Urogenital Ridge

Ureteric bud

Figure 14.2 A. Relationship of the intermediate mesoderm ofthe pronephric, mesone-phric, and metanephric systems. In cervical and upper thoracic regions intermediate mesoderm is segmented; in lower thoracic, lumbar, and sacral regions it forms a solid, unsegmented mass of tissue, the nephrogenic cord. Note the longitudinal collecting duct, formed initially by the pronephros but later by the mesonephros. B. Excretory tubules ofthe pronephric and mesonephric systems in a 5-week-old embryo.

Metanephros: The Definitive Kidney

The third urinary organ, the metanephros, or permanent kidney, appears in the fifth week. Its excretory units develop from metanephric mesoderm (Fig. 14.4) in the same manner as in the mesonephric system. The development of the duct system differs from that of the other kidney systems.

Collecting System. Collecting ducts of the permanent kidney develop from the ureteric bud, an outgrowth of the mesonephric duct close to its entrance to the cloaca (Fig. 14.4). The bud penetrates the metanephric tissue, which is molded over its distal end as a cap (Fig. 14.4). Subsequently the bud dilates, forming the primitive renal pelvis, and splits into cranial and caudal portions, the future major calyces (Fig. 14.5, A and B).

Each calyx forms two new buds while penetrating the metanephric tissue. These buds continue to subdivide until 12 or more generations of tubules have formed (Fig. 14.5). Meanwhile, at the periphery more tubules form until the

Urogenital mesentery

Mesentery Gut

Excretory tubule of mesonephros

Mesonephric duct

Parameso-nephric duct

Excretory tubule of mesonephros

Mesonephric duct

Genital Ridge

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.

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