Congenital anomalies are thus extremely important and must not only be recognized,but understood in the urological practice.

Renal Development

Basic to normal renal development is the presence of a urogenital ridge – also known as the Wolffian body (see Fig. 1.1). It contains the nephric, gonadal, and genital ductal primordia. Within this urogenital ridge, three distinct and

Mesonephros

The second stage of nephric development is the mesonephros. It is the principal excretory organ during the fourth to eighth week of embryonic life. When the mesonephric tubules from the mesonephros become connected to this pronephric duct, its name is changed to mesonephric or Wolffian duct. This single structure gives rise to the entire male genital tract and “gives birth” to the ureter, which, in turn, is essential in the development of a normal kidney. Thus, in this early embryonic stage, faulty development of the mesonephric duct is probably related to the development of many of the genitourinary anomalies.

Figure 1.1. relationship of Wolffian body with gonadal and adrenal primordia.

Metanephros

The metanephros is the third and final phase of the nephric systems and the primordium of the normal kidney. Its normal differentiation into the metanephrogenic blastema is entirely dependent upon the normal development and cranial growth of the ureteral bud into this mass of tissue. The metanephros thus caps this invaginating and branching ureteral bud and gives rise to the true renal cortex – that is, the glomeruli, proximal convoluted tubules, Henle’s loop, and the distal convoluted tubules.

Mesonephros differentiates at the end of the third week regresses in the eighth week

Development of the Collecting System

The ureteral bud arises from the dorsal surface of the mesonephric duct a short distance from the cloacal wall. It initially bifurcates into two branches, which are the forerunners of the major calyces, with the point of bifurcation eventually forming the renal pelvis (see Fig. 1.3).

Between these two primary divisions of the ureter one or two additional divisions usually occur to form additional major calyces. From the ampullary enlargement of each of these primary divisions, two to four secondary tubules develop. These in turn give rise to tertiary tubules and the process is repeated until the fifth month of fetal life, when an estimated 12 generations of tubules have developed.

The pelvis and the primary as well as secondary tubules enlarge greatly during this development. The primary divisions become the major calyces, and the secondary divisions give rise to the minor calyces. The tubules of the third and fourth order are absorbed into the walls of the enlarging secondary tubules so that the tubules of the fifth order (a total of 20–30 in number) open into the minor calyces as papillary ducts. The remaining order of tubules constitutes the collecting tubules which form the greater part of the medulla of

Ureteral bud

Metanephros

begins to differentiate in the fifth week

Mesonephric (Wolffian duct)

Metanephros

Bladder

Hind-gut

Cloacal membrane

Developing

major

Pelvis calyces

Figure 1.3. Ureteral bud and renal collecting system development.

the adult kidney. Remember, at the point the collecting tubules meet and fuse with distal convoluted tubules (ureteral bud meets metanephrogenic blastema) is where congenital renal cystic disease has its origin.