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CHARLES Ε.
WILDE
measured. Whether exhaustion of its melanin load into epidermal cells,
feather or hair, leads to the extinction of this particular serving cell with
the differentiation to maturity of succeeding series of 'reserve' melanoblasts which carry on is not known. Reserve melanoblasts exist and
there is continuing melanogenesis. Whether this reserve flow is the only
source of new melanin or whether any melanocyte can continually
synthesize for an extended period of time is a question whose answer
requires further data.
The transmission and deposition of melanin into other cells is a well
established phenomenon. The means of transfer is not well known but
it is thought to be a direct transfer or injection of melanin granules into
receptor cells from the dendritic tips of melanocytes. Actual observation
of such transfer has not been reported. This leads to the consideration
of alternative methods such as contact induction of melanin synthesis in
contiguous non-neural crest cells. This method is not proposed here as a
true interpretation but is the logical antithesis of the direct, transmembrane passage of preformed melanin granules.
Melanin granules are tolerated well by many types of differentiated
cells. Usually they appear as isolated clumps but occasionally in amphibia or in tumours, macrophages can be observed, actively moving
with their characteristic lobopodia, which are densely packed with
clumped melanin granules. In amphibians in the author's laboratory
melanin clumps or vacuoles have been observed in chondrocytes,
odontoblasts and striated muscle. The specific functional activity of
these cells does not appear to have been curtailed. Completely synthesized melanin is a very inert substance in a metabolic sense. Fully
synthesized melanin granules are probably devoid of morphogenetic
activity and are sequestered from the metabolic activities of the cell
containing them. While it is true that melanin can be bleached by
chemical processes there is no evidence that activity of this sort occurs
in living systems to any appreciable degree.
There are other pigmented cells in vertebrates. These are the cells of
the pigmented retina, (tapetum nigrum) melanophages (Niu and Twitty,
1950), and the xanthophores, guanophores and iridiophores of aquatic
or semi-aquatic lower vertebrates.
The differentiative activities of pigmented retinal cells is a distinct
subject and will not be discussed here. The reader is referred to the
reports of Stone (1957).
Melanophages are macrophages which have ingested by phagocytosis, melanin granules fabricated at some other cellular site. These
cells are already differentiated as macrophages prior, to their functional
phagocytosis of the pigment. Niu and Twitty (1950) presented some
evidence that cells of this nature became converted into pigment cells
CHARLES Ε.
WILDE
measured. Whether exhaustion of its melanin load into epidermal cells,
feather or hair, leads to the extinction of this particular serving cell with
the differentiation to maturity of succeeding series of 'reserve' melanoblasts which carry on is not known. Reserve melanoblasts exist and
there is continuing melanogenesis. Whether this reserve flow is the only
source of new melanin or whether any melanocyte can continually
synthesize for an extended period of time is a question whose answer
requires further data.
The transmission and deposition of melanin into other cells is a well
established phenomenon. The means of transfer is not well known but
it is thought to be a direct transfer or injection of melanin granules into
receptor cells from the dendritic tips of melanocytes. Actual observation
of such transfer has not been reported. This leads to the consideration
of alternative methods such as contact induction of melanin synthesis in
contiguous non-neural crest cells. This method is not proposed here as a
true interpretation but is the logical antithesis of the direct, transmembrane passage of preformed melanin granules.
Melanin granules are tolerated well by many types of differentiated
cells. Usually they appear as isolated clumps but occasionally in amphibia or in tumours, macrophages can be observed, actively moving
with their characteristic lobopodia, which are densely packed with
clumped melanin granules. In amphibians in the author's laboratory
melanin clumps or vacuoles have been observed in chondrocytes,
odontoblasts and striated muscle. The specific functional activity of
these cells does not appear to have been curtailed. Completely synthesized melanin is a very inert substance in a metabolic sense. Fully
synthesized melanin granules are probably devoid of morphogenetic
activity and are sequestered from the metabolic activities of the cell
containing them. While it is true that melanin can be bleached by
chemical processes there is no evidence that activity of this sort occurs
in living systems to any appreciable degree.
There are other pigmented cells in vertebrates. These are the cells of
the pigmented retina, (tapetum nigrum) melanophages (Niu and Twitty,
1950), and the xanthophores, guanophores and iridiophores of aquatic
or semi-aquatic lower vertebrates.
The differentiative activities of pigmented retinal cells is a distinct
subject and will not be discussed here. The reader is referred to the
reports of Stone (1957).
Melanophages are macrophages which have ingested by phagocytosis, melanin granules fabricated at some other cellular site. These
cells are already differentiated as macrophages prior, to their functional
phagocytosis of the pigment. Niu and Twitty (1950) presented some
evidence that cells of this nature became converted into pigment cells