Quantitative and histochemical aspects of the differentiation of muscle spindles in the anterior latissimus dorsi of the developing chick.

The differentiation of muscle spindles has been investigated both quantitatively and histochemically in the slow anterior latissimus dorsi (ALD) muscle during embryonic and post-hatching development of the chick. The first spindles are detected by the 13th day in ovo, and a rapid increase in number takes place until the 15th of embryonic life. Two histochemical fibre types of intrafusal fibres are distinguished as early as the 13th day of embryonic development when myofibrillar ATPase activity is demonstrated after acid preincubation. In ALD muscle from post-hatched animals, two intrafusal fibre types are also distinguished by histochemistry: one fibre type is characterized by an acid and alkali-stable myofibrillar ATPase activity while this activity is acid-labile and alkali-stable in other type. As far as the properties of myofibrillar ATPase are concerned, such types of intrafusal fibres resemble the extrafusal beta and alpha fibre types defined recently in ALD muscle. The development of spindles in the slow-tonic ALD muscle is compared to the differentiation of spindles in the fast-twitch posterior latissimus dorsi (PLD) muscle of the chick which has been previously described (Toutant et al. 1981).