URINARY SYSTEM Dr. Mujahid Khan

Urinary System It consists of: It consists of:  The kidneys, which excrete urine  The ureters, which convey urine from the kidneys to the bladder  The urinary bladder, which stores urine temporarily  The urethra, which carries urine from the bladder to the exterior

Development of Kidneys and Ureters  Three sets of kidneys develop in human embryos  The first set is pronephroi, are rudimentary  The second set is mesonephroi, well developed but function briefly  The third set is metanephroi, become the permanent kidneys

Pronephroi  These transitory, non functional structures appear early in the fourth week in the neck region  The pronephric ducts run caudally and open into the cloaca  They soon degenerate but most of the pronephric ducts persist and are utilized by the next set of kidneys

Mesonephroi  Large, elongated, excretory organ appear late in the fourth week, caudal to the rudimentary pronephroi  Are well developed and function as interim kidneys for about four weeks, until permanent kidneys develop  They consist of glomeruli and mesonephric tubules

Mesonephroi  The tubules open into the mesonephric ducts  The mesonephric ducts open into the cloaca  They degenerate toward the end of the first trimester  Their tubules become the efferent ductules of the testes  Mesonephric ducts have several adult derivatives in male

Metanephroi  These are primordia of permanent kidneys  Begin to develop early in the fifth week  Start to function about four weeks later  Urine formation continues throughout fetal life  Urine is excreted into the amniotic cavity and mixes with amniotic fluid

Metanephroi  A mature fetus swallows several hundred ml of amniotic fluid each day, which is absorbed by the intestine  The waste products are transferred through the placental membrane into the maternal blood for elimination by kidneys  They develop from metanephric diverticulum and metanephric mass of intermediate mesoderm

Metanephroi  It is an outgrowth from the mesonephric duct near its entrance into the cloaca  The metanephric mass of intermediate mesoderm is derived from the caudal part of the nephrogenic cord  Both primordia of the metanephros are of mesodermal origin

Metanephric Diverticulum  It is the primordium of the ureter, renal pelvis, calices, and collecting tubules  As it elongates, the metanephric diverticulum penetrates the metanephric mass of intermediate mesoderm  The stalk of the metanephric diverticulum becomes the ureter and its expanded cranial end forms the renal pelvis

Metanephric Diverticulum  The straight collecting tubules undergo repeated branching, forming successive generations of collecting tubules  First four generations of tubules enlarge to form the major calices  Second four generations coalesce to form the minor calices  The remaining generations of tubules form the collecting tubules

Metanephric Diverticulum  The end of each arched collecting tubule induces clusters of mesenchymal cells in the metanephric mass of mesoderm to form small metanephric vesicles  These vesicles elongate and become metanephric tubules  As these renal tubules develop, their proximal ends are invaginated by glomeruli

Metanephric Diverticulum  The renal corpuscle and its proximal convoluted tubule, loop of Henle, and distal convoluted tubule constitute a nephron  Between the tenth and eighteenth weeks of gestation, the number of glomeruli increases gradually  They increase rapidly until the thirty second week

Uriniferous Tubule  It consists of two embryologically different parts:  A nephron derived from the metanephric mass of intermediate mesoderm  A collecting tubule derived from the metanephric diverticulum

Fetal Kidneys  They are subdivided into lobes  This lobulation diminishes toward the end of the fetal period  The lobulation usually disappear during infancy as the nephrons increase and grow  The lobulated character of kidneys is normally obscured in adults

Fetal Kidneys  At term each kidneys contains 800,000 to 1,000,000 nephrons  The increase in kidney size after birth results mainly from the elongation of the proximal convoluted tubules, as well as an increase in interstitial tissue

Fetal Kidneys  Nephron formation is complete at birth, except in premature infants  Functional maturation of kidneys occurs after birth  Glomerular filtration begins around the ninth week and the rate of filtration increases after birth

Positional Changes of Kidneys  Initially the metanephric kidneys lie close to each other in the pelvis  As the abdomen and pelvis grow, the kidneys gradually come to lie in the abdomen and move apart  They attain there adult position by the ninth week

Positional Changes of Kidneys  Initially the hilum of the kidneys lie ventrally  As the kidney ascends, it rotates medially almost 90 degrees  By the ninth week the hilum is directed anteromedially

Changes in the Blood Supply of Kidneys  As the kidneys ascend from the pelvis, they receive their blood supply from vessels that are close to them  Initially the renal arteries are branches of the common iliac arteries  As they ascend further, the kidneys receive their blood supply from the distal end of the aorta  As they reach higher level, they receive new branches from the aorta

Changes in the Blood Supply of Kidneys  Normally the caudal branches disappear  Their ascent stops when they come in contact with suprarenal glands in the ninth week  The kidneys receive their most cranial branches from abdominal aorta, which becomes the permanent renal arteries  The right renal artery is longer and often more superior

Urinary Bladder The urogenital sinus is divided into three parts: The urogenital sinus is divided into three parts:  A cranial visceral part that forms most of the bladder, continuous with the allantois  Middle pelvic part becomes the urethra  A caudal phallic part that grows toward the genital tubercle

Urinary Bladder  The bladder develops mainly from the vesical part of the urogenital sinus  Its trigone region is derived from the caudal ends of the mesonephric ducts  Its epithelium is derived from the endoderm of vesical part of urogenital sinus  Other layers of its wall develop from adjacent splanchnic mesenchyme

Urinary Bladder  Initially it is continuous with the allantois  The allantois soon constricts and becomes a thick fibrous cord, called urachus  Urachus extends from the apex of the bladder to the umbilicus  In the adult the urachus is represented by the median umbilical ligament

Urinary Bladder  As the bladder enlarges, distal parts of the mesonephric ducts are incorporated into its dorsal wall  Epithelium of the entire bladder is derived from the endoderm of the urogenital sinus  As the mesonephric ducts are absorbed, the ureters come to open separately into the urinary bladder

Urinary Bladder  The orifices of the mesonephric ducts move close together and enter the prostatic part of the urethra  Caudal ends of these ducts become the ejaculatory ducts  The distal ends of the mesonephric ducts in females degenerate

Urinary Bladder  In infants and children, the urinary bladder is in the abdomen  It begins to enter the greater pelvis at about six years of age  Does not enter the lesser pelvis and become a pelvic organ until after puberty

Urethra  The epithelium of most of the male urethra and the entire female urethra is derived from endoderm of the urogenital sinus  The epithelium of the terminal part of the urethra is derived from the surface ectoderm  The connective tissue and smooth muscle of the urethra in both sexes are derived from splanchnic mesenchyme

Suprarenal Glands  The cortex develops from the mesoderm  The medulla differentiates from neural crest cells  Cortex appears during the sixth week from the mesothelium lining the posterior abdomnal wall  The cells that form the medulla are derived from an adjacent sympathetic ganglion, derived from neural crest cells

Suprarenal Glands  Differentiation of the characteristic suprarenal cortical zones begins during the late fetal period  The zona glomerulosa and zona fasciculata are present at birth  The zona reticularis is not recognizable until the end of the third year

Suprarenal Glands  The suprarenal glands of human fetus are 10 to 20 times larger than adult glands relative to body weight  This is due to the extensive size of the fetal cortex  The glands lose about one-third of their weight during the first 2 or 3 weeks after birth and do not regain their original weight until the end of the second year