MODEL SYSTEMS FOR THE STUDY OF KIDNEY DEVELOPMENT - USE OF THE PRONEPHROS IN THE ANALYSIS OF ORGAN INDUCTION AND PATTERNING

Most vertebrate organs, once formed, continue to perform the function for which they were generated until the death of the organism. The kid ney is a notable exception to this rule. Vertebrates, even those that do not undergo metamorphosis, utilize a progression of more complex ki dneys as they grow and develop. This is presumably due to the changing conditions to which the organism must respond to retain what Homer Sm ith referred to as our physiological freedom. To quote, ''Recognizing that we have the kind of blood we have because we have the kind of kid neys we have, we must acknowledge that our kidneys constitute the majo r foundation of our physiological freedom. Only because they work the way they do has it become possible for us to have bones, muscles, glan ds, and brains. Superficially, it might be said that the function of t he kidneys is to make urine; but in a more considered view one can say that the kidneys make the stuff of philosophy itself'' (''From Fish t o Philosopher,'' Little, Brown and Co., Boston, 1953). Different kidne ys are used to make the stuff of philosophy at different stages of dev elopment depending on the age and needs of the organism, rather than t he usual approach of simply making embryonic organs larger as the anim al grows. Although evolution has provided the higher vertebrates with complex adult kidneys, they continue to utilize simple kidneys in embr yogenesis. In lower vertebrates with simple adult kidneys, even more s imple versions are used during early developmental stages. In this rev iew the anatomy, development, and gene expression patterns of the embr yonic kidney, the pronephros, will be described and compared to the mo re complex kidney forms. Despite some differences in anatomy, similar developmental pathways seem to be responsible for the induction and th e response to induction in both evanescent and permanent kidney forms. Gene expression patterns can, therefore, be added to the morphologica l and functional data indicating that all forms of the kidney are clos ely related structures. Given the similarities between the development of simple and complex kidneys, the embryonic kidneys may be an ideal model system in which to investigate the genesis of multicomponent org an systems. (C) 1997 Academic Press.