Cell Lineages and Determinants of Cell Fate in Development

Abstract

The cell lineage phenomenon in ascidian embryos appears to be based in large part on the occurrence of maternally derived egg cytoplasmic determinants. These factors are localized in certain cytoplasmic regions of the zygote and are differentially segregated during early cleavages into specific later tissues and regions of the embryo, where they presumably play some role in establishing the selective developmental fate of larval tissues. Commitment to muscle differentiation is correlated with segregation of a discrete “myoplasmic” region of the zygote; developmental autonomy (self-differentiation) of isolated muscle lineage cells confirms a very early commitment to the restricted fates suggested by the classic cell lineage map. Transfer of myoplasm to other cell lineages by artificial displacement results in some conversion of those cells to muscle expression. Larval muscle acetylcholinesterase, one of the transformation markers used, originates from newly synthesized acetylcholinesterase mRNA at gastrulation; this mRNA first appears only in muscle lineage cells. Indirect evidence suggests that the muscle determinant is a positively acting control factor related to the expression of this and other muscle genes