Growth of motor end plates and histochemistry of intrafusal myofibres in neonatal piglets.

Between birth and 10 days the mean growth rate of peroneus longus motor end plates was 0-298 mum/day. A similar rate was found in seven other muscles from different anatomical regions. At birth, sartorius spindles contained three types of intrafusal myofibres; (i) large diameter with conspicuously strong acid-stable, and moderate or strong alkali-stable adenosine triphosphatase (ATPase) reactions; (ii) large diameter with weak or negligible acid-stable and alkali-stable ATPase; (iii) small diameter with weak or negligible acid-stable, and strong alkali-stable ATPase. All had a high mitochondrial enzyme content and positive PAS reaction. With the exception of some large diameter fibres, all reacted positively for phosphorylase. The frequency distribution of diameters was unimodal at birth but by 9 or 10 days was bimodal.