Observations on the differentiationin vitro of mouse metanephros.

Metanephros of mouse embryos was explanted at the 11th day of gestation and culturedin vitro on the surface of a plasma clot for up to 7 days. Microscopic sections were cut from cultures fixed every day; the differentiation process was followed and compared with development under normal conditions. The ureter branches out and gives rise by means of dichotomous division to straight canals, lined by a light, columnar epithelium, which correspond to collecting ducts. Around the blind ends of their terminal branches, mesenchyme condenses and forms caps, in which the layer of cells in contact with the ducts acquires a palisade-like appearance. Subsequently, vesicles differentiate from the caps, representing the first rudiment of the nephron. The next stage is the change of the vesicle to an S-shaped tubule, the "lower" end of which becomes crescentic or bowl-shaped and is the first anlage of Bowman's capsule: the "upper" end connects with the collecting duct.The developmentin vitro goes on at a much slower rate than in normal conditions, and the nephron does not go beyond the stage of S-shaped tubule; however, the general pattern of development is the same in both situations, except for the absence in culture of blood vessels and therefore also of the glomerular anlage. It should be assumed that glomerular vessels are not necessary in order to shape the first anlage of Bowman's capsule. The connection of the nephron with the collecting duct, generally recognized to occur at the S-tubule stage, was sometimes already found at the vesicle stage, both in cultures and in normal development.