Transient expression of insulin‐like growth factor I immunoreactivity in skeletal muscle cells during postnatal development in the rat

The immunohistochemical expression of insulin-like growth factor I (IGF-I; somatomedin C) was investigated in hindlimb skeletal muscle of rats during postnatal development. IGF-I immunoreactivity could be demonstrated in satellite fibres and myotubes 1, 2 and 3 days after birth, while no IGF-I immunoreactivity could be demonstrated in the more mature primary fibres. Five days after birth only scattered cells showed IGF-I immunoreactivity and io days after birth no specific IGF-I immunoreactivity could be demonstrated in muscle cells, i.e. the adult pattern was established. It is concluded that IGF-I immunoreactivity is expressed during a limited period of postnatal skeletal muscle maturation in rats. IGF-I is probably synthesized by IGF-I immunoreactive muscle cells and contributes to the differentiation/maturation process by autocrine and/or paracrine mechanisms.