Neuroepithelial cell contacts in 12 days old rat embryo: normal and annulated gap junctions.

Neuroepithelial cells in the neural tube of 12 days old rat embryos make contact with each other by extended junctional complexes between their lateral membranes near the luminal surface. In this stage gap junctions are not numerous. Besides these "normal" gap junctions, especially in the upper part of the neuroepithelial cells, annulated gap junctions occur. They seem to originate by an endocytotic process of normal gap junctions with participation of cytoskeletal elements. The content of these annulated gap junctions mainly consists of ribosome-like particles in a high density, but larger particles may also occur. Annulated gap junctions sometimes reveal an intimate contact with the smooth endoplasmic reticulum or with the perinuclear cistern. If present in the perinuclear region the annulated gap junctions show membrane irregularities and other signs which are thought to mark steps of breakdown. Life-cycle and possible functions of the annulated gap junctions during neurulation need further investigation.