Immunoelectron microscopic localization of HNK-1 in the embryonic rat heart

To confirm the role of HNK-1 in conduction tissue, the ultrastructural localization of monoclonal antibody HNK-1 was analyzed in developing rat hearts at embryonal day 14.5 by immunoelectron microscopic labeling procedures with post-embedding immunogold staining. Tissue sections in different planes containing the sino-atrial (SA) node, atrio-ventricular (AV) node and His bundle were used to demonstrate HNK-1. Immunogold labeling was detected on the cell surfaces and in the extracellular matrices of cells that had features common to conduction tissue cells. Non-specialized contractile myocytes were not labeled by this antibody. Furthermore, immunogold labeling was more prominent in wide intracellular spaces than in narrow intercellular spaces, and rarely observed in cell-cell contact regions. The cell surfaces and extracellular matrices of mesenchymal cells in the endocardial cushion, which contacts the His bundle, were also positive, suggesting the involvement of tract formation to the AV node. These findings may indicate that HNK-1 plays an important role in cell-cell adhesion processes both temporally and spatially in the developing conduction tissue. It was concluded, therefore, that HNK-1 is a suitable marker of the embryonic heart conduction system and might be useful in analyzing anomalous conduction systems, as in congenital heart disease.