Ultrastructural pathology in the apical neural folds was analyzed by means of tannic acid (TA) and ruthenium red (RR) cytochemistry in abnormal (vl/vl) mutant mouse embryos ranging in age from 17-35 somites. At lumbosacral levels of the spinal cord where closure fails to occur, as well as at more cranial levels where closure occurs but results in dorsal midline abnormalities, normal deposition of TA-positive and RR-positive material occurred in the space that develops between the overlying surface ectoderm (SE) and neuroepithelium (NE). However, in lumbosacral regions, pleomorphic excrescences projected abnormally from the apices of the transitional zone cells between SE and NE cells of the open neural folds. These abnormal projections consisted of enlarged cytoplasmic blebs, as well as entire cells. The cells were not necrotic nor did they show evidence of incipient degeneration. However, it is possible that they represent aberrant putative neural crest cells, as indicated by their location in the transitional zone and by the filopodia and lamellipodia projecting from their luminal surfaces.