The urinary system: embriogenesis
The urinary system is formed mainly from mesodermal and endodermal derivatives. Three separate systems form sequentially. The pronephros is vestigial; the mesonephros may function transiently, but then mainly disappears; the metanephros develops into the definitive kidney. The permanent excre tory ducts are derived from the metanephric ducts, the urogenital sinus, and surface ectoderm.
Pronephros: Segmented nephrotomes appear in the cervical intermediate mesoderm of the embryo in the fourth week. These structures grow laterally and canalize to form nephric tubules. Successive tubules grow caudally and unite to form the pronephric duct, which empties into the cloaca. The first tubules formed regress before the last ones are formed.
Mesonephros: In the fifth week, the mesonephros appears as «S-shaped» tubules in the intermediate mesoderm of the thoracic and lumbar regions of the embryo.
The medial end of each tubule enlarges to form a Bowman's capsule into which a tuft of capillaries, or glomerulus, invaginates.
The lateral end of each tubule opens into the meson-ephriс (Wolffian) duct.
Mesonephric tubules function temporarily and degenerate by the beginning of the third month. The mesonephric duct pesists in the male as the ductus epididymidis, ductus deferens, and the ejaculatory duct.
Metanephros: During the fifth week, the metanephros, or permanent kidney, develops from two sources: the ureteric bud, a diverticulum of the mesonephric duct, and the metan-ephricmas, from intermediate mesoderrn of the lumbar and sacral regions. The ureteric bud penetrates the metanephric mass, which cordenses around the diverticulum to form the metanephrogen cap. The bud dilates to form the renal pelvis. One-to-three million collecting tubules develop from the minor calyces, thus forming the renal pyramids. Penetration of collecting tubules into the metanephric mass induces cells of the tissue cap to form nephrons, or excretory units. The proximal nephron forms Bowman's capsule, wherea the distal nephron connects to a collecting tubule.
Lengtheningy of the excretory tubule gives rise to the proximal convoluted tubule, loop of Henle, and the distal convoluted tubule.
The kidneys develop in the pelvis but appear to «as-cend» into the abdomen as a result of fetal growth of the lumbar and sacral regions.
The upper and largest part of the urogenital sinus becomes the urinary bladder, which is initially continuous with the allantois. Later the lumen of the allantois becomes obliterated. The mucosa of the trigone of the bladder is formed by the incorporation of the caudal mesonephric ducts into the dorsal bladder wall. This mesodermal tissue is eventually replaced by endodermal epithelium so that the entire lining of the bladder is of endodermal origin. The smooth muscle of the bladder is derived from splanchnic mesoderm.
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Mile urethra is anatomically divided into three portions: prostatic membranous, and spongy (penile).
The prostatic urethra, membranous urethra, and proximal penile urethra develop from the narrow portion of the urogenital sinus below the urinary bladder. The distal spongy urethra is derived from the ectodermal cells of the glans penis.
Fimale urethra: The upper two-thirds develops from the esonephric ducts, and the lower portion is derived from the ogenital sinus.
New words
urinary system – мочевая система
kidneys – почки
bladder – мочевой пузырь
excretory ducts – выделительные трубочки
pronephros – первичная почка
urogenital – мочеполовой
The urinary system: kidneys
The urinary system is the major system involved in the excretion of metabolic waste products and excess water from the body It is also important in maintaining a homeostatic balance of fluids and electrolytes. The urinary system consists of two kidneys, two ureters, the urinary bladder, and the urethra. Urine is produced by the kidneys and is then transmit ted via the ureters to the bladder for temporary storage The urethra is the final pathway that conveys urine to the exterior. This system also has an important endocrine function in the production of renin and erythropoietin, which influence blood pressure and red blood cell (RBC) formation, respec tively.
Each kidney is composed of stroma and parenchyma. The stroma consists of a tough fibrous connective tissue capsule and a delicate interstitial connective tissue com posed of fibroblasts, wandering cells, collagen fibrils, and a hydrated proteoglycan extracellular matrix, which is collec tively called the renal interstitium The parenchyma consists of more than one million elaborate uriniferous tubules that represent the functional units of the kidney.
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The kidney contains a hilum, a cortex, and a medula. The hilum is located medially and serves entrance as the point of entrance and exit for the renal artery, renal veins, and ureter. The renal pelvis, the expanded upper, divides into two or three entrance into the kidney. These, in turn, divide into eight minor calyces.
The cortex forms the outer zone of the kidney.
The medulla appears as a series of medullary pyramids. Two or three pyramids may unite to form a papilla. Uriniferous tubules consist of two functionally related portions called the nephron and the collecting tubule
Glomerulus is made up of several anastomotic capillary loops interposed between an afferent and an efferent arteriole. Plasma filtration occurs in the glomerulus.
Bowman's capsule consists of an inner visceral layer and an outer parietal layer. The space between these layers, the urinary space, is continuous with the renal tubule.
Visceral layer is apposed to the glomerulus and closely follows the branches of the glomerular capillaries. The visceral layer is composed of a single layer of epithelial cells resting on a basal lamina, which is fused with the basal lamina of the capillary endothelium. The cells of the visceral layer, call podocytes.
Cytoplasmic extensions of podocytes rest on the basal lamina.
Between adjacent pedicles, a thin slit diaphragm assists in preventing large plasma proteins from escaping from the vascular system.
In fact, most of the components of the glomerular filtrate are reabsorbed in the proximal tubule. Loop of Henle is a hairpin loop of the nephron that extends into the medulla and consists of thick and thin segments. The thick proximal portion of Henle's loop, or the descending thick segment, is a direct medullary continuation of the cortical proximal convoluted tubule.
The thick distal portion of the loop of Henle, the ascending thick segment, ascends to the cortex and is continuous with distal convoluted tubule. The major function of the distal tubule is to reabsorb soduim and chloride from the tubular filtrate. Collecting tubules consist of arched and straight segments.
New words
urea – моча
stroma – строма
parenchyma – паренхима
fibrous capsule – волокнистая капсула
delicate – тонкий
interstitial – промежуточный
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