Expression of agrin isoforms in developing and adult Schwann cells. A, Expression of agrin isoforms in Schwann cells along the sciatic nerve in adult and bullfrog tadpoles at different developmental stages was examined by RT-PCR. Lane 1, The spinal cord of stage XXII bullfrog tadpoles showed all four isoforms of agrin: B0, B8, B11, and B19. Lane 2, Schwann cells at stage XXI expressed only the inactive B0 isoform.Lane 3, A trace amount of the B11 isoform was detected in Schwann cells at stage XXII. Lanes 4,5, Schwann cells at stages XXIV and XXV expressed active isoforms B11 and B19 besides B0. Lane 6, Adult Schwann cells expressed three agrin isoforms, B0, B11, and B19, but did not show B8. B, Fibroblasts in the epineurial sheath of the sciatic nerve in bullfrog tadpoles at stages XXI-XXV (lanes 2–5) and in adult (lane 6) did not express any agrin isoforms. As a positive control, the adult spinal cord expressed all four agrin isoforms (lane 1).C, RT-PCR showed neurofilament mRNA fragment (0.87 kb) in the frog spinal cord tissue (lane 1,arrow) but not in Schwann cells along the sciatic nerve (lane 2) or muscle (lane 3).