4.2 Urogenital System

Urogenital System

Urogenital system anatomy is 10% of General Anatomy on the NBCE Part I test plan. Official bullets are urinary organs, male reproductive organs, female reproductive organs, development, and histology. Pelvic-floor relations are how many viscera items are actually written (what the ureter crosses; what the levator ani supports). Renal physiology (GFR, tubular transport, acid–base) and reproductive physiology (HPG axis, cycle hormones) are separate Physiology topics.

Urinary Organs

The kidneys are retroperitoneal at about T12–L3; the right kidney sits slightly lower because of the liver. Posterior relations: diaphragm, psoas major, quadratus lumborum, transversus abdominis, and the 12th rib (plus 11th on the left). The hilum order, anterior to posterior, is renal vein, renal artery, renal pelvis (vein–artery–pelvis). Coverings from in to out: fibrous capsule, perirenal fat, renal (Gerota) fascia, pararenal fat.

Cortex contains glomeruli, proximal and distal tubules, and medullary rays. Medulla is pyramids (loops of Henle, collecting ducts) whose papillae empty into minor calyces. Renal columns of Bertin are cortical tissue between pyramids. A renal lobe is one pyramid plus its overlying cortex. Minor calyces fuse to major calyces → renal pelvis → ureter.

Renal arteries arise from the aorta at about L1–L2. Each typically gives five segmental arteries, which are end arteries—occlusion infarcts a segment. The left renal vein is long: it crosses the aorta anteriorly, under the superior mesenteric artery (nutcracker interval), and receives the left gonadal and left suprarenal veins. The right renal vein is short. Right gonadal and right suprarenal veins drain directly to the IVC. Lymph follows veins to lumbar (aortic) nodes.

The ureter is about 25–30 cm of retroperitoneal muscular tube. Abdominal ureter descends on psoas, crossed anteriorly by the gonadal vessels, and is crossed by the genitofemoral nerve. It enters the pelvis at the bifurcation of the common iliac artery (pelvic brim). In the male pelvis the ductus deferens crosses superior to the ureter; in the female the uterine artery crosses superior to the ureter lateral to the cervix (water under the bridge—the ureter is the water). Three anatomic constrictions (stone lodgment sites):

Ureteric blood is segmental (renal, gonadal, aorta/common iliac, vesical or uterine). A transplanted ureter therefore needs its adventitial vessels preserved. Innervation is autonomic; stones refer to T10–L2 (flank to groin) along the ilioinguinal and genitofemoral distribution.

The bladder lies in the true pelvis when empty; as it fills it rises into the abdomen behind the pubic bones. Apex attaches to the median umbilical ligament (obliterated urachus). Fundus (posterior wall) in the male faces the rectum (rectovesical pouch); in the female it faces vagina and uterus (vesicouterine pouch). Neck rests on the prostate in the male and on the urogenital sphincter in the female. The trigone is the smooth triangle between the two ureteric orifices and the internal urethral orifice; its mucosa is mesodermal (absorbed mesonephric ducts) and does not rugate.

Detrusor is parasympathetic (pelvic splanchnics S2–S4). Male internal urethral sphincter at the bladder neck is sympathetic (L1–L2) and prevents retrograde ejaculation. External urethral sphincter is somatic via pudendal nerve (S2–S4) in the deep perineal pouch. Superior and inferior vesical arteries (from internal iliac) supply the bladder; in the female, vaginal arteries contribute.

Male urethra: prostatic (widest, urethral crest, seminal colliculus, openings of ejaculatory ducts and prostatic utricle), membranous (shortest, in the deep perineal pouch, surrounded by sphincter urethrae—vulnerable in pelvic fracture), spongy (longest, in corpus spongiosum, bulbourethral ducts enter the bulb, navicular fossa in the glans). Female urethra is about 4 cm, embedded in the anterior vaginal wall, and opens in the vestibule; its shortness is the anatomic basis of more frequent ascending infection.

Male Reproductive Organs

The testis is covered by tunica albuginea; septa make lobules of seminiferous tubules that drain to the rete testis → efferent ductules → epididymis (head, body, tail). The tail becomes ductus (vas) deferens. Leydig (interstitial) cells sit between tubules. Left testis often hangs lower. Lymph from the testis goes to lumbar nodes, not inguinal—because the gonad descended from the abdomen. Scrotal skin lymph goes to superficial inguinal nodes.

Spermatic cord coverings: external spermatic fascia (external oblique), cremasteric fascia and muscle (internal oblique; genital branch of genitofemoral), internal spermatic fascia (transversalis fascia). Contents: ductus deferens, testicular artery, pampiniform plexus, genital branch of genitofemoral, autonomic fibers, lymphatics, remnant of processus vaginalis, and the artery of the ductus (from superior vesical). The ilioinguinal nerve travels in the inguinal canal but is not inside the cord proper.

Seminal vesicles (posterior to bladder, lateral to ductus ampullae) contribute most of the ejaculate (alkaline, fructose). Ampulla of the ductus plus the seminal-vesicle duct form the ejaculatory duct, which traverses the central zone of the prostate to open on the seminal colliculus. Prostate surrounds the prostatic urethra. Classic zones: peripheral zone (most carcinoma), transition zone (most benign prostatic hyperplasia, around the urethra), central zone (ejaculatory ducts). Inferior vesical and middle rectal arteries supply it. Bulbourethral (Cowper) glands lie in the deep perineal pouch and empty into the spongy urethra.

The penis has two corpora cavernosa (deep arteries of the penis) and one corpus spongiosum (urethra). Buck fascia wraps all three; tunica albuginea is thicker around the cavernosa. Innervation: pudendal (dorsal nerve of penis) for somatic; cavernous nerves from the prostatic plexus (parasympathetic S2–S4) for erection. Ejaculation is sympathetic.

Female Reproductive Organs and Pelvic Floor

The ovary is attached by the mesovarium to the posterior broad ligament, by the ovarian ligament to the uterus, and by the suspensory (infundibulopelvic) ligament to the pelvic wall—the last carrying ovarian vessels from the aorta / to the IVC or left renal vein. Surface is not peritonealized at the posterior free border (ovulation). Lymph follows ovarian vessels to lumbar nodes.

Uterine tube: infundibulum with fimbriae, ampulla (usual site of fertilization and of ectopic pregnancy), isthmus, intramural (uterine) part. The tube is in the mesosalpinx.

Uterus: fundus, body, isthmus, cervix (internal os, cervical canal, external os). Typical position is anteverted (angle of cervix to vagina) and anteflexed (angle of body to cervix). Broad ligament is peritoneum (mesometrium, mesosalpinx, mesovarium). Round ligament is a gubernaculum remnant through the inguinal canal to the labia majora. Cardinal (transverse cervical) ligaments at the base of the broad ligament carry the uterine arteries. Uterosacral ligaments pass back around the rectum. Uterine artery from the internal iliac runs in the cardinal ligament and crosses over the ureter. Venous plexus drains to internal iliac veins.

Vagina has fornices; the posterior fornix is related to the rectouterine pouch of Douglas—the most dependent peritoneal recess in the female. Vestibule contains the urethral and vaginal orifices and greater vestibular (Bartholin) glands (homologues of Cowper glands). Paraurethral (Skene) glands are prostatic homologues.

Pelvic floor (pelvic diaphragm): levator ani (pubococcygeus, puborectalis, iliococcygeus) plus coccygeus (ischiococcygeus). Puborectalis slings the anorectal junction and maintains continence. The urogenital hiatus transmits urethra (and vagina). Perineal body is the central tendon between urogenital and anal triangles—attachment for bulbospongiosus, superficial transverse perineal, and the external anal sphincter. Deep perineal pouch: sphincter urethrae, deep transverse perineal muscle, bulbourethral glands (male), and the membranous urethra. Superficial perineal pouch: erectile tissues, ischiocavernosus, bulbospongiosus, superficial transverse perineal, Bartholin glands (female). Inferior rectal and perineal branches of pudendal (S2–S4) innervate the perineum; levator ani also takes direct S3–S4 twigs and the nerve to levator ani.

Development

The urinary and genital systems both arise from intermediate mesoderm (urogenital ridge). Three kidneys appear in craniocaudal sequence.

Pronephros (week 4, cervical) is vestigial and degenerates. Mesonephros (thoracolumbar) functions briefly in the embryo; its mesonephric (Wolffian) ducts persist and open into the cloaca. Metanephros is the definitive kidney. It forms when the ureteric bud (an outgrowth of the mesonephric duct near the cloaca) contacts metanephric mesenchyme (blastema) in the sacral region.

The metanephros ascends from the pelvis, picking up and then discarding successive aortic sprouts until the definitive renal arteries at L1–L2 remain. Failed ascent is a pelvic kidney. Fusion of lower poles before ascent produces a horseshoe kidney, which catches on the inferior mesenteric artery and stays low. Accessory renal arteries are common because old segmental sprouts persist. The kidney rotates so the hilum faces medial.

Gonads are bipotential. Primordial germ cells migrate from the yolk-sac endoderm along the dorsal mesentery into the genital ridge. SRY on the Y chromosome drives testis-determining programs. Sertoli cells secrete anti-Müllerian hormone (AMH) that regresses paramesonephric ducts in the male. Leydig cells secrete testosterone that stabilizes mesonephric ducts. Without SRY, the gonad becomes ovary and paramesonephric (Müllerian) ducts persist.

Testicular descent through the inguinal canal is complete near term; the processus vaginalis should obliterate (failure → congenital hydrocele or indirect hernia). Cryptorchidism leaves the testis in abdomen or canal. Incomplete fusion of paramesonephric ducts produces uterus didelphys or bicornuate uterus. Incomplete fusion of urethral folds is hypospadias. The vaginal plate (sinovaginal bulbs from the urogenital sinus) canalizes to form the lower vagina; the hymen marks that junction.

Histology

Renal corpuscle: glomerular capillaries (fenestrated endothelium), glomerular basement membrane, and visceral epithelial podocytes whose slit diaphragms complete the filtration barrier. Mesangial cells support the tuft and are phagocytic. Parietal Bowman capsule is simple squamous. Macula densa of the DCT abuts the afferent arteriole as part of the juxtaglomerular apparatus (JG cells = modified afferent smooth muscle, renin).

Ureter and bladder are lined by transitional epithelium (urothelium) with surface umbrella cells that stretch. Ureteric muscularis is inner longitudinal and outer circular; a third outer longitudinal layer appears near the bladder. Bladder detrusor is an interlacing meshwork, not a simple three-layer gut wall.

Seminiferous tubules: stratified germinal epithelium plus Sertoli cells (columnar, blood–testis barrier at tight junctions, support spermatogenesis, secrete inhibin and AMH). Germ cells progress from spermatogonia on the basal lamina to spermatids at the lumen. Leydig cells in the interstitium are eosinophilic, lipid-rich, and make testosterone under LH. Epididymis: pseudostratified columnar with stereocilia. Prostate: tubuloalveolar glands in fibromuscular stroma; corpora amylacea are concentric concretions in lumina.

Ovary: cortex with follicles in stroma; medulla with vessels. Follicle sequence: primordial (flat follicle cells) → primary (cuboidal granulosa, zona pellucida) → secondary (antral fluid, theca) → Graafian. After ovulation the follicle becomes corpus luteum (granulosa-lutein and theca-lutein cells) or, if no pregnancy, corpus albicans. Uterus: endometrium (stratum functionalis shed, stratum basalis retained), thick myometrium, perimetrium. Cervix: simple columnar endocervix meets stratified squamous ectocervix at the transformation zone. Vagina: nonkeratinized stratified squamous epithelium rich in glycogen; no glands (lubrication is cervical and Bartholin).

Recite bud versus blastema, the three ureteric narrowings, left-versus-right venous tributaries, and the homologous duct table, then mix items on /practice/nbce-part1.