UROGENITAL SYSTEM DEVELOPMENT

There are three parts of mesoderm: Paraxial Intermediate Intermediate Mesoderm Forms Much of the Urogenital System There are three parts of mesoderm: Paraxial Intermediate Lateral plate. Most of the urinary and genital systems develop from the intermediate mesoderm.

Mesoderm in the Chick Embryo Paraxial mesoderm (somite) Intermediate mesoderm Body cavity Lateral plate mesoderm

Three consecutive kidney systems formed Pronephros Mesonephros metanephros. They form in the intermediate mesoderm that extends from cervical to pelvic regions along the posterior wall.

Pronephros: segmented, forms in cervical regions, rudimentary, with little or no significance. (4th w) Mesonephros: (4th w) unsegmented, forms in thoracic and upper lumbar regions, may function briefly, consists of filtration units and a duct, degenerates almost completely in females, but contributes to the duct work of the male reproductive system. Metanephros: (5th W) definitive kidney system, forms in the pelvic region, begins functioning in the 10th to 12th weeks. 4.5 weeks All three systems are never present at the same time

Intermediate Mesoderm Forms the Urogenital Ridge

Intermediate Mesoderm Forms the Urogenital Ridge In lumbar region, the gonads develop from a proliferation of epithelium and condensation of underlying mesenchyme. Together the gonads and mesonephros form the urogenital ridge. (retroperitoneal) Excretory tubules form in the mesonephros and differentiate into Bowman’s capsule around a tuft of capillaries called the glomerulus (renal corpuscle). This is the filtration region for the kidney.

Intermediate Mesoderm Forms the Urogenital Ridge The nephron consists of filtration units (Bowman’s capsule plus glomerulus) together with the excretory tubule. Two ducts : mesonephric (wolfian) and paramesonephric (müllerian) ducts. Both ducts enter into the anterior portion of the cloaca that will form the urogenital sinus.

Intermediate Mesoderm Forms the Urogenital Ridge mesonephric duct is the collecting duct for the mesonephric kidney and will contribute to the male reproductive ducts. The paramesonephric duct parallels the mesonephric duct and contributes to the female reproductive system.

ureteric bud grows off the The Ureteric Bud Forms as an Outgrowth from the Mesonephric Duct ureteric bud grows off the caudal end of the mesonephric duct and into intermediate mesoderm(metanephric Blastema)in the pelvic region. The bud is responsible for inducing mesoderm to form kidney.

Collecting System Forms from the Ureteric Bud 6-7 Weeks The ureteric bud forms the collecting system for the definitive kidney in addition to ureter. The ureteric bud lengthens and then starts to divide, eventually forming the renal pelvis, major and minor calyces, and collecting tubules for the definitive kidney.

The Ureteric Bud Induces the Metanephric Blastema to Form Filtration Units As the ureteric bud branches in the metanephric blastema, it induces this mesoderm to form filtration units (nephrons). Thus, the tubules formed and eventually developed into Bowman’s capsules around the glomeruli and proximal and distal convoluted tubules and Henle’s loops.

Kidneys are initially formed in the pelvic region and are said to “ascend” into their definitive location in the lumbar area Renal function: Time excreation

Kidneys ascend from the pelvis to the lumbar region. Ascent of the Kidneys Kidneys ascend from the pelvis to the lumbar region. This is mostly a passive process created by growth of the caudal region of the embryo and diminution of the body curvature.

Pelvic and Horseshoe Kidneys Ureter Pelvic kidney: kidney sticks in the pelvis during caudal growth of the embryo and fails to ascend. Horseshoe kidney: in early development in the pelvis, the kidneys are in close proximity, and sometimes their tissues fuse. It is blocked in its ascent by the inferior mesenteric artery. Wilms tumor: cancer of the kidneys affects children by 5 years age

Polycystic Kidney Recessive form: 1:5000; cysts from collecting tubules; renal failure in infancy Dominant form: 1:500-1000: cysts from anywhere (all regions of the nephrons); renal failure in adulthood

Complete and partial duplications may occur. Duplications of the Ureters Complete and partial duplications may occur. Results from early splitting of the ureteric bud They are not usually symptomatic but may make it more difficult to pass kidney stones.

Bladder Development 6 Weeks 7 Weeks 8 Weeks Proctodeum Initially, both the urinary system and the gut enter into a common chamber called the cloaca. During 4th-7th week the urorectal septum will completely divide cloaca into rectal canal posteriorly and urogenital sinus anteriorly. The tip of the urorectal septum forms the perineal body.

urogenital sinus: Three portions : Bladder Development urogenital sinus: Three portions : Upper (largest part) is the urinary bladder pelvic part: in male gives rise to prostatic and membranous urethra phallic part: as genital tubercle grows, this part of the sinus will be pulled ventrally allantois connects bladder to umbilicus which later forms urachus (fibrous cord), ultimately be median umbilical ligament.

Consequently, ureters will enter bladder separately Bladder Development: Incorporation of the Mesonephric Duct bladder expansion: caudal mesonephric ducts absorbed into the posterior wall of urinary bladder (trigone : its mucosa is mesodermal then covered by endoderm ). Consequently, ureters will enter bladder separately

Bladder Development: Incorporation of the Mesonephric Duct ascent of the kidneys: orifices of ureters move farther cranially while those of mesonephric ducts enter prostatic urethra and in the male become vas deferens and ejaculatory ducts. Notice that vas deferens crosses over the ureters. If the embryo is female, the mesonephric ducts degenerate.

Urethra Epithelium: endoderm C.T. and muscular tis: visceral mesoderm end of third month: prostatic urethra epithelium proliferate and penetrate the surrounding mesenchyme. In male, these buds form the prostate gland In female, gives rise to urethral and paraurethral glands.

Possible Positions for Ectopic Ureters

Final Position of the Ureters and Mesonephric Duct (Vas Deferens) In male: mesonephric ducts differentiate into the vas deferens, seminal vesicle and ejaculatory duct.

Urorectal Septal Defects and Imperforate Anus

bladder Abnormalities

Genital System Relationship of the Genital Ridge to the Mesonephric System

3rd W. formed in yolk sac endoderm (dorsal wall). Migration of the Germ Cells Germ cells: 3rd W. formed in yolk sac endoderm (dorsal wall). 4th W. migrate to the genital ridge along the hindgut through the dorsal mesentery. 5th W. They reach the ridge 6th W. they penetrate the ridge

Relationship of Germ Cells to the Epithelium of the Genital Ridge 6 Weeks

Formation of the Seminiferous Tubules 8 weeks 16 weeks Sertoli cells: derived from epithelium; serve as nurse cells for sperm and produce antimüllerian hormone (AMH; also called müllerian-inhibiting substance [MIS]) Interstitial cells (of Leydig): derived from mesenchyme of genital ridge; secrete testosterone (mesonephric ducts, tubules, etc.; dihydrotestosterone, external genitalia)

Male Tubules and Ducts 16 Weeks 9 Months

Formation of Primary Oocytes and Follicular Cells

Derivatives of the Paramesonephric Ducts 8 Weeks 9 Months

Induction of the Vagina by the Paramesonephric Ducts 9 Weeks 12 Weeks Newborn

Vaginal Development

Uterine Abnormalities

Testis and Ovary Development

Genetic Signals: Male vs. Female

External Genitalia: Indifferent Stage 4 Weeks 6 Weeks

Penis and Scrotal Development 10 Weeks Newborn

Male 8 Weeks

Male 12 Weeks

Hypospadias Glandular Hypospadius Urethral and Glandular Hypospadias

Female 11 Weeks

Female External Genitalia Newborn 6 Weeks 5 Months

Female Pseudohermaphrodism (Adrenogenital Syndrome)

Androgen Insensitivity Syndrome (Testicular Feminization)

Signals for Sexual Differentiation SRY –SRY, +WNT4

Descent of the Testes 8 Weeks 19 Weeks 28 Weeks Newborn Cryptorchidism: Failure of the testes to reach the scrotal sac (3% of newborns)

Layers Covering the Testis

Indirect Inguinal Hernias and Hydroceles

Remnants of the Mesonephric Duct in Females