Uro-genital system
|
The urinary and genital system both arises from the mesoderm. Both drain into a common urogenital sinus and hence these two systems are intimately associated in their origin, development and relation.
Urinary organs
-
Nowhere in the development of the individual can be found a better example of the principle of recapitulation than in the development of kidneys of mammals. The earliest excretory organ is the pronephros (functional in adult amphioxus). In fishes and amphibians, the pronephros is replaced in the adult condition by the mesonephros. This is present in the embryos of reptiles, birds and mammals but the final permanent kidney in these classes is the metanephros. All the three types of kidneys are formed in succession, one behind the other during development of a mammal.
-
The kidney arise from the mesoderm of the intermediate cell mass or nephrotome which lies next to the mesodermal somites. The series of nephrotomes from the urino-genital ridge which splits up into an lateral nephric and a medial genital ridge, which project into the coelom from the dorsal wall.
-
The pronephros arises as a series of about seven simple paired pronephric tubules arranged segmentally. One end of each tube opens into the coelom while the other end opens into a longitudinal pronephric duct which is excretory in function and drains into the cloaca. The arterial tuft projecting into the coelom is the external glomerulus which serves to filter the waste products from the blood into the coelom. These degenerate early in foetal life.
-
The mesonephros or Wolffian body is larger and it consists of about 30 tubes which are longer and complicated, situated about the middle of the ridge. These mesonephric tubules are tortuous and are connected to the pronephric duct which is now termed as the mesonephric or Wolffian duct. At one end each tubule is invaginated like a gastrula. Bowman’s capsule and is associated with a glomerulus (internal glomerulus) and at the other end with the duct. These tubules degenerate about the fourth month.
-
The permanent kidney or metanephros arises far behind in the body. The final kidney consists of an aggregate of tubules which drain into a common duct. The excretory ducts (ureter, pelvis calyces, papillary ducts and straight collecting tubules are derived from a bud called ureteric-bud which is growing off the mesonephric duct. On the other hand the secretory unit, the nephron (Bowman’s capsule, both convoluted tubules and Henle’s loop) differentiates from the substance of the nephrogenous cord (metanephrogenic mass), which surrounds the pelvic dilation as a cap. The collecting (drainage duct) and secretory tubules then unite secondarily to complete a continuous uriniferous tubule. In structure and function, therefore these two components remain as different as was their origin.
-
The mesonephric duct makes a sharp bend before opening into the cloaca. At this angle, the ureteric primordium (ureteric-bud) appears as a hollow outgrowth from the mesonephric duct and grows forwards. The proximal, rapidly elongating portion of this invagination is the future ureter, while the blind end expands into the renal pelvis. This ureteric-bud pushes itself into a mass of condensed tissue, which is the caudalmost portion of the nephrogenous cord, which also forms a cap over the ureteric-bud. The ureteric-bud differentiates into the calyx’s majores, calyx’s minores, papillary ducts and collecting tubules for making up a large part of the medulla of the kidney. The metanephrogenous tissue differentiates into the secretory tubules forming the cortex. Then the formation of capsule and interstitial tissue are established between the secretory and excretory portions of the tubules. Soon the metanephros migrates toward the sublumbar region.
Anomalies
|
Last modified: Tuesday, 8 May 2012, 7:23 AM
Participants