274
CHARLES
Ε.
WILDE
cell division in a broad sense. This statement is rarely accepted with
reserve. Any cell which retains a viable nucleus should be considered
capable of division, regardless of its cytoplasmic load. The author has
published serial photographs of nuclear division with incomplete
cytokinesis in mature differentiated striated muscle (Wilde, 1959).
Figure 1 is a photomicrograph of metaphase in a living young melanocyte with a heavy cytoplasmic load of previously synthesized melanin
granules. Following completion of cytokinesis both cells returned to the
dendritic form characteristic of mature melanocytes with an even
redistribution and dispersion of the granules.
It is well known that cells whose normal mature state is not characterized by evidence of cell division, do increase by orderly multiples the
load of nuclear DNA (Mirsky and Ris, 1951. See Brächet, 1957 for
a discussion). Mature neurones fall into this category. Such orderly
increases in DNA loading are best explained by intranuclear duplication
and reduplication of chromosomes (Brächet, 1957 for critical discussion)
without the co-ordinate development of a functioning division apparatus.
Other examples such as mitosis with cytokinesis in mature cartilage
cells could also be cited. The co-ordinated processes which are required
for cellular division are not necessarily broadly antagonistic to cellular
differentiation processes but they modify, hasten or slow, or render
incomplete mitosis and cytokinesis. Surely the one considered as a
discrete process does not absolutely impede the other.
IV. The Morphological Characteristics of Pigment Cells
A. The Melanoblast
The classical description of melanoblasts in their early stages of
differentiation is to be found in the reports of the activities of the early
urodele neural crest. Descriptive material stems, in large part, from
explantation studies into simple saline solutions or into nutrient solutions
(DuShane, 1935). Early neural crest cells are indistinguishable from
other dorsal cells of the embryo in form or movement. Cells in the outer
ectodermal layers often show a moderate amount of amorphous pigment,
derived in part from the 'surface coat' (Holtfreter, 1943), or at least
closely associated with it. While this material is granular and exists
characteristically in small irregular clumps, it does not show the
homogeneity of granule size and distribution characteristic of mature
melanocytes. Indeed, it is not known whether this material is chemically
identical with the granular melanin of the mature pigment cell. Early
neural crest cells are, like all other urodele cells at this time, ameboid
and are not mutually adhesive. They wander readily on glass by the
CHARLES
Ε.
WILDE
cell division in a broad sense. This statement is rarely accepted with
reserve. Any cell which retains a viable nucleus should be considered
capable of division, regardless of its cytoplasmic load. The author has
published serial photographs of nuclear division with incomplete
cytokinesis in mature differentiated striated muscle (Wilde, 1959).
Figure 1 is a photomicrograph of metaphase in a living young melanocyte with a heavy cytoplasmic load of previously synthesized melanin
granules. Following completion of cytokinesis both cells returned to the
dendritic form characteristic of mature melanocytes with an even
redistribution and dispersion of the granules.
It is well known that cells whose normal mature state is not characterized by evidence of cell division, do increase by orderly multiples the
load of nuclear DNA (Mirsky and Ris, 1951. See Brächet, 1957 for
a discussion). Mature neurones fall into this category. Such orderly
increases in DNA loading are best explained by intranuclear duplication
and reduplication of chromosomes (Brächet, 1957 for critical discussion)
without the co-ordinate development of a functioning division apparatus.
Other examples such as mitosis with cytokinesis in mature cartilage
cells could also be cited. The co-ordinated processes which are required
for cellular division are not necessarily broadly antagonistic to cellular
differentiation processes but they modify, hasten or slow, or render
incomplete mitosis and cytokinesis. Surely the one considered as a
discrete process does not absolutely impede the other.
IV. The Morphological Characteristics of Pigment Cells
A. The Melanoblast
The classical description of melanoblasts in their early stages of
differentiation is to be found in the reports of the activities of the early
urodele neural crest. Descriptive material stems, in large part, from
explantation studies into simple saline solutions or into nutrient solutions
(DuShane, 1935). Early neural crest cells are indistinguishable from
other dorsal cells of the embryo in form or movement. Cells in the outer
ectodermal layers often show a moderate amount of amorphous pigment,
derived in part from the 'surface coat' (Holtfreter, 1943), or at least
closely associated with it. While this material is granular and exists
characteristically in small irregular clumps, it does not show the
homogeneity of granule size and distribution characteristic of mature
melanocytes. Indeed, it is not known whether this material is chemically
identical with the granular melanin of the mature pigment cell. Early
neural crest cells are, like all other urodele cells at this time, ameboid
and are not mutually adhesive. They wander readily on glass by the