Kidney Systems

Three slightly overlapping kidney systems are formed in a cranial to caudal sequence during intrauterine life in humans: the pronephros, mesonephros, and metanephros. The first of these systems is rudimentary and nonfunctional; the second may function for a short time during the early fetal period; the third forms the permanent kidney.

Pronephros

At the beginning of the fourth week, the pronephros is represented by 7 to 10 solid cell groups in the cervical region (Figs. 14.1 and 14.2).

These groups form vestigial excretory units, nephrotomes, that regress before more caudal ones are formed. By the end of the fourth week, all indications of the pronephric system have disappeared.

Paraxial mesoderm

Intermediate mesoderm

Somite

Paraxial mesoderm

Intermediate mesoderm

Somite

Somatic meso-derm

Paraxial Lateral Intermediate Mesoderm

Internal glomerulus

Endoderm

Figure 14.1 Transverse sections through embryos at various stages of development showing formation of nephric tubules. A. 21 days. B. 25 days. Note formation of external and internal glomeruli and the open connection between the intraembryonic cavity and the nephric tubule.

Somatic meso-derm

Internal glomerulus

Nephric tubule

External glomerulus

Endoderm

Figure 14.1 Transverse sections through embryos at various stages of development showing formation of nephric tubules. A. 21 days. B. 25 days. Note formation of external and internal glomeruli and the open connection between the intraembryonic cavity and the nephric tubule.

Mesonephros

The mesonephros and mesonephric ducts are derived from intermediate mesoderm from upper thoracic to upper lumbar (L3) segments (Fig. 14.2). Early in the fourth week of development, during regression of the pronephric system, the first excretory tubules of the mesonephros appear. They lengthen rapidly, form an S-shaped loop, and acquire a tuft of capillaries that will form a glomerulus at their medial extremity (Fig. 14.3A). Around the glomerulus the tubules form Bowman's capsule, and together these structures constitute a renal corpuscle. Laterally the tubule enters the longitudinal collecting duct known as the mesonephric or wolffian duct (Figs. 14.2 and 14.3).

In the middle of the second month the mesonephros forms a large ovoid organ on each side of the midline (Fig. 14.3). Since the developing gonad is on its medial side, the ridge formed by both organs is known as the urogenital ridge (Fig. 14.3). While caudal tubules are still differentiating, cranial tubules and glomeruli show degenerative changes, and by the end of the second month the majority have disappeared. In the male a few of the caudal tubules and the mesonephric duct persist and participate in formation of the genital system, but they disappear in the female (see Genital System; p. 337).

Segmented intermediate mesoderm (pronephric system)

Segmented intermediate mesoderm (pronephric system)

Vitelline duct

Vitelline Duct

Allantois Cloaca

Mesonephric duct

Unsegmented mesoderm (metanephric system)

Vestigial pronephric system

Vitelline duct

Allantois Cloaca

Unsegmented intermediate mesoderm (mesonephric system)

Mesonephric duct

Unsegmented mesoderm (metanephric system)

Vestigial pronephric system

Unsegmented intermediate mesoderm (mesonephric system)

Allantois Cloaca

Meso-nephric excretory units

Mesonephric duct

Ureteric bud

Meso-nephric excretory units

Mesonephric duct

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