Differential deposition of basement membrane components during formation of the caudal neural tube in the mouse embryo.

The distribution of basement membrane and extracellular matrix components laminin, fibronectin, type IV collagen and heparan sulphate proteoglycan was examined during posterior neuropore closure and secondary neurulation in the mouse embryo. During posterior neuropore closure, these components were densely deposited in basement membranes of neuroepithelium, blood vessels, gut and notochord; although deposition was sparse in the midline of the regressing primitive streak. During secondary neurulation, mesenchymal cells formed an initial aggregate near the dorsal surface, which canalized and merged with the anterior neuroepithelium. With aggregation, fibronectin and heparan sulphate proteoglycan were first detected at the base of a 3- to 4-layer zone of radially organized cells. With formation of a lumen within the aggregate, laminin and type IV collagen were also deposited in the forming basement membrane. During both posterior neuropore closure and secondary neurulation, fibronectin and heparan sulphate proteoglycan were associated with the most caudal portion of the neuroepithelium, the region where newly formed epithelium merges with the consolidated neuroepithelium. In regions of neural crest migration, the deposition of basement membrane components was altered, lacking laminin and type IV collagen, with increased deposition of fibronectin and heparan sulphate proteoglycan.