Differentiation of the avian latissimus dorsi primordium: Analysis of fiber type expression using the myosin ATPase histochemical reaction

Abstract

AbstractTypically, fast twitch fibers of the adult avian posterior latissimus dorsi (PLD) muscle express alkali stable myosin ATPase histochemical activity, whereas slow tonic fibers of the anterior latissimus dorsi (ALD) muscle exhibit dual acid and alkali stability. Since PLD fibers cross‐reinnervated by the nerve of the ALD muscle also demonstrate dual stability, we sought to determine whether the ALD nerve induces this new phenotype de novo or de‐represses an embryonic phenotype repressed during further development in ovo. Chick embryos from Day 2 in ovo onward were frozen in toto, and the development of myosin ATPase histochemical profiles following alkali and acid preincubation was determined for myo‐genic cells destined to form the ALD and PLD muscles. In addition, the silver cholinesterase reaction was employed on serial sections to investigate early muscle‐nerve interaction in ovo. Our results indicate that all myogenic cells of the primary dorsal muscle mass, including those of the future ALD and PLD muscles, demonstrate their first myosin ATPase response at Stage (St) 25. At this time (4.5 days in ovo), only alkali stable enzymic activity is expressed. By St 28 (5.5 days) a distinct latissimus dorsi (LD) primordium has cleaved from the dorsal muscle mass and the entire primordium still exhibits only alkali stability. As development proceeds and the LD primordium separates into recognizable anterior and posterior regions (St 29, 6.0 days through St 30, 6.5 days), the future PLD muscle continues to express only alkali stability, whereas the future ALD muscle now demonstrates dual stability. By St 32 (7.5 days), these two muscles are completely separated, and during the remainder of in ovo development the profiles characteristic of adult slow tonic versus fast twitch fibers remain unaltered. Thus, our results indicate that, during cross‐reinnervation ex ovo, the ALD nerve induces the expression of a new phenotype never exhibited by potential fast twitch PLD fibers during the entire embryonic period, including their incipient formation. Moreover, our results indicate that the histochemical reaction for myosin ATPase activity provides an excellent enzymic marker both for the identification of myogenic cells and individual muscle primordia, and, for the detection of muscle fiber type differentiation during the first week of development in ovo.