Models of the segmental organization of the Xenopus pronephric and mammalian metanephric nephrons. The color coding of analogous nephron segments is based on the comparison of marker gene expression as shown in Figure 7. (a) Schematic representation of the stage 35/36 Xenopus pronephric kidney. The glomerular filtration apparatus (G; also known as glomus) is derived from the splanchnic layer of the intermediate mesoderm and receives blood from vessels that branch from the dorsal aorta. All other parts of the pronephric nephron are derivatives of the somatic layer of the intermediate mesoderm. On the basis of molecular markers, four distinct tubular compartments can be recognized. Each tubule may be further subdivided into distinct segments: proximal tubule (PT, yellow; PT1, PT2, and PT3), intermediate tubule (IT, green; IT1 and IT2), distal tubule (DT, orange; DT1 and DT2), and connecting tubule (CT, gray). The nephrostomes (NS) are ciliated peritoneal funnels that connect the coelomic cavity (C) to the nephron. The scheme was adapted from Reggiani and coworkers . (b) Scheme depicting a short-looped and a long-looped nephron of the adult mammalian metanephric kidney. The figure was taken and adapted from Kriz and Bankir . Abbreviations used for the mammalian nephron segments are as follows: ATL, ascending thin limb; CD, collecting duct; CNT, connecting tubule; DCT, distal convoluted tubule; DTL, descending thin limb; S1, S2, and S3, segments of the proximal tubule; TAL, thick ascending limb.