284
CHARLES Ε.
WILDE
These experiments were interpreted as conclusive of negative chemotactic responses among melanoblasts stemming from metabolic processes
integral to the cells themselves. The initial spacing in vitro and in vivo
was concluded to be due to properties inherent in the differentiating
melanoblasts.
It must be borne in mind, in considering these experiments that the
starting cellular material consisted of already migrating melanoblasts
which were, at the start of the capillary experiment, recognizable to
the experimenters as melanoblasts or propigment cells. Thus inferences
regarding the kinetics of migration of neural crest cells prior to this
time cannot be made. Furthermore, Twitty (1945) has previously
reported that pigment cells differentiate far more completely in coelomic
fluid than in simple saline solutions. Also it must be considered that the
maintenance of the improvised diffusion gradient throughout the
experiment was not tested. Finally, the spacing of the cells was confined
by the design of the experiment to a linear pattern. No evidence was
adduced concerning two or three dimensional spacing. As a consequence,
the action of the micro-environment itself was not subject to test
(extrinsic factors), nor was any pattern other than rectilinear reported.
Even within the confines of a capillary tube, which is larger by far than
the least diameter of the included cells, a spacing pattern referable to a
helix of radius identical to the internal radius of the tube could occur,
whereby the spacing referable to some chord along the gyres would be
equally expected, as well as a rectilinear pattern along one side of the
capillary. For the moment however, there is ground for assuming that
metabolic influences stemming from the differentiating cells themselves
may be a factor in their intermediate migratory behaviour.
Melanoblasts, as all other cells, migrate along some substratum. The
physical nature of these surfaces is varied. In artificial systems, glass or
an air-fluid interface is an effective substitute. In embryos these surfaces
are irregular and consist of cells, intercellular materials and cellular
products. The substratum itself is a highly active metabolic material
with rapid exchanges between protoplasm and micro-environment, as
well as exchange between migrating cell and substratum. It must be
assumed that exchanges between migrating melanoblasts and microenvironment will influence the migratory behaviour of the cells as well
as their differentiation. There is evidence of this nature which will next
be examined.
Twitty (1945, 1949; Twitty and Niu, 1948), in the course of his studies
on pigment cell migration, has compared both the initial and later
activities of pigmented cells in urodele species with widely differing
larval colour patterns. He has shown, in species with concentrated bars
of pigment cells, that following the initial migratory dispersion discussed
CHARLES Ε.
WILDE
These experiments were interpreted as conclusive of negative chemotactic responses among melanoblasts stemming from metabolic processes
integral to the cells themselves. The initial spacing in vitro and in vivo
was concluded to be due to properties inherent in the differentiating
melanoblasts.
It must be borne in mind, in considering these experiments that the
starting cellular material consisted of already migrating melanoblasts
which were, at the start of the capillary experiment, recognizable to
the experimenters as melanoblasts or propigment cells. Thus inferences
regarding the kinetics of migration of neural crest cells prior to this
time cannot be made. Furthermore, Twitty (1945) has previously
reported that pigment cells differentiate far more completely in coelomic
fluid than in simple saline solutions. Also it must be considered that the
maintenance of the improvised diffusion gradient throughout the
experiment was not tested. Finally, the spacing of the cells was confined
by the design of the experiment to a linear pattern. No evidence was
adduced concerning two or three dimensional spacing. As a consequence,
the action of the micro-environment itself was not subject to test
(extrinsic factors), nor was any pattern other than rectilinear reported.
Even within the confines of a capillary tube, which is larger by far than
the least diameter of the included cells, a spacing pattern referable to a
helix of radius identical to the internal radius of the tube could occur,
whereby the spacing referable to some chord along the gyres would be
equally expected, as well as a rectilinear pattern along one side of the
capillary. For the moment however, there is ground for assuming that
metabolic influences stemming from the differentiating cells themselves
may be a factor in their intermediate migratory behaviour.
Melanoblasts, as all other cells, migrate along some substratum. The
physical nature of these surfaces is varied. In artificial systems, glass or
an air-fluid interface is an effective substitute. In embryos these surfaces
are irregular and consist of cells, intercellular materials and cellular
products. The substratum itself is a highly active metabolic material
with rapid exchanges between protoplasm and micro-environment, as
well as exchange between migrating cell and substratum. It must be
assumed that exchanges between migrating melanoblasts and microenvironment will influence the migratory behaviour of the cells as well
as their differentiation. There is evidence of this nature which will next
be examined.
Twitty (1945, 1949; Twitty and Niu, 1948), in the course of his studies
on pigment cell migration, has compared both the initial and later
activities of pigmented cells in urodele species with widely differing
larval colour patterns. He has shown, in species with concentrated bars
of pigment cells, that following the initial migratory dispersion discussed