Development and evolution of the adrenal gland and its homologs in teleosts, anurans, chelonians and birds

The morphology, histology, histochemistry and immunohistochemistry of the adrenal gland or its homolog were examined during development and in the adults of species belonging to the teleost fishes, anuran amphibians, chelonian reptiles and birds (chicken). The first objective was to clarify the ontogenetic origin of the anatomical differences observed among the adrenal glands of vertebrates. A second objective was to investigate the anatomical relationships of the gland with the renal system, which plays an important role during development. In the course of vertebrate evolution the adrenal gland emerges from the renal parenchyma and becomes more concentrated. Three main points may be stressed concerning this evolutionary process: 1- During ontogenesis, the kidneys show different patterns for controlling water loss and excreting waste through embryonic and adult life: pronephros, mesonephros and metanephros, which develop from intermediate mesoderm following a precise temporal and spatial sequence, each kidney being formed as a result of an inductive interaction with the previous form. 2- In teleosts the ontogenetic origin of the adrenocortical cells has been found in the mesoderm on the surface of the pronephros; later these cells become internal to the head kidney and make contact with the chromaffin cells deriving from the neural crests. The latter may also spread inside the kidney. In tetrapods, on the contrary, the peritoneal mesoderm gives rise to steroidogenic tissue at a more caudal level with respect to the pronephros, and chromaffin cells reach this area. The adrenal gland always retains the original position external to the mesonephric or metanephric kidney. 3- The anatomy of the adrenal gland varies independently of the renal system evolution. Teleosts and amphibians have a definitive mesonephric kidney, while they display different adrenal structural patterns. Conversely, amphibians and chelonians share the same type of gland, which is scattered over the ventral renal surface, although they do have, respectively, definitive mesonephric and metanephric kidneys. In the other reptiles, in birds and in mammals, the adrenal gland constitutes a discrete organ with little or no contact with the metanephric kidney. Structural differences among the adrenal glands can be explained by developmental modifications of the site of origin of the steroidogenic cells, the possible differentiation of connective tissue cells isolating the developing adrenal gland from the kidney and the length of the glandular blastema. Chromaffin cells migrating from neural crests also play a role in the final configuration pattern of the gland. Possible evolutionary advantages of the various structural adrenal types are suggested.