An ultrastructural analysis of abnormal otic development in exencephalic mutant mice.

Homozygous loop-tail (Lp/Lp) mice exhibit defects in the otocyst as well as extensive neural dysraphism. At 19 days of gestation, cells in the otic pit of abnormal embryos are flattened and lack the rounded luminal contours characteristic of otic cells in their normal littermates. Apical filaments also are not as prominent as in normal embryos, and there is an increase in densely stained globular material in cells at the ventral lip of the otic pit. With glutaraldehyde-tannic acid fixation, the basal lamina of the otic pit cells shows differences from that of the normal otic pit. In abnormal specimens, the lamina densa is irregular and clumped, and the adjacent less dense area is spotty and lacks the more uniformly arranged and delicate fibrils characteristic of the normal basal lamina. These defects may reflect faulty developmental interactions between the dysraphic neural tube, mesenchymal cells, and otic anlage.