Early development of the brain and spinal cord in dysraphic mice: a transmission electron microscopic study.

The hindbrain and spinal cord were studied by transmission electron microscopy with and without lanthanum nitrate treatment in nine-day embryos of the loop-tail (Lp) mutant mouse. Homozygous (Lp/Lp) individuals exhibit dysraphism from the hindbrain caudally throughout the embryo; in +/+ and Lp/+ individuals, the brains and spinal cords are normal. In contrast, the ventricular cells in the abnormal hindbrain and spinal cord showed increased amounts of intercellular space in an area intermediate between the luminal border and basal zone, and a flattening which occurs variably in their luminal surfaces. A most striking difference occurred in the frequency of gap junctional vesicles, circular structures bounded by a double membrane and containing ribosome-like material. A quantitative analysis of the distribution of these organelles revealed that they are more numerous in the dysraphic hindbrain and lumbosacral spinal cord of the abnormal animals than in comparable regions of the normal; however, in the cervicothoracic spinal cord, the frequency of these vesicles is similar. In specimens treated with lanthanum, the tracer freely penetrated the luminal junctional complexes in both normal and abnormal animals, but was not present between the membranes of the gap junctional vesicles. The developmental significance of gap junctional vesicles frequently located in juxtaluminal regions of mitotic cells, but also found deeply in dividing cells, is not known; they may relate to cell-to-cell attachment and/or communication. In any event, further study of them may prove valuable in understanding normal and abnormal development of the neural tube.