Formation of the chick mesonephros

Summary

The post-induction pattern of nephron organization was analysed by microdissection of 6- and 8-day chick embryo mesonephroi marked in two transversal planes by a standard technique allowing the exact determination of homologous portions. It was proved possible to distinguish four nephron populations with Malpighian corpuscles situated on the surface of the kidney, which were arranged in a ventrodorsal sequence according to age. These populations formed practically two thirds of the mesonephros. A fifth (the youngest and very heterogenous) population was localized deep in the mesonephros forming most of the dorsal half of the organ. The correct determination of the sequence of the nephron populations according to their origin was verified by the corresponding gradient of the lengths of their tubules. The oldest population (V1) constituting a ventral surface of mesonephros was the largest; it comprized in average 29±1 nephrons and stretched from the cranial to the caudal pole. The cranial narrower part of the mesonephros (crp) was composed exclusively of the V1 nephrons. The other nephron populations occurred only in the caudal, growing part of the mesonephros. The total number of nephrons per mesonephros was close to one hundred (97±7). In all isolated nephrons, the glomerulus was found to be firmly connected to the intermediate segment of the tubule at the site of the second main flexure. Developmental irregularities, as rudimentary or hypertrophic nephrons occured in crp only, but the abnormality known as “two-headed nephron” was distributed more or less at random in the mesonephros with the mean frequency 6.5% of the isolated nephrons.