Absence of basement membranes after targeting the LAMC1 gene results in embryonic lethality due to failure of endoderm differentiation

The LAMC1 gene coding for the laminin yl subunit was targeted by homologous recombination in mouse embryonic stem cells. Mice heterozygous for the mut ation had a normal phenotype and were fertile, whereas homozygous mutant em bryos did not survive beyond day 5.5 post coitum. These embryos lacked base ment membranes and although the blastocysts had expanded, primitive endoder m cells remained in the inner cell mass, and the parietal yolk sac did not develop. Cultured embryonic stem cells appeared normal after targeting both LAMC1 genes, but the embryoid bodies derived from them also lacked basemen t membranes, having disorganized extracellular deposits of the basement mem brane proteins collagen IV and perlecan, and the cells failed to differenti ate into stable myotubes. Secretion of the linking protein nidogen and a tr uncated laminin oil subunit did occur, but these were not deposited in the extracellular matrix. These results show that the laminin yl subunit is nec essary for laminin assembly and that laminin is in turn essential for the o rganization of other basement membrane components in vivo and in vitro. Sur prisingly, basement membranes are not necessary for the formation of the fi rst epithelium to develop during embryogenesis, but first become required f or extra embryonic endoderm differentiation.