Lymph vessels of the rabbit heart: distribution and fine structure in ventricles.

We examined the distribution and fine morphology of small lymph channels in the rabbit heart, and particularly in the ventricular myocardium. Light microscopy showed many small subendocardial lymphatics in both ventricles and especially in the subendocardium of papillary muscles and right side of the septum. Electron microscopy revealed that the endothelial ultrastructure of these lymphatics was similar at different sites. The cytoplasm exhibited frequent adluminal expansions and contained numerous micropinocytotic vesicles opening both into the lumen and surrounding interstitium. Bundles of microfilaments, commonly arranged in a parallel fashion, were also visible. Between laminations of the endothelial wall were relatively dense, finely granular material. Intercellular contacts were characterized by cell superimpositions and interlockings, specialized junctions and occasionally maculae adherentes. The basal membrane was discontinuous and often of variable thickness. In the surrounding connective tissue, thin filaments appeared adjacent to the abluminal surface of the endothelial wall. Although the fine structure of the walls of small lymphatics in the rabbit ventricular myocardium was similar to that of small lymphatics at other sites, their distribution varied from other mammalian species.