VI.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
283
the physical parameters of the in vitro system, and on a glass substratum
there are some indications of the formation of a tissue system.
In the melanocyte zone the pigmented cells appear to be evenly
dispersed and in general are more or less equidistant from each other.
They become intensely black and dendritic. In ageing cultures certain
melanocytes may show a strong recondensation of the pigment in the
perinuclear cytoplasm, repeating the state known from in vivo experiments as 'punctate' (Dalton, 1953). The foregoing description is made
from personal observations in the author's laboratory on cultures of
Ambystoma maculatum. In other species a certain variance from this
pattern is seen (Twitty, 1945, 1949).
A migratory habit is therefore established for all neural crest derivative cells with the hint of a differential developing between the maturing
cell types.
The migratory pattern described above obviously cannot account for
any but the simplest pigmentary patterns. Patterns of bars, spots, and
stripes are commonly observed in nature. A fruitful field of analysis
has developed concerning the nature of the responses in melanoblasts,
leading to their dispersal or selective concentration in particular
anatomic areas or structures. The now classical studies of V. H. Twitty
are germane and fundamental to this area of pigment cell kinetics. In a
long series of studies Twitty and his collaborators and students have
reported on the development of pattern both in vitro and in vivo (see
Lehman and Youngs, 1959, for a literature list). According to their
studies the initial migration of melanoblasts is independent of future
patterning. The cells tend to disperse themselves peripherally yet
equidistant from each other. If a number of the cells are placed in a
restrictive environment, such as that imposed by a film supporting a
fragment of cover glass the dispersion pattern is similar but with wider
spacing. It was inferred from these studies that a mutual antagonism,
probably of a metabolic nature, was active in the initial spatial patterning of the differentiating cells. These observations were followed by a
delicate series of experiments whereby one, two or three cells, following
initial lobopodial migration and onset of differentiative processes, were
taken up mechanically in small capillary tubes (Twitty and Niu, 1954).
The medium of the tubes was coelomic fluid. The experimenter had the
option of sealing one end, spacing the cells in various positions along the
tube, or creating arbitrary diffusion gradients at the open end of the
tube. A characteristic gradient was coelomic fluid in the tube diffusing
into simple saline solution (Flickinger, 1949). Single cells were found to
carry out aimless movements before settling down to begin to complete
their differentiation. Two or three cells moved so as to space themselves
widely along the tube, both into and away from the diffusion gradient.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
283
the physical parameters of the in vitro system, and on a glass substratum
there are some indications of the formation of a tissue system.
In the melanocyte zone the pigmented cells appear to be evenly
dispersed and in general are more or less equidistant from each other.
They become intensely black and dendritic. In ageing cultures certain
melanocytes may show a strong recondensation of the pigment in the
perinuclear cytoplasm, repeating the state known from in vivo experiments as 'punctate' (Dalton, 1953). The foregoing description is made
from personal observations in the author's laboratory on cultures of
Ambystoma maculatum. In other species a certain variance from this
pattern is seen (Twitty, 1945, 1949).
A migratory habit is therefore established for all neural crest derivative cells with the hint of a differential developing between the maturing
cell types.
The migratory pattern described above obviously cannot account for
any but the simplest pigmentary patterns. Patterns of bars, spots, and
stripes are commonly observed in nature. A fruitful field of analysis
has developed concerning the nature of the responses in melanoblasts,
leading to their dispersal or selective concentration in particular
anatomic areas or structures. The now classical studies of V. H. Twitty
are germane and fundamental to this area of pigment cell kinetics. In a
long series of studies Twitty and his collaborators and students have
reported on the development of pattern both in vitro and in vivo (see
Lehman and Youngs, 1959, for a literature list). According to their
studies the initial migration of melanoblasts is independent of future
patterning. The cells tend to disperse themselves peripherally yet
equidistant from each other. If a number of the cells are placed in a
restrictive environment, such as that imposed by a film supporting a
fragment of cover glass the dispersion pattern is similar but with wider
spacing. It was inferred from these studies that a mutual antagonism,
probably of a metabolic nature, was active in the initial spatial patterning of the differentiating cells. These observations were followed by a
delicate series of experiments whereby one, two or three cells, following
initial lobopodial migration and onset of differentiative processes, were
taken up mechanically in small capillary tubes (Twitty and Niu, 1954).
The medium of the tubes was coelomic fluid. The experimenter had the
option of sealing one end, spacing the cells in various positions along the
tube, or creating arbitrary diffusion gradients at the open end of the
tube. A characteristic gradient was coelomic fluid in the tube diffusing
into simple saline solution (Flickinger, 1949). Single cells were found to
carry out aimless movements before settling down to begin to complete
their differentiation. Two or three cells moved so as to space themselves
widely along the tube, both into and away from the diffusion gradient.