorly, the bladder sits on the uterus separated by the uterovesical pouch, which is a fold of peritoneum. On the posterior surface, the rectum separates the uterus by a deep fold of peritoneum that extends down to the upper vagina, creating the pouch of Douglas. The small intestine sits on the uterus and can extend into the pouch of Douglas if the walls are not fused. Laterally, the tubes, round ligament, ligament of the ovary, the uterine artery and vein, and the ureter all sit in close proximity. The ureters run parallel to the cervix as they enter the bladder. The uterine vessels cross over the ureter 1.5 cm from the lateral fornix of the vagina (water under the bridge).
The ligaments (Fig. 1.20) that support the uterus include the uterosacral ligaments, the cardinal ligaments (also called the transverse ligaments of the cervix and the ligamentum transversum colli, and the ligament of Mackenrodt), the round ligaments, and the broad ligaments. Although the cervix is fixed, the body and fundus of the uterus move freely with changes in the bladder and with pregnancy. The uterosacral ligaments are folds of peritoneum that coalesce at the cervix. Although they have some involvement in fixing the cervix to the sidewall, they mostly carry sympathetic, parasympathetic, and C fibers from the inferior hypogastric nerve. Supporting the cervix, the cardinal ligaments fan out from the cervix to the lateral sidewall. They contain the uterine artery, vein, and nerves, which perforate the ligament at the internal os. The round ligament is a vestige of the gubernaculum and enters the inguinal ring with the ilioinguinal and the genitofemoral nerves. Providing no support, the broad ligament drapes over most of the uterus.
Uterine ligaments and relationships to ovaries and fallopian tubes
The uterine wall consists of three layers: serous, muscular, and mucous. The serous layer (parametrium) is the peritoneal covering over the uterus. The muscular layer (myometrium) is a thick layer of muscle that is continuous with the vaginal muscle layer, the muscle layer of the tubes, and with the round, ovarian, cardinal, and uterosacral ligaments. While the outer layer of muscle is weak, the inner layer contains strong multidirectional fibers interspersed with a large venous plexus. The cervix has no smooth muscle layer beyond the internal os. The inner layer, the endometrium, is composed of soft, spongy connective tissue containing multiple tubular glands lined by ciliated columnar epithelium.
Blood Supply (Fig. 1.21)
The blood supply to the uterus primarily comes from the ovarian artery, a branch of the aorta, and the uterine artery, a terminal branch of the internal iliac (hypogastric) artery. Giving off branches as it descends, the uterine artery runs laterally along the parametrium to the cervix, where it travels over the ureter about two centimeters lateral to the cervix. Taking a tortuous course through the organ, the uterine artery gives off spiral branches that provide nutrients to the placenta during pregnancy. Branches of the uterine artery encircle the cervix and anastomose with branches of the vaginal artery, which is a terminal branch of the hypogastric or superior vesical artery (a branch of the hypogastric artery as well). Cephalad, the uterine artery, becomes the tubal artery, where it supplies the mesosalpinx and anastomoses with the ovarian artery, a direct branch of the aorta.
Blood suppply to the uterus
The visceral organs of the pelvis are mainly innervated by the autonomic nervous system, which carries both motor and sensory nerve fibers. Divided into three groups, the innervation to the uterus is comprised of the superior hypogastric plexus, the middle hypogastric plexus, and the inferior hypogastric plexus. Located below the inferior mesenteric artery over the middle sacral vessels at the level of L4- L5, the superior hypogastric plexus connects with the inferior hypogastric plexus through intermesenteric nerves, which also receive branches from the lumbar sympathetic ganglia. Not always present, the middle hypogastric plexus sits at the sacral promontory. The inferior mesenteric plexus, also known as the hypogastric nerves, derives innervation from S2-S4 (nervi erigentes—the parasympathetic component of the inferior mesenteric plexus), while it dives into the pelvis through the sacrouterine ligament into the upper vagina, where it becomes the pelvic plexus.
Located lateral to the uterus, the hypogastric ganglia supply nerves to the uterus through the internal os, the isthmus, and the broad ligament, where they enter the body of the uterus. Both myelinated and unmyelinated fibers innervate the uterus and run along the same course as the blood vessels with a higher concentration of fibers at the isthmus and a lower concentration at the fundus. Vader–Pacini corpuscles (corpuscles lamellose) and Dogiel and Krause corpuscles are seen within nerve bundles in the region of the endocervix, the broad ligament, and where the uterine artery meets the uterus. They induce uterine contractions during labor through modulation of the stretch response.
The Oviducts (Fallopian Tubes)
Embryology (Fig. 1.18)
The oviducts arise from the unfused, cranial end of the paramesonephric duct. The ostium of the oviduct is formed from the open end of the same segment. The mesoderm gives rise to the urogenital ridge, which gives rise to the mesonephric (Wolffian ducts) and paramesonephric (Mullerian) ducts. The mesonephric ducts give rise to the male genital ducts and the paramesonephric ducts give rise to the uterine tubes, the uterus, and the upper vagina.
Each oviduct is 7–14 cm long and is divided into three parts: the isthmus, the ampulla, and the infundibulum. Joined to the uterus, the isthmus is straight, with a long intramural segment. The ampulla is the longest part of the tube. It terminates into the infundibulum, which extends into finger-like projections, called fimbriae, the longest of which communicate with the ovary. The mouth of the infundibulum sits inside the peritoneum.
Each tube is covered by peritoneum on three sides, and the mesosalpynx on the fourth side. Blood vessesl and nerves run in the subserous layer of each tube. The muscle layer has smooth muscle fibers arranged in an outer longitudinal and an inner circular layer. The mucosal layer is lined with ciliated columnar epithelium.
Both the ovarian and the uterine artery supply the tubes. The blood supply coming off of the uterine artery runs on the inferior aspect of the tube up to the fimbria where it meets branches of the ovarian artery.
Both sympathetic and parasympathetic nerve fibers originating in the pelvic plexus innervate the fallopian tubes. The ampulla receives innervation from nerves of the ovarian plexus, while the uterovaginal plexus supplies the isthmus. The nerves enter the tubes through the mesosalpinx.
Embryology (Fig. 1.18)
Derived from the mesoderm, the urogenital ridge gives rise to the gonadal ridge, which differentiates into the testis or the ovary at 8 to 10 weeks of gestation. In the absence of the Y chromosome, the embryo does not produce testis-determining factor, anti-mullerian hormone, or testosterone. Without these substances, the gonad will become an ovary by default. Although formation of the ovary forms by default, without two X chromosomes, the ovaries will not develop into functioning organs. Around the fourth week of gestation, the germ cells migrate from the allantoic region to the urogenital ridge.
From weeks 9 to 10, the germ cells organize in the cortical region of the ovary, with interspersed epithelial and somatic cells. The primordial follicles contain diplotene oocytes. The ovarian medulla is composed of mesenchymal cells, somatic cells, connective tissue, and blood vessels. The mesovarian joins the ovary with the vestigial urogenital ridge.
Located immediately below the pelvic brim, the ovaries are 2.5–5 cm in length and weigh from 4 to 8 g each. The anterior surface of each ovary is tethered to the mesovarian (a peritoneal covering that connects the ovary to the broad ligament) and the posterior aspect is free. The upper pole sits next to the ipsilateral tube and the lower pole abuts the uterus. The ovaries are covered by the small intestine. The blood vessels, lymphatics, and nerves that supply the ovaries enter through the mesovarian at the hilum.
The ligaments that support the ovaries are the mesovarian, the suspensory ligament of the ovary, the ovarian ligament and the infundibulopelvic ligament. The mesovarian is made up of a double layer of peritoneum, containing branches of the uterine and ovarian arteries, a nerve plexus, the pampiniform vein plexus, and the lateral end of the ovarian ligament. Crossing the iliac vessels, the suspensory ligament is a peritoneal attachment at the lateral aspect of the ovary and contains the ovarian vessels. The ovarian ligament attaches the ovary to the medial aspect of the uterus. The infundibulopelvic ligament attaches the ovary to the sidewall. It contains the ovarian artery, veins, and nerves as they pass through the mesovarian into the hilum of the ovary.
The ovary is divided into the cortex and the medulla. The cortex contains the ova, which mature at different rates. Follicles form around the developing ova and project onto the free surface of the ovary. These are called graafian follicles. Fully mature ova within a follicle transform into corpus luteum. Once the ova is released or involutes, the corpus luteum becomes a corpus albicans, or scarred follicle. The medulla contains connective tissue, which supports the blood supply, lymphatics, and nerves to the cortex.
Emanating from the abdominal aorta, the ovarian blood vessels are the main blood supply to the ovary. The left ovarian artery often comes off of the left renal artery. After crossing the common iliac artery, they turn medially over the ureter and into the pelvis where they are covered by the folds of the suspensory ligament of the ovary and enter the hilum through the mesovarian. The ovarian branch of the uterine artery supplies the ovary from the medial aspect, creating an anastomosis with the ovarian vessels.
The nerves run along the same course as the arteries. They originate from the lumbosacral sympathetic chain.
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