Embryology of the gonads. Which one of the following is TRUE?
The sex of the embryo is determined genetically at the time of fertilisation. However, it is not until the 7th week that the gonads develop features of male or female morphology. Primordial germ cells migrate during the 5th week of gestation from the yolk sac along the hindgut and its dorsal mesentery towards the mesenchyme of the posterior body wall. The mesonephros and coelomic epithelium proliferate to form a pair of genital ridges. During the 6th week, cells of the genital ridge invade the mesenchyme to form primitive sex cords, consisting of cortical and medullary regions that will eventually invest the germ cells and support their development. At the same time, paramesonephric ducts develop lateral to the mesonephric ducts in a cranio-caudal direction. Primordial germ cells carry the XY sex chromosome if the embryo is genetically male. The SRY gene located on the Y chromosome encodes for testis determining factor. This acts to stimulate proliferation of cells in the medulla and degeneration of cells in the cortical regions of the primitive sex cords. During the 7th week, the cells differentiate into Sertoli cells that become organised into testis cords and eventually to seminiferous tubules around the time of pubescence. These Sertoli cells secrete Müllerian Inhibiting Substance (MIS) that cause regression of the paramesonephric ducts between the 8th and 10th weeks, some of which remain as the Hydatid of Morgagni. Near the hilum, the testis cords break up into a network of thin strands that eventually become the rete testis. Some of these thin walled ducts connect to the mesonephric ducts from the medial aspect of the gonads. With further development, the testis cords become separated from the surface epithelium by the fibrous tunica albuginea. By the 10th week, Leydig cells develop from the mesenchymal cells of the genital ridge and secrete testosterone, initially under the influence of placental chorionic gonadotropin and later by the pituitary gonadotrophins. By the 12th week, the mesonephric (Wolffian) duct is transformed into the vas deferens under the influence of testosterone. The cranial nephric duct also degenerates whilst the part adjacent to the presumptive testis becomes the epididymis. During the 7th week, the testes are located near the kidneys, held in position by the dorsal cranial suspensory ligament and the ventral ligament, which becomes the gubernaculum. During the 12th week, the testes descend to the inguinal canal and the suspensory ligament regresses. By the 28th week, there is an outgrowth from the gubernaculum that passes through the external ring and enters the scrotum. It is hollowed out by a tongue of peritoneum (processus vaginalis), that allows the testes to descend into the scrotum by 33 weeks. The peritoneal layer covering the testis becomes the visceral layer of the tunica vaginalis and the remainder of the peritoneal sac forms the parietal layer. The canal connecting the peritoneal cavity with the vaginal process becomes obliterated shortly before or after birth. The lack of testis determining factor in the female embryo causes the primitive sex cords to degenerate and the mesothelium of the genital ridge forms the secondary cortical sex cords. They invest the primordial germ cells to form ovarian follicles, which differentiate into oogonia and then primary oocytes that remain in this phase until puberty. The mesonephric ducts regress but the paramesonephric ducts develop into fallopian tubes, uterus and upper two-thirds of the vagina. The remainder of the inferior third of the vagina forms as a result of the caudal ends of the paramesonephric ducts, which come together and form a common channel (uterovaginal canal). It then fuses with the thickened tissue on the posterior urogenital sinus known as the sinovaginal bulb. The vaginal plate then develops at the inferior part of the canal, which then elongates and canalises between 12 and 20 weeks to form the inferior vaginal lumen.
Anatomy of the kidneys. Which one of the following is TRUE?
Answer E
Each kidney measures 10–12 cm in length and weighs between 135 and 150 g. The right kidney is shorter and wider than the left and lies between the levels of the L1–L3 vertebrae from displacement by the liver. The left kidney lies up to 2 cm higher between T12 and L3. The dromedary hump is a bulge on the lateral contour of the kidney, more common on the left, caused by downward pressure from either the liver or spleen and is of no clinical significance. Each kidney is orientated in such a way that the upper poles are more medially and posteriorly located than the lower poles. Much of the surrounding relationship of each kidney is similar. The posterior upper third is covered by the diaphragm, the medial lower two-thirds lie against the psoas and the lateral contours are in contact with quadratus lumborum and the aponeurosis of transversus abdominis. Additionally, on the right, much of the medial aspect lies opposite the descending duodenum and the hepatic flexure of the colon cross over the anterior surface of the lower pole. On the left, the pancreatic tail reaches the upper pole and above this is the posterior gastric wall. The spleen is found on the outer upper aspect and is attached to the kidney by the splenorenal ligament. The splenic vessels lie adjacent to the hilum and the splenic flexure of the colon lies anterior to the lower pole. Gerota’s fascia surrounds the kidney except inferiorly. Each kidney is divided into an outer thinner cortex and deeper thicker medulla. The medulla typically contains 7–9 renal pyramids whose apex (renal papillae) point towards the renal pelvis. Each papillae is cupped by a minor calyx and each drains via an infundibulum into two or three major calyces. These coalesce to form the renal pelvis that gives rise to the ureter. The cortex extends between the pyramids as columns of Bertin that contain the branching renal vasculature. Closely associated with these blood vessels are lymphatics that empty into larger trunks in the renal sinus before eventually reaching the lymph nodes closely associated with the renal veins. On the left, the lymphatic drainage is mainly to the lateral paraaortic nodes whilst on the right it is to the interaortocaval and paracaval nodes. The autonomic supply to the kidney is mainly concerned with vasomotor control; the sympathetic fibres originate from spinal nerves T8 to L1 and cause vasoconstriction whilst the vagal parasympathetic fibres cause vasodilation.
Which of the following is TRUE regarding the prostate?
The prostate develops during the 10th–12th week of gestation under influence of testosterone. The urogenital sinus gives off a prostatic bud comprised of solid epithelial cords, which canalise into solid prostatic ducts, and the epithelial cells become luminal and basal cells. The prostatic mesenchyme differentiates into smooth muscle cells to surround the ducts. There is a marked increase in the size of the prostate at puberty under the influence of testosterone and secretion of prostate specific antigen occurs. The gland is ovoid in shape, lies at the base of the bladder and weighs roughly 18 g in the normal adult. A fibrous capsule surrounds the gland composed of predominant glandular tissue (70%) and fibromuscular stroma (30%). It has an anterior, posterior and lateral surface with an inferior apex that lies on the urogenital diaphragm and a superior base. The prostate is further anatomically divided the gland into four zones based on the glandular elements.
The rectum is posterior to the gland, separated by Denonvilliers’ fascia and a loose layer of areolar tissue, the endopelvic fascia anterolaterally and the pubococcygeal muscles of levator ani laterally. Near the apex, the puboprostatic ligaments are on either side of the midline that extend from the prostate to the pubic bone. The base is continuous with the bladder neck where the detrusor muscle fibres merge with the capsule. The prostatic urethra is about 3 cm long and runs through the prostate from the bladder neck to become the membranous urethra. On the posterior wall, is a longitudinal ridge (urethral crest) that runs the length of the gland in between two grooves (prostatic sinus) into which the glandular elements open into the urethra. The crest widens distally to form the verumontanum. At its apex lies the slit-like prostatic utricle and represents a Müllerian remnant. The ejaculatory ducts open on either side of this. The main blood supply to the prostate is from the inferior vesical artery, which becomes the prostatic artery and divides into two main branches. The urethral arteries enter the prostate at the junction with the bladder and approach the bladder neck at the 1–11 (Flock) and 5–7 (Badenoch) o’clock positions. From here they run parallel to the urethra to supply the transition zone and periurethral glands. The capsular arteries supply the capsule and glandular tissues. The veins of the prostate drain into the dorsal vein complex lying just beneath the pubic symphysis; the deep dorsal vein leaves the penis under Buck’s fascia and penetrates the urogenital diaphragm dividing into three major branches – the superficial branch and the right and left lateral plexus. The superficial branch pierces the endopelvic fascia and runs over the neck and anterior bladder surface to drain the anterior prostate, bladder and retropubic fat. The lateral branches pass down the sides of the prostate to join the vesical plexus, which subsequently drains into the internal iliac veins.
Regarding the bladder, which of the following is FALSE?
Answer B
The bladder has a dual function of storing and voiding urine with a capacity of 500 mL. Its shape changes with the amount of urine: a pyramidal shape when empty but ovoid when full. When empty, the apex faces anteriorly and lies just behind the pubic symphysis. It is the point of attachment for the urachus, a fibrous cord and remnant of the allantois, which suspends the bladder from the anterior abdominal wall by fusing with one of the obliterated umbilical arteries near the umbilicus. The base faces posteriorly, is triangular in shape and is in contact with a pair of seminal vesicles separated by the vas deferens. The inferior angle gives rise to the urethra. The bladder neck is 3–4 cm directly behind the pubic symphysis, and its position remains fixed during bladder distention. The superior surface is covered with peritoneum, which in women is further reflected over the uterus (vesicouterine pouch) and continues further back to cover the rectum (rectouterine pouch). On filling, the superior surface rises into the abdominal cavity, pushing the peritoneum away to come into direct contact with the posterior surface of the abdominal wall. The layers of bladder wall include an innermost transitional cell epithelium, made up to 6 cells thick overlying a basement membrane, the lamina propria and the outermost detrusor (smooth) muscle. The fibres of the detrusor are arranged into an inner longitudinal, circular and outer longitudinal fashion that forms the internal (proximal) sphincter. In men, the inner fibres are continuous with those of the urethra. The middle fibres form a ring around the bladder neck with the anterior fibromuscular stroma of the prostate. The outer fibres are abundant in the bladder base and they also form a loop around the bladder neck to provide continence. In females, the arrangement of the inner layer is similar but less is known about the arrangement of the middle and outer fibres. On approaching the bladder, the ureters become covered by a fibromuscular Sheath of Waldeyer before passing obliquely through the wall for about 2 cm and ending at the ureteral orifice. The intramural portion is narrow from compression by the detrusor muscle. This angle of entry and the surrounding detrusor muscle acts to prevent retrograde reflux of urine up the ureter. The two ureteric orifices form a triangular-shaped trigone with the apex at the bladder neck. The longitudinal muscle fibres of the ureter fan out over trigone and deep to this is the fibromuscular sheath of Waldeyer that inserts into the bladder neck. The urothelium is thinnest on the trigone. The fibres between the two orifices are thickened to form the interureteric ridge. The blood supply to the bladder is the superior and inferior vesical arteries. Venous drainage is to the vesical plexus, which drains into the internal iliac vein.
Regarding the urethra, which of the following is TRUE?
The urethra extends from the bladder neck to the external meatus. It is longer in men (up to 20 cm) than in women (4 cm). In men, it is broadly divided into the anterior and posterior urethra. The latter is further subdivided into prostatic and membranous while anteriorly it is bulbar and pendulous. There are bends at the junctions between the membranous and bulbar and the bulbar and pendulous urethra. The membranous urethra is short and narrow as it passes through the urogenital diaphragm. It is surrounded by the striated external urethral sphincter whose inner smooth muscle is continuous with that of the proximal sphincter at the bladder neck. The outer muscle extends from the bladder neck and anterior prostate. The sphincter has a horseshoe configuration on cross section due to a broad anterior and a deficient posterior surface. Some of the fibres also attach to the perineal body causing the urethra to be pulled backwards when the sphincter contracts. The sphincter is also suspended from the pubic bones through attachments to the puboprostatic and suspensory ligament of the penis. Neural supply is from the somatic pudendal nerve and an autonomic branch from the sacral plexus. The anterior urethra is longer (15 cm), beginning at the level of the perineal membrane and transversing the whole length of the spongiosum to end at the narrow external meatus. It is dilated at the bulbar urethra and fossa navicularis in the glans. Mucus is secreted into the urethra by bulbourethral glands in the base of the external urethral sphincter and the glands of Littre in the submucosa. Proximally, the anterior urethra is lined by columnar epithelium but distally within the glans penis it is lined by stratified squamous epithelium. In contrast, the prostatic urethra is lined by transitional cell epithelium. The female urethra opens into the vestibule at the external meatus. On either side of the meatus are two ducts that drain mucus from the paraurethral (Skene) glands. It is surrounded by a striated external urethral sphincter. At the proximal and distal ends, the slow twitch muscle fibres completely surround the urethra whilst the fibres in the middle do not meet posteriorly and instead attach to the vagina. The urethra is pulled against the vagina when the sphincter contracts. The urethra is suspended beneath the pubic bone by the suspensory ligament of clitoris and the pubourethral ligament. It is lined proximally by transitional cell epithelium, which becomes stratified squamous epithelium distally. Somatic nerve innervation is similar to that in the male.
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