Kidney and Urinary Tract Development - Anatomy & Physiology
|
The Kidneys
The kidneys are formed from the intermediate mesoderm. Firstly it begins to thicken longitudinally in its caudal domain. This is called the nephrogenic thickening. From this thickening three so called attempts at producing excretory orgins arise. All the attempts have at the histological level a series of excretory tubules however only one of these attempts forms the kidney in adult life.
The Pronephros
This structure forms in the neck region of the embryo and is not functional in mammals. Its tubules attach and form the continous pronephric duct. The caudal edge of this duct then opens into the cloaca. The duct survives the regression of the tubules and actually helps drain the mesonepohric tubules which begin to appear at this point.
The Mesonephros
This structure forms in the thoraic and lumbar regions and is important in embryonic life. It has more tubules than the pronephros and the structure of these tubules is similar to that of the adult kidney. They are closed and one end and open at the other. The open end drains into the pronephric duct which is now more appropriately called the mesonephric duct. The closed end has a bundle of capillaries which form a filtration mechanism. This organ is variable in size in relation to how efficent the placenta is at removing waste. The worse the placenta the bigger the organ. As the metanephros develops it supersedes this organ and it begins to regress as a result. Parts of it do however remain in the male and form part of the reproductive tract.
The Metanephros
The metanephros forms from 2 primordia. One is an out growth of the ureteric bud which grows from the lower part of mesonephric duct near the cloaca. The bud grows up into the metanephros and divides. The early divisions later go on to be reabsorbed to a variable degree accounting for the inter-species differances in the renal pelvis and calyces. The later divisions form the collecting ducts. A mass of tissue is seen which is termed the metanephric mass. The outer part of this mass goes on to form the renal capsule and interstitum. The inner part goes on to form the nephrons via intermediates called cell cords. Each cell cord joins with a collecting duct. The other end invaginates around a tuft of capillaries arising from the aorta to form the glomerulus.