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

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 (alpha' and beta') differentiate from a pool of acid and alkali-stable myofibrillar ATPase fibres. Intermediate stages of the transformation from beta' to alpha' 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 alpha R or fast oxidative-glycolytic fibres and are assumed to be focally innervated.