Genetic regulation of myelin basic protein expression in the mouse brain

Both the early embryo of the leech and the segmental ganglia of this annelid worm contain relatively large, identifiable cells accessible to physiological manipulation. In early development, the leech egg undergoes a stereotyped series of cleavages generating five bilateral pairs of segmental precursor blastomeres, the M (mesodermal) and N, O, P and Q (ectodermal) teloblasts. Subsequently, each teloblast contributes a column of progeny blast cells to a longitudinally arrayed set of cells called the germinal plate, from which the ventral nerve cord ganglia and other segmental tissues arise. By injecting tracer substances such as horseradish peroxidase or fluorescently labeled peptides into various teloblasts in the early embryo, it has been shown that each teloblast, including even the mesodermal precursor teloblasts (M), contribute a characteristic subset of neurons to the ipsilateral half-ganglia in normal development. This indication of a highly determinate development for the leech is largely supported by the neuronal deficits observed in teloblast ablation experiments. However, a number of observations suggest that the O and P teloblasts differ from the others in that they are of equal developmental pluripotency, and that the fates of their progeny are determined on the basis of a positional hierarchy within the developing embryo.