VI.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
275
extrusion of colourless, clear ectoplasmic lobopodia. Holtfreter (1946),
has studied these phenomena in extenso. The point of interest here lies
in the fundamental change in locomotory activity which takes place in
these cells as they undergo their differentiation. The output of blunt,
rolling pseudopodia gradually ceases and the former activity is replaced
by the output of filopodia which may become exceedingly long. The
establishment of filopodia and the cessation of output of lobopodia is
used in the author's laboratory as a convenient criterion of the onset of
differentiative processes. It is convenient to consider this point in the
process as indicating the onset of overt differentiation but it should not
therefore be inferred that invisible preceding metabolic steps have not
been carried out; nor is the development of filóse pseudopodia unique
to neural crest cells but is quite a standard performance for all cells
with the probable exception of macrophages. These tend to retain the
lobopodial type of ameboid extension in their mature state.
Melanoblasts are at this time indistinguishable from the other
differentiating cells of the crest, although neuroblasts will shortly begin
to produce their characteristic axon outgrowths. Ectomesenchyme cells
also become flattened and produce filopodia. As will be discussed below,
there ensues a characteristic pattern of migratory movement in vitro
and a moderate sorting out of melanoblasts from ectomesenchyme and
neuroblasts takes place. Migratory movement next slows, and in the
perinuclear area of appropriate cells discrete, yellow-brown, homogeneously sized melanin granules are synthesized. These granules often
lie in parallel rows in the immediate perinuclear cytoplasm (Fig. 2).
This orientation is not essential to melanin granule synthesis but is a
common finding. It perhaps reflects orientational stresses in the late
migrating cytoplasm, or the positioning of filamentous mitochondria.
The melanoblasts are not at this time truly dendritic. This phenomenon
is of somewhat later onset and apparently somewhat separable from
melanogenesis itself.
The perinuclear onset of melanogenesis is followed by further synthesis
of melanin granules. The granules themselves are to be found further
and further outward in the extending cytoplasm. At this time the cell
becomes dendritic with ramifying processes and the withdrawal of the
colourless filopodia. Mitosis may still take place.
It has not been established that synthesis of melanin granules
(although uniformly initiated in the perinuclear cytoplasm) is solely
seated in this region of the cell or whether a more generally dispersed
synthesis occurs during later differentiation. The author inclines to the
latter view although a crucial experimental test has yet to be made.
The reasons for this opinion lie in analysis of time-lapse motion pictures
of differentiating melanoblasts. Following initial perinuclear synthesis
K
A.M.I
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
275
extrusion of colourless, clear ectoplasmic lobopodia. Holtfreter (1946),
has studied these phenomena in extenso. The point of interest here lies
in the fundamental change in locomotory activity which takes place in
these cells as they undergo their differentiation. The output of blunt,
rolling pseudopodia gradually ceases and the former activity is replaced
by the output of filopodia which may become exceedingly long. The
establishment of filopodia and the cessation of output of lobopodia is
used in the author's laboratory as a convenient criterion of the onset of
differentiative processes. It is convenient to consider this point in the
process as indicating the onset of overt differentiation but it should not
therefore be inferred that invisible preceding metabolic steps have not
been carried out; nor is the development of filóse pseudopodia unique
to neural crest cells but is quite a standard performance for all cells
with the probable exception of macrophages. These tend to retain the
lobopodial type of ameboid extension in their mature state.
Melanoblasts are at this time indistinguishable from the other
differentiating cells of the crest, although neuroblasts will shortly begin
to produce their characteristic axon outgrowths. Ectomesenchyme cells
also become flattened and produce filopodia. As will be discussed below,
there ensues a characteristic pattern of migratory movement in vitro
and a moderate sorting out of melanoblasts from ectomesenchyme and
neuroblasts takes place. Migratory movement next slows, and in the
perinuclear area of appropriate cells discrete, yellow-brown, homogeneously sized melanin granules are synthesized. These granules often
lie in parallel rows in the immediate perinuclear cytoplasm (Fig. 2).
This orientation is not essential to melanin granule synthesis but is a
common finding. It perhaps reflects orientational stresses in the late
migrating cytoplasm, or the positioning of filamentous mitochondria.
The melanoblasts are not at this time truly dendritic. This phenomenon
is of somewhat later onset and apparently somewhat separable from
melanogenesis itself.
The perinuclear onset of melanogenesis is followed by further synthesis
of melanin granules. The granules themselves are to be found further
and further outward in the extending cytoplasm. At this time the cell
becomes dendritic with ramifying processes and the withdrawal of the
colourless filopodia. Mitosis may still take place.
It has not been established that synthesis of melanin granules
(although uniformly initiated in the perinuclear cytoplasm) is solely
seated in this region of the cell or whether a more generally dispersed
synthesis occurs during later differentiation. The author inclines to the
latter view although a crucial experimental test has yet to be made.
The reasons for this opinion lie in analysis of time-lapse motion pictures
of differentiating melanoblasts. Following initial perinuclear synthesis
K
A.M.I