VI.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
285
above, there is a secondary reaggregation of pigmented cells into dense
clumps in vitro, which mimics the behaviour of the cells in their in vivo
pattern formation. Several other investigators have presented evidence
that pattern formation of this nature is influenced by factors resident
in topographically localized areas of the embryo. The predominating
concept which has arisen from these investigations (Delanney, 1941;
Lehman, 1950, 1952, 1957; Lehman and Youngs, 1952, 1959; Finnegan,
1955, 1958), is that the negative Chemotaxis or 'population pressure'
effect encourages migration of melanoblasts in a dorso-ventral direction
while an antagonistic effect, centred in the ventral midline and extending
in the hypomeric mesoderm ventro-dorsálly, provides an environment
antagonistic to the migration of pro-pigment cells into this area. The
strength of this antagonistic effect apparently varies between species.
The effect is strong in Tarichia torosa, very weak in Triturus rivularis
and intermediate in Ambystoma maculatum. These are the embryos
upon which most of the experimental work has been done.
Migration and pattern formation (anti-migration?) are to be considered
in this context as the result of intrinsic 'drives' in the melanoblasts
balanced against an extrinsic 'brake' in the ventral hypomere. In contrast to this concept, Rosin (1943) and Holtfreter (1939), have produced
evidence from which they interpret migration as due to 'attractive' or
positive chemotactic forces.
Finally, in this context there is evidence from DuShane (1943),
Delanney (1941) and Dalton and Krassner (1959), that extrinsic systems in epidermis and mesodermal cells strongly influence migration
and melanogenesis in melanoblasts. Thus an inhibitory process present
in white axolotl epidermis prevents melanization of widely spread
melanoblasts. The melanoblasts can be shown by reciprocal transplantations to be fully capable of melanogenesis under appropriate
conditions.
In the avian embryo evidence indicates (Rawles, 1948), that certain
skin organelles such as feather germs may serve as centres to which
migrating melanoblasts are attracted. This appears to be distinct from
the general location of melanoblasts in the dermis. The dichotomy of
locations is under genetic control (Rawles, 1945).
There is, furthermore, evidence that pigment cells show no peculiar
affinities for the anatomical areas they colonize. Internally migrating
cells may be transplanted to subsurface areas where they colonize and
pigment external structures such as feathers and hair (Rawles, 1944).
At the present time there is experimental evidence in support of at
least seven agencies active in melanoblast migration; an intrinsic factor
(negative affinity), a hypomeric factor, an epidermal factor, a mesodermal factor, attraction to specific organs or parts, a genetic dichotomy
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
285
above, there is a secondary reaggregation of pigmented cells into dense
clumps in vitro, which mimics the behaviour of the cells in their in vivo
pattern formation. Several other investigators have presented evidence
that pattern formation of this nature is influenced by factors resident
in topographically localized areas of the embryo. The predominating
concept which has arisen from these investigations (Delanney, 1941;
Lehman, 1950, 1952, 1957; Lehman and Youngs, 1952, 1959; Finnegan,
1955, 1958), is that the negative Chemotaxis or 'population pressure'
effect encourages migration of melanoblasts in a dorso-ventral direction
while an antagonistic effect, centred in the ventral midline and extending
in the hypomeric mesoderm ventro-dorsálly, provides an environment
antagonistic to the migration of pro-pigment cells into this area. The
strength of this antagonistic effect apparently varies between species.
The effect is strong in Tarichia torosa, very weak in Triturus rivularis
and intermediate in Ambystoma maculatum. These are the embryos
upon which most of the experimental work has been done.
Migration and pattern formation (anti-migration?) are to be considered
in this context as the result of intrinsic 'drives' in the melanoblasts
balanced against an extrinsic 'brake' in the ventral hypomere. In contrast to this concept, Rosin (1943) and Holtfreter (1939), have produced
evidence from which they interpret migration as due to 'attractive' or
positive chemotactic forces.
Finally, in this context there is evidence from DuShane (1943),
Delanney (1941) and Dalton and Krassner (1959), that extrinsic systems in epidermis and mesodermal cells strongly influence migration
and melanogenesis in melanoblasts. Thus an inhibitory process present
in white axolotl epidermis prevents melanization of widely spread
melanoblasts. The melanoblasts can be shown by reciprocal transplantations to be fully capable of melanogenesis under appropriate
conditions.
In the avian embryo evidence indicates (Rawles, 1948), that certain
skin organelles such as feather germs may serve as centres to which
migrating melanoblasts are attracted. This appears to be distinct from
the general location of melanoblasts in the dermis. The dichotomy of
locations is under genetic control (Rawles, 1945).
There is, furthermore, evidence that pigment cells show no peculiar
affinities for the anatomical areas they colonize. Internally migrating
cells may be transplanted to subsurface areas where they colonize and
pigment external structures such as feathers and hair (Rawles, 1944).
At the present time there is experimental evidence in support of at
least seven agencies active in melanoblast migration; an intrinsic factor
(negative affinity), a hypomeric factor, an epidermal factor, a mesodermal factor, attraction to specific organs or parts, a genetic dichotomy