Characterization of intercellular junctions in the caudal portion of the developing neural tube of the chick embryo.

The types of intercellular junctions present within caudal levels of the chick neural tube (i.e., future lower thoracic and lumbosacral regions of the spinal cord) were determined by freeze-fracture of stage 14 to 16 embryos. Two levels of the developing neural tube were examined: the region of the neurulation overlap zone--consisting of primary neural tube dorsally and secondary neural tube ventrally--and the portion of the primary neural tube located just cranial to the overlapping region. Gap junctions were the most numerous type of intercellular junction present within the primary neural tube. These junctions were located primarily in juxtaluminal areas, near the apices of neuroepithelial cells, and sometimes also at the bases of these same cells. In addition, focal, poorly defined tight junctions occasionally occupied juxtaluminal regions of the primary neural tube. The medullary cord (i.e., the immediate precursor of the secondary neural tube) and secondary neural tube contained gap junctions exclusively. Gap junctions were first found in these areas at the lateral borders of the medullary cord, concomitant with formation of this structure, and then at the interface between the elongated, peripheral cells of the cord and the irregularly shaped and loosely arranged central cells of this structure. Finally, gap junctions were distributed radially around secondary lumina formed by cavitation. The precise spatial and temporal correlation between the appearance of gap junctions and the specific changes occurring in cellular morphology and arrangement during secondary neurulation strongly suggest that gap junctions may have a role in coordinating cellular activities during formation of both the medullary cord and the secondary neural tube.