Abstract
Horseradish peroxidase injected into individual blastomeres of 128-, 256-, and 512-cell embyros of Xenopus laevis was identified in cells of the central nervous system (CNS) at early to middle larval stages. Labeled cells were dispersed, mingled with unlabeled cells. Four boundaries in the CNS could be defined by the behavior of clones of labeled cells: in the transverse plane at the level of the isthmus; in the horizontal plane between dorsal and ventral regions extending the entire length of the CNS; in the dorsal midline extending the entire length; and in the ventral midline of rhombencephalon and spinal cord but absent more rostrally. Cells injected with HRP at the 512-cell stage produced clones that, with rare exceptions, did not cross any boundary, whereas labeled clones initiated at earlier stages frequently crossed boundaries. Axons and dendrites were not restricted by these boundaries. These boundaries subdivided the CNS into seven compartments, each of which was occupied exclusively by the descendants of a group of 14 to 26 blastomeres in the 512-cell embryo. These groups of blastomeres formed a bilaterally symmetrical pattern composed of a single anterior median group straddling the dorsal midline near the animal pole and three groups on each side. Because cells mingled in each compartment but not across compartmental boundaries, there was a one-to-one relationship between individual blastomeres and CNS compartments but one-many and many-one relationships between individual blastomeres and neuroanatomical subdivisions smaller than a compartment. There was no constant relationship between phenotypes of nerve cells and their ancestry from individual blastomeres of the 512- cell or earlier stages.
