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CHARLES Ε.
WILDE
C. The Migration of Melanoblasts
One öf the major attributes of the melanoblast is its profound ability
to migrate over relatively long distances from its origin in the neural
crest. The migratory activity of these cells is one of their most outstanding physiological and developmental attributes. In general the
cells carry out the greater part of their migratory activities during their
colourless melanoblastic phase. Evidence of their final location in the
embryo or adult is in general dependent upon the synthesis of pigment.
This criterion necessarily does not identify reserves of melanoblasts, nor
does it give any insight into further migration of these reserves.
Evidence concerning the migration of melanoblasts comes from in
vitro studies and from genetic and transplantation studies in a variety of
vertebrate forms. It will be convenient here to discuss selected experiments illustrating the data from the several types of analysis. A
beginning can be made by discussing in vitro behaviour of the cells
migrating from a fragment of neuro-epithelium of the urodele explanted
to nutrient medium. Differentiation in nutrient media is superior to
that seen in simple saline solutions (Wilde, 1952; Twitty, 1945), even
though the early cells have a store of yolk.
Cells wander by ameboid activity with broad lobopodia from a
fragment adherent to glass in a hanging or 'standing' drop. The cells
find the glass substratum sufficient for movement. In general the movement tends in a peripheral direction away from the cell mass of the
explant. Disaggregated cells placed at random on glass do not show
peripheral migration, rather their movement is completely at random.
In peripheral movement away from an explant the lobopodial form of
movement, after two or three days, is replaced by fine filopodia and the
cells increase their surface in contact with the glass. Mitosis is frequent.
Migration by filopodial extension continues until major melanin granule
synthesis begins. Migratory activity slows as the cells complete their
differentiation. In a standard explant culture the most mature pigment
cells tend to be found at the periphery and it is here, in general (and in
the explant itself), that the first melanocytes can be seen. In a majority
of cases the pigment cells are most dense in the peripheral zone while
ectomesenchymal cells lie between them and the explant. This area is
often traversed by nerve axones upon which many ectomesenchyme
cells wrap themselves in differentiating as Schwann cells. An overall
view then would show the central explant, a zone relatively free of
pigment cells but filled with ectomesenchyme, Schwann cells and
axons, and a peripheral zone where melanocytes predominate. Xanthophores are to be found in mature cultures interspersed with melanocytes
but tending to lie at the inner edge of the melanocyte zone. Thus within
CHARLES Ε.
WILDE
C. The Migration of Melanoblasts
One öf the major attributes of the melanoblast is its profound ability
to migrate over relatively long distances from its origin in the neural
crest. The migratory activity of these cells is one of their most outstanding physiological and developmental attributes. In general the
cells carry out the greater part of their migratory activities during their
colourless melanoblastic phase. Evidence of their final location in the
embryo or adult is in general dependent upon the synthesis of pigment.
This criterion necessarily does not identify reserves of melanoblasts, nor
does it give any insight into further migration of these reserves.
Evidence concerning the migration of melanoblasts comes from in
vitro studies and from genetic and transplantation studies in a variety of
vertebrate forms. It will be convenient here to discuss selected experiments illustrating the data from the several types of analysis. A
beginning can be made by discussing in vitro behaviour of the cells
migrating from a fragment of neuro-epithelium of the urodele explanted
to nutrient medium. Differentiation in nutrient media is superior to
that seen in simple saline solutions (Wilde, 1952; Twitty, 1945), even
though the early cells have a store of yolk.
Cells wander by ameboid activity with broad lobopodia from a
fragment adherent to glass in a hanging or 'standing' drop. The cells
find the glass substratum sufficient for movement. In general the movement tends in a peripheral direction away from the cell mass of the
explant. Disaggregated cells placed at random on glass do not show
peripheral migration, rather their movement is completely at random.
In peripheral movement away from an explant the lobopodial form of
movement, after two or three days, is replaced by fine filopodia and the
cells increase their surface in contact with the glass. Mitosis is frequent.
Migration by filopodial extension continues until major melanin granule
synthesis begins. Migratory activity slows as the cells complete their
differentiation. In a standard explant culture the most mature pigment
cells tend to be found at the periphery and it is here, in general (and in
the explant itself), that the first melanocytes can be seen. In a majority
of cases the pigment cells are most dense in the peripheral zone while
ectomesenchymal cells lie between them and the explant. This area is
often traversed by nerve axones upon which many ectomesenchyme
cells wrap themselves in differentiating as Schwann cells. An overall
view then would show the central explant, a zone relatively free of
pigment cells but filled with ectomesenchyme, Schwann cells and
axons, and a peripheral zone where melanocytes predominate. Xanthophores are to be found in mature cultures interspersed with melanocytes
but tending to lie at the inner edge of the melanocyte zone. Thus within