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Journal of Neuroscience, Vol 13, 3193-3210, Copyright © 1993 by Society for Neuroscience
The retinal fate of Xenopus cleavage stage progenitors is dependent upon blastomere position and competence: studies of normal and regulated clones
S Huang and SA Moody
Department of Anatomy and Neuroscience Program, George Washington University Medical Center, Washington, D.C. 20037.
The clonal origin of the stage 43-44 Xenopus retina from cleavage stage
precursors was quantitatively assessed with lineage tracing techniques. The
retina descends from a specific subset of those blastomeres that form
forebrain. The most animal dorsal midline cell (D1.1.1) produced about half
of the retina, the three other dorsal ipsilateral blastomeres each produce
about an eighth of the retina, and the four contralateral dorsal
blastomeres and an ipsilateral ventral-animal cell together produce the
remaining eighth of the retina. There was no significant spatial
segregation of the clones derived from different progenitors in either the
anterior-posterior or dorsal-ventral axes of the retina and no boundaries
between clones were observed. Instead, the clones intermixed to form
multiple radial subclones that were equivalent to those demonstrated by
marking optic vesicle progenitor cells (Holt et al., 1988; Wetts and
Fraser, 1988). This mosaic pattern was initiated by the beginning of
gastrulation, advanced in the neural plate, and virtually complete in the
optic vesicle. At optic vesicle stages cell movement within subclones was
restricted, resulting in the formation of lineally related columns of cells
in the mature retina. To determine if the blastomere progenitors are
determined to produce these retinal lineage patterns, the major retinal
progenitor (D1.1.1) was deleted bilaterally. About 60% of the tadpoles
developed normal- appearing eyes; of these the retinas in two-thirds were
normal in size and the rest were smaller. The blastomeres surrounding the
deleted D1.1.1 progenitors changed their contributions to retina in
different ways to effect a complete or partial restoration. Ventral
blastomeres, which normally contribute mainly to the tail, produced
substantial amounts of the retina while dorsal blastomeres, which normally
contribute mainly to the head, decreased their contribution to the retina.
To determine whether these changes in retinal lineage were due to changes
in blastomere position after the surgery, various other blastomeres were
deleted prior to lineage mapping. Dorsal-animal blastomeres took over the
retinal fate of their dorsal-vegetal neighbors after those neighbors were
deleted, but did not change fate after the deletion of their ventral-animal
neighbors. This result suggests that dorsal-animal blastomeres change
positional values in only one direction (dorsal to vegetal) after neighbor
cell deletion, and that retinal fate is dictated by blastomere position. To
test this hypothesis directly, different ventral and vegetal blastomeres,
which normally do not produce retina, were transplanted to the position of
D1.1.1.(ABSTRACT TRUNCATED AT 400 WORDS)
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