Cellular differentiation in the anuran pronephros

Ribonucleic acid (RNA) is detectable histochemically by means of methyl green pyronin staining (controlled with ribonuclease) in the cytoplasm of the proximal tubules of the frog (Rana pipiens). This is seen at Shumway stage 23 when the proximal tubules become differentiated from the remainder of the tubular complex. RNA levels remain high during the process of pronephric differentiation, dropping when this process is completed. The onset of specific renal functions (phenol red transport; protein resorption) during the period of high RNA levels raises the question of the role of RNA in the development of renal function as well as cellular differentiation. Histochemical events accompanying differentiation in the pronephros (periodic acid‐Schiff staining of the brush borders and the appearance of protein resorption droplets) are similar to those found in differentiating meso‐ and metanephric kidneys. Mitochondria in the undifferentiated pronephros are found to be granular in form by vital staining with Janus green B and with iron hematoxylin staining. These organelles do not consolidate into rodlets as found in differentiated meso‐ and metanephric kidneys but remain in their original granular form, as demonstrated by the techniques noted above. Granular mitochondria have been demonstrated in the differentiated pronephros of a month old tadpole.