It is well known that renal development occurs in three stages, each one countersigned by a transient, tubular organ. These stages are pronephros, mesonephros, and metanephros. To avoid spending too long describing this topic, here we will only consider data specifically to do with the subject of segmentation.

In very low stages of the zoological scale there are tubes that are for the external discharge of toxic materials. In invertebrates, like some worms (annelid), there are nephridia in each disc or body segment, which are able to excrete the toxic portion of protein metabolism. At a higher level of the zoological scale, it becomes necessary to also eliminate excess water from the body. This new function is performed independently from the excretory function. The filtration ducts run independently and are isolated compared to the excretory ones. Only vertebrates have a true and proper organ for those two (and other) functions. The kidney does not discharge products outside the body, but inside at cloacal level (Fig. 6.1). The phylogenetic transition of the urinary apparatus is evident in sharks, in which the nephridia of every somite does not open outside but flows into a discharging duct (Wolff duct) that opens in the cloaca. The fusion of those two structures and functions in a single organ are seen clearly in reptiles, birds, and mammals.

The pronephros, or primordial kidney, works only in fish, with nephridia for every somite in the body. It is the first of three similar tubular organs involved in the genito-urinary apparatus. The pronephros disappears in all vertebrates and is substituted by the mesonephos. The mesonephos is definitive in some animals, such as amphibians, while it is intermediate in man and in other superior animals (mid kidney). It transforms in shape and function only partially and is replaced by the metanephros, which is the last of the three similar tubular organs to be transformed. The metanephros is located in the pelvic zone and will evolve into the definitive kidney.

In mammals the definitive kidney works to filtrate and excrete simultaneously. The two tubular structures are separated and dislocated over a very short period of time in the metanephric evolution and then in quick succession they come together after forming the definitive morphological and functional unit for filtration and excretion. This ensures that products are ejected from the body and a constant water balance with the normal amount of water in fluids and tissues is maintained.

In man the transformation of metanephros in the definitive kidney occures within the fifth week of intrauterine life (i.e., fetal life) and it is complete at the second month of gestation. This increase and improvement in the kidney lasts beyond birth.

The aforementioned transformation is countersigned by two fundamental stages that concern kidney segmentation. At the cloacal level a diverticulum appears from the Wolff duct (residue from the pronephros and mesonephros duct). This is initially solid and later channeled, growing postero-laterally until it reaches the metanephrogenic bonnet or metanephric blastema of the posterior mesodermic derivation. The meeting of those two structures induces a strong stimulus of differentiation with S-shaped tubular formations, which spread cranially embracing capillary branches, and primitive arterious glomeruli. Distally they increasingly grow in the direction of the Wolff duct diverticulum, which is now completely patent and tubular. The superior end of this tube, after having spread, enlarges in ampullary form and creates the primitive pelvis. It divides into primitive major calyces (Bellini collecting ducts), eventually becoming nephrogenic structures, which come from the metanephrogenic cup [166–168].

Phylogenetic studies show the formation of several renal lobules, each one containing parenchyma drained by a calyx, and vascularized by one or two arteries and veins (frog kidney). In other animals, such as young elephants, a lobar constitution is recognizable (Fig. 6.2).

In the majority of animals, man included, the kidney appears compact with the external area being generally smooth and conglobate. However, the intrinsic structure of mammal kidneys reveals the parenchyma is formed by an external part, the cortex, which encloses the internal part, the pyramid or medulla, and is drained by a minor calyx. The cortex contains glomeruli and tubules while the pyramid contains only tubules. The whole pyramid, together with the surrounding cortex, and the discharging calyx constitute a lobe that is not externally visible. A very thin connective layer with some smooth muscle cells invisibly separates one lobe from its neighbors [169–171]. Two, or in exceptional cases more, pyramids may be drained by the same calyx, especially at the poles. Commonly in human kidneys there are 12 or 13 lobes (Fig. 6.3).