RESUMO
In the urodele Ambystoma mexicanum, the pronephric duct (PND) is formed from a coherent group of cells that migrate from the pronephros to the cloaca along a pathway immediately ventral to the developing somites. The guidance cues used by the migrating PND primordium to find the cloaca are a local property of the migration substratum, are temporally regulated, and are both polarized and oriented. Since the pronephric duct migrates between two tissues--the underlying lateral mesoderm and the overlying epidermis--we performed a study to identify the tissue(s) in which PND guidance cues originate. Through direct manipulation of the epidermis overlying the duct pathway, we show that the migrating PND reads epidermally derived cues (1) along the anterior-posterior axis that direct migration from anterior to posterior and (2) along the dorsal-ventral axis that constrain migration to the duct pathway. Heterochronic grafting experiments reveal that the ability to direct PND migration is a stable property of flank epidermis throughout the period of PND migration. Epidermal cues are, therefore, not responsible for the observed temporal restrictions on PND migration. Thus, the region of the embryo within which the advancing PND tip can migrate actually represents an area where two distinct but required sets of PND migration cues overlap. The epidermis overlying the duct pathway provides directional information; temporal restriction of duct migration is hypothesized to be a property of the flank mesoderm.
