Schwann cells as a source of insulin-like growth factor-1 for extraocular muscles.

Precise force regulation is fundamentally important for extraocular muscle (EOM) function. Insulin-like growth factor-1 (IGF-1) plays a major role in EOM force regulation, but the source of endogenous IGF-1 is unclear. Multiple IGF-1 sources may supply EOMs, including: the EOM itself; the systemic circulation; innervating motoneurons; and Schwann cells within nerves. IGF-1 expression was measured in chicken during oculomotor system maturation by using real-time polymerase chain reaction (PCR). Accumulation of radiolabeled IGF-1 in EOMs was compared after either injection into the vascular circulation or into the trochlear nerve. Schwann cells were the most prominent IGF-1 source. A microtubule-dependent mechanism exists to anterogradely transport IGF-1 to EOMs. EOMs were significantly more efficient in extracting IGF-1 from the nerve than from the systemic circulation. Therefore, Schwann cells are the most prominent and potentially the most important source of IGF-1 for EOMs. These findings may contribute to a better understanding of EOM force regulation and its failure in strabismus.