Growth changes in the myelin sheath of peripheral nerve fibres in fishes

A quantitative study of the growth of the myelin sheath of fibres of the lateral line nerve in Salmo trutta and Raia clavata has been made. Contrary to earlier reports, but in agreement with a recent investigation on mammalian nerves, the relative thickness of the myelin sheath is found to decrease progressively during growth as fibre size increases. Comparison between fishes of differing length demonstrates that the thickness of the myelin sheath for a given axon diameter is constant, irrespective of the size of the fish, for comparable sites in the nerve. Measurement of the internodal length of the largest fibres in the nerve in specimens of differing length confirms the close dependence of the length of the myelin segments on growth. The form of the relationship between internodal length and fibre diameter depends upon their relative growth rates. Growth of internodal distance follows that of the part in which the nerve lies, but this is not true of diameter. The changes in the relationship between internodal length and diameter during growth have been analyzed and the applicability of the equation employed to other vertebrate groups considered. The contribution that quantitative investigations of the changes taking place during the growth of nerve fibres can make towards the understanding of the way in which their structural organization is achieved and maintained is discussed.