Histochemical properties of the biventer cervicis muscle of the chick: a relationship between multiple innervation and slow-tonic fibre types.

Chick biventer cervicis muscle fibres have been studied histochemically. Fast-twitch, focally innervated (alpha) fibres represent 70-80% of the total fibres in this muscles. Two histochemical profiles of slow-tonic multi-innervated (beta) fibres have been observed from embryonic life the adult (three-months) stage. These two slow-tonic types differ in the activity of their histochemically demonstrated myofibrillar ATPase after either acid or alkaline preincubation, and after formalin fixation. Both slow-tonic fibre types have a high oxidative metabolism and are PAS-negative. They are referred as to beta 1 and beta 2R fibre types (slow-tonic oxidative) in an expansion of Ashmore's nomenclature, and compared to avian slow-tonic sub-types that have been described in recent reports. beta 1 and beta 2 fibre types exhibit a similar pattern of innervation. Possible explanations of the origin of histochemical heterogeneity in multiple innervated fibres are discussed.