• Urinary and genital systems develop from intermediate mesoderm along dorsal body wall of embryo on each side of aorta
• kidneys develop in the pelvis and migrate superiorly; gonads develop in the abdomen and migrate inferiorly

• initially both the digestive and urinary tracts terminate in cloaca ("sewer") of the common gut tube
• during development, the urorectal septum divides the cloaca into two parts:
• (1) ventral urogenital sinus – divided into cranial (vesicle), middle (pelvic), and caudal portions
• vesicle Þ bladder – mesonephric duct empties into this and forms triangular region on posterior wall (trigone)
• initially bladder is continuous with allantois (connects to yolk sac) –becomes urachas (median umbilical ligament)
• (2) dorsal rectum – forms rectum and anus, most distal parts of the digestive tract
• both of these are covered by a membrane that prevents passage of fluids; this opens before birth
• Kidneys – intermediate mesoderm forms three kidneys
• (1) pronephros – forms in 4^th week – never functional
• (2) mesonephros – begins formation during 4^th week; functional from week 6 until week 10
• mesonephric ducts appear day 24 as longitudinal rods dorsolateral to mesonephric tubules – they form rostrally and develop caudally, moving closer together and eventually fusing with the ventrolateral walls of the cloaca
• becomes caudal ends of ejaculatory ducts in male; degenerates in female
• (3) metanephros – definitive kidney; formation begins day 28, from metanephric blastema; induced by ureteric buds
• ureteric buds originate from distal portion of the mesonephric ducts (future bladder); these in turn are induced by the metanephric blastema to become the renal pelvis, major and minor calyces, and the collecting tubules
• metanephros migrates superiorly to meet the adrenal gland and carries the ureter with it
• Urethra – from lower portion of urogenital sinus; distal part of male urethra arises from glandular plate
• Developmental Abnormalities
• (1) Cyst Formation
• (2) Ureter Abnormalities – e.g., second ureter that empties into main ureter, bladder, or directly into urethra
• (3) Migration Abnormalities – kidney may not ascend correctly and remain in the pelvis; usually nonfunctional
• (4) Horseshoe Kidney – inferior poles fuse during ascent, forming a single U-shaped kidney
• this becomes caught under the inferior mesenteric artery during ascent and does not reach its normal location
• (5) Double Renal Arteries – blood supply changes as kidney ascends; two renal arteries may be present
• (6) Patent Urachus – presents as urination through umbilicus

Genital System – female is the default pathway

• Pairs of genital ducts develop in both males and females; in undifferentiated stage, both pairs are present
• (1) Mesonephric (Wolffian) Duct – male reproductive system – originally drains urine from mesonephros
• proximal part forms epididymis; remainder forms ductus deferens and ejaculatory ducts – disappears in women
• (2) Paramesonephric (Mullarian) Duct – female reproductive system – lateral to gonads and metanephric ducts
• cranial end is open to peritoneum; passes caudally parallel to mesonephric ducts, then crosses over mesonephros and fuses to form Y-shaped uterovaginal primodium
• Early Development – until week 7, embryo is indifferent and is anatomically neither male nor female
• week 4 – primordial germ cells develop in the endodermal yolk sac
• week 5 – germ cells migrate to mesodermal endothelium on both sides of the hindgut
• these induce the gonadal ridge just medial to the developing mesonephroi
• week 6 – cells from the mesonephros and coelomic gonadal ridge form the primitive sex cords which invest the germ cells
• Male Development – testis determining factor (TDF) gene (on Y ch.) causes sex cords to form seminiferous tubules
• TDF induces primary sex cords to become prominent and convoluted; they then separate from the epithelium
• a fibrous thick capsule forms around the cords (Tunica albuginea); they become known as the seminiferous cords
• enlarged testes separate from the degenerating mesonephros and become suspended by mesentery (mesorchium)
• by week 8, interstitial cells of Leydig develop and produce testosterone that induces masculine differentiation
• Sertoli cells produce Mullarian Inhibiting Factor (MIF), which suppresses development of the paramesonephric duct
• seminiferous cords derived from the primary sex cords develop into seminiferous tubules, tubuli recti and rete testes
• rete testes becomes continuous with 15-20 mesonephric tubules that become the efferent ductules, which are connected to the part of the mesonephric duct near the testis that becomes the ductus epididymis
• the remainder of the mesonephric duct develops into the ductus deferens, seminal vesicle and ejaculatory duct
• the prostate arises from urethral endodermal outgrowths
• the bulbourethral glands arise from spongy tissue of the urethra just inferior to the prostate
• Female Development – absence of TDF causes gonads to develop into ovaries
• in absense of TDF, primary sex cords degenerate and disappear
• during weeks 10-12, secondary sex cords (cortical cords) develop and extend into the medullary mesenchyme
• by week 16, cortical cords break into isolated cell cultures called primordial follicles
• the developing ovary separates from the regressing mesonephros and becomes suspended by mesentery (mesovarium)
• mesonephric ducts regress (due to lack of testosterone) and the paramesonephric ducts develop (due to lack of MIF)
• inferior, fused paramesonephric ducts form uterovaginal (canal) primordium – becomes uterus and superior vagina
• superior, unfused paramesonephric ducts become fallopian tubes (oviducts); superior openings become infundibula
• fusion of the uterovaginal primordium with the urogenital sinus brings together two folds of the peritoneum and forms the broad ligament; this creates the vesicouterine pouch and the rectouterine pouch
• contact of uterovaginal primordium with urogenital sinus induces paired endodermal outgrowths –forms vaginal plate
• central vaginal cells eventually break down forming the vaginal cavity; a membrane (hymen) persists at the inferior end
• urethral and paraurethral glands develop from urethra
• vestibular glands develop as outgrowths from the urogenital sinus
• Clinical Abnormalities
• (1) double uterus and vagina (normal condition in marsupials)
• (2) doubles uterus and single vagina (non-fusion of paramesonephric ducts)
• (3) incomplete lumen formation of paramesonephric duct
• Remnants of the Genital Ducts in Males
• mesonephros becomes:
• appendix of epididymis – blind cranial end of duct
• paradidymis – tubules caudal to efferent ductules
• paramesonephros becomes:
• appendix testis – cranial end of duct
• prostatic utrical – corresponds to vagina
• seminal colliculus – corresponds to hymen
• Remnants of the Genital Ducts in Females:
• mesonephros becomes:
• appendix vesiculosa – cranial end of duct
• epoophoron – correspond to efferent ductules, epididymis
• paroophron – blind tubules near uterus
• ducts of Gartner – correspond to ductus deferens and ejaculatory duct (in broad ligament)
• paramesonephros becomes:
• hydatid of Margagni – cranial end of duct
• External Genitalia is similar (undifferentiated) until week 7; can be distinguished by week 9, fully developed by week 12
• proliferating mesenchyme produces a genital tubercle at cranial end of cloacal membrane by week 4 – forms phallus
• labioscrotal swellings and urogenital folds develop on each side of the cloacal membrane
• Male – phallus elongates and enlarges; urogenital folds form the urethral groove along the ventral surface of the penis
• groove is lined by endoderm of the urethral plate
• urogenital folds fuse to form spongy urethra, which is enclosed by penile raphe (fusion of surface ectoderm in midline)
• ectodermal glandular plate forms at tip of penis and grows caudally until it meets the spongy urethra; this then canalizes
• circular ingrowth of ectoderm occurs at the periphery of the glans during week 12; breaks down and forms prepuce
• corpora cavernosa and corpus spongiosum develop from mesenchyme within the penis
• labial scrotal swellings fuse to form the scrotum; line of fusion Þ scrotal raphe
• hypospadias – incomplete fusion of urethral or labioscrotal folds and opening of urethra onto ventral surface of the penis
• Female – phallus growth ceases and it becomes the clitoris
• urogenital folds do not fuse except for small portion that forms the frenulum of the labia minora
• unfused urogenital folds become labia minora
• labioscrotal folds fuse and form poster. labial commissure posteriorly, anter.labial commisure and mons pubis anteriorly
• middle portion of the folds remains unfused and forms labia majora
• Inguinal Canals – gubernaculum ligament is attached to the gonads and descents on each side of the abdomen
• this passes through anterior wall and attaches to inferior surface of the labioscrotal swelling
• process vaginalis, an invagination of the peritoneum, develops ventral to the gubernaculum and pushes through the
• abdominal wall along path formed by the gubernaculum
• process vaginalis pulls all layers of the abdominal wall with it; layers cover testes in males
• opening in transversalis fascia Þ deep inguinal ring; opening in external oblique aponeurosis Þ superficial inguinal ring
• Testes Descend through deep inguinal ring and inguinal canal – enter scrotum with ductus deferens and testicular vesicles
• abdominal fascia follows testis:
• transversalis fascia Þ internal spermatic fascia
• internal oblique Þ cremaster muscle
• external oblique aponeurosis Þ external spermatic fascia
• Ovaries Descend from posterior abdominal wall to just below the pelvic brim
• gubernaculum attaches to uterus near entrance to uterine tube – proximalÞ ligament of ovary, distalÞ round ligament
• round ligament passes through inguinal canal and ends in the labium majus