Assembly and disassembly of gap junctions during mesenchymal cell condensation and early chondrogenesis in limb buds of mouse embryos.

During early chondrogenesis of the limb skeleton, cell condensations occur which are characterised by an increased density of cell packing, a decrease in the extent of the intercellular space and a drastic increase in the number of gap junctions. Electron microscopical investigations have been performed on the assembly and disassembly of these gap junctions in limb buds of mouse embryos during Days 10-12 of pregnancy. In undifferentiated mesenchyme, focal cell contacts occurred showing a gap of about 10 nm between adjacent cell membranes. During further development these contacts became broader, the intercellular gap decreased to 5 nm and gap junction structures were first formed at the ends of such cellular contacts. At the stage of maximum cell condensation, the blastemal cells were interconnected by large gap junctions, but there were also a considerable number of coiled gap junction structures present intracellularly. Near these coiled gap junctions, a well developed Golgi apparatus and many coated vesicles were generally detectable, indicating a fast turnover of cell membrane. The results suggest an insertion of gap junction components into the membrane during contact formation. Deposition of new cell membrane and coiling and invagination of gap junctions are discussed in the context of cell separation at the beginning of chondrogenesis.