Developmental loss of laminin from the interstitial extracellular matrix correlates with decreased laminin gene expression.

The expression of the polypeptide subunits of the glycoprotein laminin in developing mouse tissues was analysed by immunoblots and Northern blots, and by immunohistochemistry at the ultrastructural level. In the neonate, almost all the laminin of the sciatic nerve was freely extractable and was located mainly in the mesenchymal interstitial extracellular matrix, rather than in basement membranes. During the first two postnatal weeks, the distribution of laminin shifted to assume the adult pattern, most being located in basement membranes and insoluble under physiological conditions. Analysis of laminin subunit expression showed that both the mRNA for the laminin B chains and the corresponding polypeptides are widely expressed in nerve and other tissues, the mRNA levels decreasing during the first two postnatal weeks as the amount of laminin in the tissue increased. In contrast, the A chain mRNA and polypeptide were undetectable in nerve at any age studied, although they were present in perinatal kidney and placenta. It is proposed that the large amount of soluble laminin present in the developing interstitial extracellular matrix is a consequence of the high levels of expression of laminin mRNA, the subsequent decrease in expression resulting in the adult distribution where most laminin is insoluble within the basement membrane.