Cell proliferation and morphogenesis of the apical ectodermal ridge in the pectoral fin bud of the trout embryo (Salmo trutta fario L.).

Cell proliferation has been studied in the pectoral fin bud of the brown trout (Salmo trutta fario L.). A single injection of tritiated thymidine into the coelom was performed at stages 160, 180 or 200. Fixations were made at increasing intervals between 30 min and 12 days after the injection. The incubation temperature was 7° C, stage 244 being attained 12 days after injection at stage 160.Graphs of the variations of the labelling index, the mitotic index and the percentage of labelled mitoses as a function of the time after injection of tritiated thymidine at stage 160 have been drawn for the periderm, the basal layer of the ectoderm and the mesoderm. The duration of the cell cycles and of their phases were deduced from the graphs. Results are summarized in the table.Knowledge of the parameters of cell proliferation in the mesoderm and the ectoderm of the fin bud is of great interest for the analysis of morphogenetic mechanisms leading to the formation of the apical ectodermal ridge. The dorsal ectoderm of the bud proliferates intensely between stages 158 and 167, and this might lead to the initiation of the formation of the apical ectodermal ridge. Following that first period, the ventral ectoderm, in turn, proliferates more actively than the dorsal one between stages 170 and 209. In the subjacent mesoderm two proliferative crises occur at stages 161.75 and 170, thus probably causing or permitting the extension of the dorsal and ventral parts of the ectoderm. This alternative proliferative activity of dorsal and ventral ectoderm finally results in the formation of an apical ectodermal ridge that becomes elongated into an apical fold, which itself will give rise to the swimming paddle of the fin including its skeletal elements (lepidotrichia andactinotrichia).