Journal of Neuroscience, Vol 13, 1022-1034, Copyright © 1993 by Society for Neuroscience
Replacement of lateral line sensory organs during tail regeneration in salamanders: identification of progenitor cells and analysis of leukocyte activity
JE Jones and JT Corwin
Department of Neuroscience, School of Medicine, University of Virginia, Charlottesville 22908.
It has been proposed that supporting cells may be the progenitors of
regenerated hair cells that contribute to recovery of hearing in birds, but
regeneration is difficult to visualize in the ear, because it occurs deep
in the skull. Hair cells and supporting cells that are comparable to those
in the ear are present in lateral line neuromasts, and in axolotl
salamanders these cells are accessible to microscopic observation in vivo.
After amputation of a segment of the tail that contains neuromasts, cells
from the posteriormost neuromast on the tail stump divide rapidly and form
a migratory regenerative placode. The cells of the regenerative placode
represent a lineage that eventually produces both hair cells and supporting
cells in replacement neuromasts. We sought to identify the progenitors of
the regenerative placode by using differential interference contrast
microscopy combined with time-lapse video recording in living axolotl
salamanders. In response to amputation, the mantle-type supporting cells at
the posteroventral edge of the neuromast that is nearest to the wound
increased their frequency of cell division, and gave rise to the first
cells of the placode. The increase in mitotic activity of mantle-type
supporting cells was accompanied by an unexplained decrease in the
frequency of divisions in the same neuromast's population of internal
supporting cells. The time-lapse records suggested that the changes in the
mitotic activity of supporting cells might have been linked to the presence
of phagocytic leukocytes in the vicinity of the neuromast that was nearest
to the wound. Leukocytes were evenly distributed around control neuromasts,
but during regeneration leukocyte activity increased significantly in the
vicinity of the posterior half of the posteriormost neuromast. The
redistribution of leukocytes occurred early in the regenerative response,
but a causal role for the leukocytes has not been conclusively established.
It is possible that the leukocytes could contribute to the formation of the
regenerative placode at that location by breaking down the glycocalyx that
ensheaths the outermost cells of the neuromast, or through the secretion of
mitogenic growth factors.
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