Clonal analysis and cell lineages of the vertebrate central nervous system.

Abstract

Very little progress in tracing cell lineages in the CNS of vertebrates had been made before the introduction of horseradish peroxidase (HRP) as an intra­ cellular lineage tracer in the frog embryo (Jacobson & Hirose 1978, 1981, Hirose & Jacobson 1979). Conceptual weaknesses and lack of adequate tech­ niques reinforced one another to ensure slow progress (Jacobson 1978, p. 98). Thus, a distinction has traditionally been made between embryos that have development, in which lineage determines cell fate, and those with regulative development, in which cell interactions determine cell fate, but, in fact, most if not all embryos show a combination of both (Harrison 1933, Weiss 1939, Davidson 1976). I shall propose that there are several lineage modes with different degrees of autonomy. In addition to the determinate lineage mode. which is completely autonomous and in which the relationship between a given progenitor and the phenotype(s) of its descendants is all-or­ none, there are others called indeterminant lineage modes in which the rela­ tionship between ancestral cells and the phenotypes of their descendants is contingent on external conditions to some degree. It is now clear that cell interactions can result in changes in of cellular fates in Uroand Cephalo­ chordates (Tung et a11958, 1960, Nakauchi & Takashita 1983), in which there is rigorous proof that materials localized in the egg and differentially distrib­ uted into different lineages determine the ultimate fates of the differentiated cells (Whittaker 1973, 1979. Jeffery 1983, Jeffery et al 1983). Such unim­ peachable evidence of extensive regulation in mosaic embryos should help to open the minds of those who have been unwilling to admit that determination