Histochemical differentiation of extrafusal muscle fibres of the anterior latissimus dorsi in the chick

Abstract Histochemical differentiation of the chick anterior latissimus dorsi (ALD) muscle was studied during embryonic development and after hatching. The two types of adult ALD tonic fibres (α′ and β′) differentiate from a pool of acid and alkali-stable myofibrillar ATPase fibres. Intermediate stages of the transformation from β′ to α′ were observed. At all developmental stages studied, a low percentage of formalin-resistant, alkali-stable and acid-labile ATPase fibres were observed. Such fibres have the histochemical properties of the αR or fast oxidative-glycolytic fibres and are assumed to be focally innervated.