ABSTRACT
The pronephros (Figure 5.1) The human pronephros is a transient rudimentary organ present in embryos 25-32 days old, which are 2.5-4 mm long. Pronephric nephrons are clusters of mesodermal cells attached laterally to the stalks of somites C1-T1. In each segment, there is only one pronephric cell cluster. In segments C5-T1, ectodermal buds invaginate from surface ectoderm to the pronephric clusters, become detached from the surface, become attached to the mesodermal pronephric cells and then interconnect craniocaudally into a longitudinal tubule, which represents the primary ureter (Wolffian duct). The primary ureters grow caudally and enter the hindgut in embryos 28-32 days old. In this way, the terminal portion of the hindgut changes into the cloaca. By the end of the 5th week the pronephric cell clusters (rudimentary nephrons) disappear, while the primary ureters remain preserved, and are known as the mesonephric, Wolffian ducts.
