The development of the human kidney is a complex process that requires interactions between epithelial and mesenchymal cells, eventually leading to the coordinated growth and differentiation of multiple highly specialized stromal, vascular and epithelial cell types. The application of molecular biology and immunocytochemistry to the study of cell types involved in renal morphogenesis is leading to a better understanding of nephrogenesis, which requires a fine balance of many factors that can be disturbed by various prenatal events in humans. The aim of this paper is to review human kidney organogenesis, with particular emphasis on the sequence of morphological events, on the immunohistochemical peculiarities of nephron progenitor populations and on the molecular pathways regulating the process of mesenchymal to epithelial transition. Kidney development can be subdivided into five steps: 1) the primary ureteric bud; 2) the cap mesenchyme; 3) the mesenchymal-epithelial transition; 4) glomerulogenesis and tubulogenesis; 5) the interstitial cells. Complex correlations between morphological and molecular events from the origin of the ureteric bud and its branching to the metanephric mesenchyme, ending with the maturation of nephrons, have been reported in different animals, including mammals. Marked differences, observed among different species in the origin and the duration of nephrogenesis, suggest that morphological and molecular events may be different in different animal species and mammals. Further studies must be carried out in humans to verify at the morphological, immunohistochemical and molecular levels if the outcome in humans parallels that previously described in other species.
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