AbstractThe laminin α2 chain is a major component of basal lamina in both skeletal muscle and the peripheral nervous system. Laminin α2 chain deficiency causes merosin‐deficient congenital muscular dystrophy, which affects not only skeletal muscles, but also the peripheral and central nervous systems. It has been reported that the formation of basal lamina is required for myelination in the peripheral nervous system. In fact, the spinal root of dystrophic mice (dy/dy mice), whose laminin α2 chain expression is greatly reduced, shows lack of basal lamina and clusters of naked axons. To investigate the role of laminin α2 chain and basal lamina in vivo, we examined the peripheral nervous system of dy3K/dy3Kmice, which are null mutants of laminin α2 chain. The results indicate the presence of myelination although Schwann cells lacked basal lamina in the spinal roots of dy3K/dy3K mice, suggesting that basal lamina is not an absolute requirement for myelination in vivo. Immunohistochemically, the expression of laminin α4 chain was increased and laminin α5 chain was preserved in the endoneurium of the spinal root. Laminin α4 and α5 chains may play the critical role in myelination instead of laminin α2 chain in dy3K/dy3Kmice. In addition, the motor conduction velocity of the sciatic nerve was significantly reduced compared with that of wild‐type littermate. This reduction in conduction velocity may be due to small axon diameter, thin myelin sheath and the patchy disruption of the basal lamina of the nodes of Ranvier in dy3K/dy3Kmice. GLIA 35:101–110, 2001. © 2001 Wiley‐Liss, Inc.