An analysis of in vivo cell migration during teleost fin morphogenesis

In the teleost embryo the pectoral fin bud initially displays an apical ectodermal ridge along its entire distal margin. The ridge subsequently becomes transformed into an apical fold as the distal ectodermal epithelium grows and folds to enclose an extracellular space between the apposed basal surfaces of the epithelium. Collagen fibrils up to 2 micron in diameter, termed 'actinotrichia', are deposited along the proximo-distal axis in two (dorsal and ventral) arrays. The actinotrichia are aligned parallel to one another with a regular spacing along the greater part of their length. Mesenchymal cells migrating distally from the base of the fin bud encounter the dorsal and ventral arrays of actinotrichia and move between them apparently using the fibrils as a substratum. The entire structure is transparent and, using the killifish Aphyosemion scheeli, we have investigated the migration of the mesenchymal cells between 135 and 220 h of development, using Nomarski interference contrast microscopy and time-lapse video recording. The number of cellular processes per cell increased significantly during the period of observation. These processes could be graded according to their diameters. Processes of diameter greater than 2 micron were not usually aligned along actinotrichia and arose at any aspect of the cell body. In contrast, processes with diameters less than 2 micron appeared to be confined to the distal aspects of the migrating cells and showed an increasing tendency to become aligned as development progressed. Time-lapse video recordings revealed that such aligned processes move faster (mean speed 17.98 (+/- 2.25) micron/h) than non-aligned processes (mean speed 4.66 (+/- 0.67) micron/h). Whole cell translocation was generally slower than rates of process movement: the lowest mean value (1.52(+/- 0.36) micron/h) was recorded between 135 and 160 h of development rising to a maximum mean rate (4.72(+/- 0.42) micron/h) between 195 and 220 h; the period of the fastest rate of cell translocation correlated with maximum process alignment along actinotrichia. Thin 1 micron plastic sections revealed that, generally, aligned processes were in close association with the surface of the actinotrichial fibrils and not the spaces between them.