Urinary system

1. Urinary system • 1- Pronephros : It begin in 4 wk and represented by 7 – 10 group of cells in the cervical region • these cell form excretory unit Nephrotomes that regress before more caudal ones formed , by the end of the 4 wk all disappear . • 2- Mesonephros : The mesonephros and mesonephric ducts derived from intermediate mesoderm from upper thoracic to upper lumber segment L3 .During the regression of pronephric system the first excretory tube of mesonephric tube appear, They length rapidly form S –shap

2. And acquire a tuft of capillaries that will form the glomerulus , around the glomerulus the tube form Bowmans capsule . Together these structure form renal corpuscle , Laterally the tube enters the longitudinal collection duct known as the mesonephric or wolffian duct .By the end of 2 ed month the maturity have disappeared . • 3-Metanephros : • The definitive kidney It appear in the 5 th wk. • a- collecting system: it developed from the urteric bud it is an out grothgrowth of the mesonephric duct close to its entrance to the cloaca,

4. The bud pentrates the metanephric tissue and dilated form renal pelvis and split into crinal and caudal portions to major calyces ;,These subdivided to 12 or more generation forming minor calyces ,The minor calyces will form the renal pyramid in which the collecting tube open So Urteric bud give rise to the ureter , renal pelvis . Major calyces , minor calyces and 1-3 million collecting tubules

7. Excretory System • Each newly formed collecting tubule is covered at its distal end by a metanephric tissue cap (Fig. 15.6A). Under the inductive influence of the tubule, cells of the tissue cap form small vesicles, the renal vesicles, which in turn give rise to small S-shaped tubules (Fig. 15.6B,C). Capillaries grow into the pocket at one end of the S and differentiate into glomeruli. These tubules, together with their glomeruli, form nephrons, or excretory units.

8. The proximal end of each nephron forms Bowman's capsule, which is deeply indented by a glomerulus (Fig. 15.6C,D). The distal end forms an open connection with one of the collecting tubules, establishing a passageway from Bowman's capsule to the collecting unit. Continuous lengthening of the excretory tubule results in formation of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule (Fig. 15.6E,F). Hence, the kidney develops from two sources: (a) metanephric mesoderm, which provides excretory units and (b) the ureteric bud, which gives rise to the collecting system.

9. Nephrons are formed until birth, at which time there are approximately 1 million in each kidney. Urine production begins early in gestation, soon after differentiation of the glomerular capillaries, which start to form by the 10th week. At birth, the kidneys have a lobulated appearance, but the lobulation disappears during infancy as a result of further growth of the nephrons, although there is no increase in their number

10. Position of the Kidney • The kidney, initially in the pelvic region, later shifts to a more cranial position in the abdomen. This ascent of the kidney is caused by diminution of body curvature and by growth of the body in the lumbar and sacral regions (Fig. 15.10). In the pelvis, the metanephros receives its arterial supply from a pelvic branch of the aorta. During its ascent to the abdominal level, it is vascularized by arteries that originate from the aorta at continuously higher levels. The lower vessels usually degenerate, but some may remain. • P.242

11. Function of the Kidney • The definitive kidney formed from the metanephros becomes functional near the 12th week. Urine is passed into the amniotic cavity and mixes with the amniotic fluid. The fluid is swallowed by the fetus and recycles through the kidneys. During fetal life, the kidneys are not responsible for excretion of waste products, since the placenta serves this function.

13. Bladder and Urethra • During the fourth to seventh weeks of development, the cloaca divides into the urogenital sinus anteriorly and the anal canal posteriorly (Fig. 15.12) (see Chapter 14, p. 231). The urorectal septum is a layer of mesoderm between the primitive anal canal and the urogenital sinus. The tip of the septum will form the perineal body (Fig. 15.12C). Three portions of the urogenital sinus can be distinguished: The upper and largest part is the urinary bladder (Fig. 15.13A). Initially, the bladder is continuous with the allantois, but when the lumen of the allantois is obliterated, a thick fibrous cord, the urachus,

15. remains and connects the apex of the bladder with the umbilicus (Fig. 15.13B). In the adult, it forms the median umbilical ligament.The next part is a rather narrow canal, the pelvic part of the urogenital sinus, which in the male gives rise to the prostatic and membranous parts of the urethra. The last part is the phallic part of the urogenital sinus. • Development of the phallic part of the urogenital sinus differs greatly between the two sexes.)

16. Clinical Correlates • 1-Renal Tumors and Defects Wilms' tumor is a cancer of the kidneys that usually affects children by 5 years of age but may also occur in the fetus. Wilms' tumor is due to mutations .in the WT1 gene . 2-Renal dysplasias and agenesis . 3-congenital polycystic kidney disease . 4-Abnormal Location of the Kidneys, ectopic . 5-Horseshoe kidney