Changes in number and distribution of orthogonal arrays during postnatal muscle development.

Quantitative freeze-fracture electron microscopy was utilized to study the changes in number and distribution of orthogonal arrays (aggregates of 6-7 nm particles) of the sarcolemmas of the fast twitch extensor digitorum longus (EDL) and slow twitch soleus muscles during the first few weeks of postnatal development in the rat. In the adult rat, orthogonal arrays are present in high densities in the fast twitch type II fibers but only in low densities in slow twitch type I fibers. In this study, the changing histochemical profiles of fiber types in the EDL and soleus muscles were also determined for the first month of postnatal development and correlated with the changing number and distribution of orthogonal arrays during the same time frame. At day 3 postpartum, EDL and soleus fibers possessed few orthogonal arrays. The developing EDL fibers rapidly acquired additional orthogonal arrays until the approximate adult number and distribution were attained at postpartum day 25. In contrast, the slow twitch soleus fibers rapidly acquired orthogonal arrays and type IIA fibers until day 35 when both were in excess of adult values. Subsequently, the number of arrays and type IIA fibers declined to normal adult ranges. We suggest that the patterns of development of orthogonal arrays and fiber types are different in the EDL and soleus because the types of innervating motor units are different in the two muscles. The EDL is innervated almost entirely by fast motor units throughout early development and maturity. The soleus, however, is initially innervated by a more heterogeneous population of motor neurons. Thus, during the period of polyneuronal innervation which occurs normally during the first weeks of postnatal development, many individual soleus fibers may possess simultaneous innervation by axons from different motor neuron types. These dual influences may be responsible for the irregular pattern of development of orthogonal arrays and type IIA fibers in developing soleus fibers. Later, as the adult pattern of monosynaptic innervation is developed, expected adult values of orthogonal arrays and fiber types are attained.