The relationship between abnormalities of pigmentation and of the inner ear

Abstract

Abnormalities of pigmentation and of the inner ear occur together in inherited syndromes in several mammals, including man. The pigmentary abnormalities are always of the white spotting kind, which is distinct from albinism, although the entire skin and fur may be affected. The inner ear abnormalities are confined to the cochlea and the saccule, and follow a certain pattern. Recently, the author obtained some evidence suggesting that this association might be based on the common origin of the melanocytes and a part of the acoustic ganglion in the neural crest: the abnormalities of the ear were looked for in two spotted mutants in the mouse in which the neural crest had been experimentally shown to be abnormal, and they were found. The questions arose: how widespread were abnormalities of the ear in animals with spotting, and what were the requirements in size and locality of spots for the involvement of the ear? To answer these the inner ear was examined in mice of 15 genotypes covering a wide range with regard to the degree and locality of spotting. Abnormalities were found in all heavily spotted genotypes. Of much greater interest were the observations that the pigmentation of the inner ear was also always affected; that the spotting of the coat was not a good guide to this internal 'spotting'; and that the severity and extent of the inner ear abnormalities were much more closely related to the internal than to the external spotting. Moreover, most genotypes had their own pattern of internal 'spotting'. No observation was made that was incompatible with the hypothesis that the neural crest might be the link between the two components of the syndrome. The chief alternative explanation, that the pigment or the pigment cells have some unknown function in the inner ear, cannot be entirely ruled out, but it has several weighty arguments against it. This study has also explained why the two components of the syndrome sometimes appear to be separated. It has shown that the problem of the nature and origin of spotting in mammals is even more complex than was thought before. The evolutionary significance of the relationship between pigmentation and the inner ear is pointed out.