VI.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
281
(melanocytes) in the urodele. This has been criticized by Dalton (1953)
and by Wilde (1955c) among others. Such a conversion implies a morphogenetic activity on the part of fully synthesized melanin granules
sufficient to overcome the macrophage habitus of the reacting cell and
the adoption of a completely different morphological and metabolic
pattern by the cell. As will be seen below, such conversions are on record,
but their demonstration requires far more stringent experimental
technique than observations on already pigmented animals. Furthermore, the occurrence of melanin vacuoles in the many cell types
mentioned previously, without concomitant change in morphology or
biochemistry of the differentiated cell type argues, albeit inconclusively,
against an active morphogenetic role for previously synthesized melanin
granules. Further discussion of this important point must be reserved
pending the acquisition of more conclusive data.
Xanthophores are yellowish pigment cells, commonly seen in
urodeles. They differentiate readily in neural crest explants. These cells
are morphologically similar to mature melanocytes but contain a sparse
distribution of melanin granules. Their colouration is due to a yellowish
pigment present in their cytoplasm. The identity of the yellow pigment
is obscure. In Ambystoma maculatum the pigment is apparently in
solution in the cytoplasm of the cell. In A. tigrinum it is more or less
particulate. The pigment in A. maculatum is soluble in ethanol. It can
be eluted from fixed cells in processes of routine cy tological preparation
(Wilde, 1955b). There is evidence from several sources (Niu, 1954;
Wilde, 1955b) that urodele xanthophores may continue to synthesize
melanin granules and become converted (with the disappearance of the
yellow pigment) into forms indistinguishable from melanocytes. Much
more information concerning this interesting cell group is required.
Guanophores and iridiophores are pigmented cells whose dynamics
and differentiation have been poorly studied. This is true also for certain
variously pigmented cells of teleost fish.
The more exotically pigmented cells are limited to certain groups of
'lower vertebrates' particularly teleost fish, amphibia and reptiles.
Colour variations in birds are due to differential reflections and absorptions of light quanta, the location of pigmented cells, the degree of
oxidation of melanin and the presence of accessory filtering cells.
However, all vertebrates have a common characteristic in the differentiation of melanin laden cells stemming from the migratory cells of the
neuro-epithelium. The melanoblast and mature melanocyte are common,
therefore, to all vertebrate forms.
DIFFERENTIATION OF VERTEBRATE PIGMENT CELLS
281
(melanocytes) in the urodele. This has been criticized by Dalton (1953)
and by Wilde (1955c) among others. Such a conversion implies a morphogenetic activity on the part of fully synthesized melanin granules
sufficient to overcome the macrophage habitus of the reacting cell and
the adoption of a completely different morphological and metabolic
pattern by the cell. As will be seen below, such conversions are on record,
but their demonstration requires far more stringent experimental
technique than observations on already pigmented animals. Furthermore, the occurrence of melanin vacuoles in the many cell types
mentioned previously, without concomitant change in morphology or
biochemistry of the differentiated cell type argues, albeit inconclusively,
against an active morphogenetic role for previously synthesized melanin
granules. Further discussion of this important point must be reserved
pending the acquisition of more conclusive data.
Xanthophores are yellowish pigment cells, commonly seen in
urodeles. They differentiate readily in neural crest explants. These cells
are morphologically similar to mature melanocytes but contain a sparse
distribution of melanin granules. Their colouration is due to a yellowish
pigment present in their cytoplasm. The identity of the yellow pigment
is obscure. In Ambystoma maculatum the pigment is apparently in
solution in the cytoplasm of the cell. In A. tigrinum it is more or less
particulate. The pigment in A. maculatum is soluble in ethanol. It can
be eluted from fixed cells in processes of routine cy tological preparation
(Wilde, 1955b). There is evidence from several sources (Niu, 1954;
Wilde, 1955b) that urodele xanthophores may continue to synthesize
melanin granules and become converted (with the disappearance of the
yellow pigment) into forms indistinguishable from melanocytes. Much
more information concerning this interesting cell group is required.
Guanophores and iridiophores are pigmented cells whose dynamics
and differentiation have been poorly studied. This is true also for certain
variously pigmented cells of teleost fish.
The more exotically pigmented cells are limited to certain groups of
'lower vertebrates' particularly teleost fish, amphibia and reptiles.
Colour variations in birds are due to differential reflections and absorptions of light quanta, the location of pigmented cells, the degree of
oxidation of melanin and the presence of accessory filtering cells.
However, all vertebrates have a common characteristic in the differentiation of melanin laden cells stemming from the migratory cells of the
neuro-epithelium. The melanoblast and mature melanocyte are common,
therefore, to all vertebrate forms.