Ultrastructural analysis of the midaxial extracellular matrix in spinal dysraphism

Ultrastructural aspects of the extracellular matrix (ECM) in the midaxial region of dysraphic embryos of the loop-tail (Lp) mutant mouse were analyzed by means of electron microscopy. In 17-23 somite embryos, ultrastructural differences in the ECM occurred with respect to the presence of a pair of long trailing basal laminar strands extending continuously from the ventral notochordal cells to the gut in abnormal (Lp/Lp) embryos, in contrast to short, ragged, discontinuous strands in normal (+/+; Lp/+) embryos. The ultrastructural localization and configuration of fibronectin (FN) and laminin (L) associated with these strands, however, were similar in normals and abnormals. In addition, FN occurred over interstitial bodies, fibrils, and sporadically along the basal laminae of the neural tube (or folds), notochord, gut, and vessels, whereas L was largely confined to the basal laminae. The results indicate that although the ultrastructural pattern of FN and L reactivity are similar in normal and abnormal embryos, a disturbance in the manner whereby the notochord detaches from the gut in dysraphic embryos may be of causal significance in the etiology of dysraphism in this mutant.