Structure and distribution of interstitial cells (presumptive interrenal cells) in the opisthonephric kidneys of larval and adult sea lamprey, Petromyzon marinus L

The opisthonephric kidneys of larval and adult sea lamprey, Petromyzon marinus L., possess diffusely arranged interstitial cells having all the morphological features of steroid-secreting cells, namely abundant smooth endoplasmic reticulum, lipid droplets, and mitochondria with vesiculate to tubular cristae. The kidney does not show any 3β-hydroxysteroid dehydrogenase activity but the cells are sudanophilic and stain with Nile blue. These cells are always closely associated to the arterial circulation and can be found within the renal corpuscle and among the kidney tubules. Morphological evidence suggests a resemblance to interrenal cells of other vertebrates. Lack of any demonstration of 3β-hydroxysteroid dehydrogenase activity may be explained by the fact that only small numbers of cells are normally present in lampreys. The evolutionary implications of these findings are discussed in relation to the distribution of interrenal tissue in other vertebrates.