Background: In vertebrate embryos, migration of trunk neural crest cells (NCC) proceeds mainly in two streams: a dorsoventral path between the neural tube and somites, and a dorsolateral one between somites and ectoderm. This last pathway is taken by melanocyte precursor cells (MPC) homing the skin, while pigment cells seeding internal organs and the peritoneal wall follow the dorsoventral pathway. Early routes taken by subpopulations of NCC have been well documented using the quail-chick chimaera system and monoclonal antibodies to NCC. However, very little is known about the advanced migratory behavior of MPC, which determines their late distribution patterns at different embryonic axial levels.
Methods: Histological sections of neck, thorax, and abdomen of 6.5 to 9 day quail embryos submitted to DOPA reaction (tyrosinase activity) were used. In four concentric areas--dorsal and ventrally subdivided--the relative density of MPC was determined by morphometric methods.
Results: The relative regional density of MPC from their individualization as DOPA-positive putative pigment cells until their definitive seeding in the epidermis showed a progressively higher cell density from deeper to peripheral zones in all three levels studied, with peaks of cell density suggesting a centrifugal pattern occurring in at least two waves of migratory cells.
Conclusions: The spatial distribution of the MPC varies according to both the axial level and the developmental stage of the embryo. Furthermore, the general pattern of centrifugal distribution observed might be attributed to a different timing of cell differentiation closely related to their migratory behavior.