Metanephric Blastema

Figure 14.3 A. Transverse section through the urogenital ridge in the lower thoracic region of a 5-week embryo showing formation of an excretory tubule of the mesonephric system. Note the appearance of Bowman's capsule and the gonadal ridge. The mesonephros and gonad are attached to the posterior abdominal wall by a broad urogenital mesentery. B. Relation of the gonad and the mesonephros. Note the size of the mesonephros. The mesonephric duct (wolffian duct) runs along the lateral side of the mesonephros. C. Scanning electron micrograph of a mouse embryo showing the genital ridge (arrow) and mesonephric duct (arrowheads).K, kidneys.

Hindgut

Allantois

Mesonephric tic-oí i r\

Mesonephric duct

Hindgut

Mesonephric tic-oí i r\

Mesonephric duct

Allantois

Urorectal Septum Malformation

Metanephric blastema

Figure 14.4 Relation of the hindgut and cloaca at the end of the fifth week. The ureteric bud penetrates the metanephric mesoderm (blastema).

Urorectal septum

Cloaca Ureteric bud

Metanephric blastema

Figure 14.4 Relation of the hindgut and cloaca at the end of the fifth week. The ureteric bud penetrates the metanephric mesoderm (blastema).

Metanephric Mesoderm Kidney
Figure 14.5 Development of the renal pelvis, calyces, and collecting tubules of the metanephros. A. 6 weeks. B. At the end of the sixth week. C. 7 weeks, D. Newborn. Note the pyramid form of the collecting tubules entering the minor calyx.

end of the fifth month. The tubules of the second order enlarge and absorb those of the third and fourth generations, forming the minor calyces of the renal pelvis. During further development, collecting tubules of the fifth and successive generations elongate considerably and converge on the minor calyx, forming the renal pyramid (Fig. 14.5D). The ureteric bud gives rise to the ureter, the renal pelvis, the major and minor calyces, and approximately 1 million to 3 million collecting tubules.

Metanephric Tissue Cap
Figure 14.6 Development of a metanephric excretory unit. Arrows, the place where the excretory unit (blue) establishes an open communication with the collecting system (yellow), allowing flow of urine from the glomerulus into the collecting ducts.

Excretory System. Each newly formed collecting tubule is covered at its distal end by a metanephric tissue cap (Fig. 14.6A). 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. 14.6, B and 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. The proximal end of each nephron forms Bowman's capsule, which is deeply indented by a glomerulus (Fig. 14.6, C and 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. 14.6, E and 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.

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 lobula-tion disappears during infancy as a result of further growth of the nephrons, although there is no increase in their number.

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