Development of the ocular motor system in mouse embryos

The ocular motor system enables organisms to move their eyes to obtain precise visual feedback. In humans, six extraocular muscles form an oppositional pulley system to mediate slow, smooth tracking events as well as fast saccades. Their movement is dependent on three cranial nerves: the oculomotor, trochlear, and abducens. This study focuses on the oculomotor nerve (OMN), which innervates four of the extraocular muscles. In early development, cranial nerves send pioneering axons into the peripheral tissue surrounding the neural tube. Axons must then respond to exogenous guidance cues to find their muscle targets. Precise axon projections are required, and miswiring or other abnormal development in the ocular system leads to strabismus, amblyopia, and impaired vision. Strabismus alone affects up to 2% of the population, and studies have shown that it greatly affects self-image, employment, and public perception. Studying the development of the ocular motor system can help identify the underlying cause of these defects. Previous studies have explored oculomotor guidance in chick and zebrafish embryos; however, a time course of mammalian development has not been established. This study establishes a time course of the critical stages of oculomotor nerve outgrowth to the periocular area in mice at embryonic day (e) 9.5 - e14.5. In previous studies, targeting was thought to occur when the nerve selected between the 6 differentiated populations of the extraocular muscles. Via immunolabeling and tissue-specific genetic markers, we find that the oculomotor nerve grows out to and spreads out within a mass of muscle precursor cells expressing the myogenic determinants Pitx2 and Myf5 near the eye. Contact between nerves and muscle precursors occurs days before mature muscle fibers have formed. The nerve remains in a plexus with these muscle precursor cells past the point of primary myogenesis, then infiltrates the mature fibers at e14.5. To examine whether EOM primordia were necessary for nIII guidance, we ablated muscle precursor cells by driving diphtheria toxin fragment A (DTA) with the early muscle differentiation factor Myf5. We found that the oculomotor nerve displayed ectopic branches, some of which projected beyond the eye, or a lack of plexus formation altogether. Our research integrates the parallel fields of oculomotor guidance and extraocular muscle development to form a more complete understanding of the choate ocular motor system in mammals. Our results also identify extraocular muscle precursor cells as an intermediate target for the OMN, thus providing evidence for an additional pathfinding step in this poorly understood process