Laminin is required for the production of a basement membrane in both the early embryo and in the embryoid body. The absence of the gamma 1 chain leads to different processing of the other subunits in the trimer and prevents the production of a polymerizing laminin molecule. Collagen IV and perlecan are not able to compensate for the loss of the laminin molecule, and nidogen, which would normally bind to the gamma 1 chain, is almost completely lost from the embryoid body. In the embryo the failure of the basement membrane results in embryonic lethality at embryonic day 5.5, a time when the primitive endoderm is differentiating to visceral and parietal endoderm, and the latter is migrating out of the inner cell mass over the trophectoderm. It is likely that failure in one or both of these events leads to the death of the embryo. The absence of the trophectodermal basement membrane normally present in the blastocyst appears not to be necessary for the formation of the blastocystic cavity, which depends on the formation of a polarized epithelium. Hence the first epithelium produced in the developing embryo does so independently of a basement membrane.