Prenatal morphogenesis of primate extraocular muscle: neuromuscular junction formation and fiber type differentiation.

Extraocular muscle represents a distinctive class among mammalian skeletal musculature in exhibiting the full range of muscle fiber type variability found in vertebrate species. To better understand the basis for the unique structural/functional diversity of extraocular muscle, the ontogeny of lateral rectus muscles was studied in Macaca nemestrina fetuses of 62-156 days gestation using light and electron microscopy. At E62, myotubes and myofibers are evident, but fiber-type differentiation has not yet occurred and neuromuscular junctions are primitive. By E92, presumptive singly and multiply innervated fiber types could be distinguished on the basis of myofibril delineation. Like other skeletal muscles, extraocular myogenesis proceeds through at least two generations of myofibers. All primary and secondary myofibers were generated and were maturing by E121. The phylogenetically "old" global multiply innervated fiber type was the first to attain adult form. This was followed by maturation of global layer singly innervated fiber types, which are developed by E156, except for attainment of definitive size and mitochondrial content. Orbital layer fiber types, particularly the orbital singly innervated fiber, are the last to mature. Neuromuscular junction maturation paralleled the changes observed during fiber-type differentiation. In summary, the sequential development of their constituent muscle fiber types may reflect the functional pressures the extraocular muscles are exposed to by maturing visual and visuomotor systems. In particular, ontogenic and phylogenic changes observed in the orbital singly innervated fiber type may have direct implications for the types, range, and precision of eye movements used by different species and at different gestational ages.