ORIGIN OF MELANOPHORES AND THEIR RÔLE IN DEVELOPMENT OF COLOR PATTERNS IN VERTEBRATES

Abstract

Author''s conclusion: "From the data reviewed in this paper we are led to conclude that in the vertebrates, in general, melanin pigmentation in its great variety of forms is produced by one type of highly specialized cell, the melanophore, which originates in the embryonic neural crest. That the melanophores found in all of the vertebrates are homologous cells is evidenced by their origin, by their capacity for extensive migration in their early undif-ferentiated (melanoblast) stages when indistinguishable from mesenchyme cells, by their general morphology and physiology and, above all, by their capacity to synthesize melanin pigment granules and deposit them in epidermal cell. The data indicate strongly that all melanoblasts of any one individual are initially alike and potentially capable of synthesizing melanin granules. Whether or not this potentiality is realized depends to a great extent upon the particular tissue substrate with which they become associated, in other words, their locus. In the lower, coldblooded vertebrates, differentiation of melanoblasts into melanophores is, in general, wide-spread and occurs in a variety of deeper visceral tissues, in the dermis and in the epidermis. In the higher, warm-blooded vertebrates melanophore differentiation is much more limited and occurs primarily in the epidermis and its specialized derivative, feathers and hair. Associated with the development of these keratinized epidermal structures is the loss of the ability of the melanophore to shift its contents in effecting color changes. Like any other embryonic cell of the individual, the melanoblast is endowed with a complement of genes and its development and ultimate differentiation are dependent upon the activity of these genes, subject at all times to correlative influences from adjacent or contiguous tissues and to influences from the external environment. While the action of genes in the control of morphological and color patterns in general has not been completely clarified, there is much evidence that genes act by determining the presence or absence of specific enzymes which guide metabolic processes along particular paths or channels. The interpretation, towards which recent investigations on the development of melanin pigmentation in the vertebrates appear to be leading, is that color patterns arise through a constant interaction between melanoblasts and their tissue substrates and to reactions between the melanoblasts themselves. The complement of genes with which a melanoblast of a particular genotype is endowed, provide it with certain reaction potencies which determine the particular type of response that can be elicited by a particular tissue substrate. The physiological condition of the substrate may be influenced or conditioned by numerous factors, hormones, vitamins and various stimuli from the external environment, such as temperature, light, etc. Changes in the physiological condition of the substrate can elicit changes or modifications in melanophore response, but the specific response is always in accordance with the genie constitution of the melanophoire.