A scanning electron microscope survey of the origin of the primordial pronephric duct cells in the avian embryo.

In the avian embryo, the pronephric duct derives from the anterior part of a ridge that develops just lateral to the somites and segmental plate. The ridge extends from the sixth somite to Hensen's node and begins to form as the sixth somite is condensing. By the nine-somite stage, the cranial end of the ridge (for a length roughly equivalent to four or five somite diameters) is seen as a duct primordium of smooth, elongated cells that resemble the migrating cells of the caudal tip of the duct as it extends to the cloaca. These cells show a decreased attachment to the fibers of the interstitial matrix and an increased adhesion to other duct cells. By 10 somites, there is a well-formed pronephric duct rudiment at a time when the pronephric tubules have not yet begun to develop. Therefore, the avian pronephric duct has a separate origin from the pronephric tubules and may play an inductive role in the formation of pronephric tubules.