Intracytoplasmic filaments in pulmonary lymphatic endothelial cells. Fine structure and reaction after heavy meromyosin incubation.

The cytochemistry and ultrastructure of intracytoplasmic filaments of pulmonary lymphatic endothelial cells of neonatal rabbits were studied by comparison with myofilaments of the peribronchial and pulmonary vascular smooth muscle cells. Two types of endothelial filaments were observed: thin filaments (diameter: 50 A) which lie close to the abluminal cell membrane; and thick filaments (diameter: 90 A) which are dispersed throughout the cell cytoplasm. Following heavy meromyosin (HMM) treatment, characteristic arrowhead complexes formed in the thin lymphatic endothelial filaments as well as in the actin filaments of the smooth muscle cells. There was no detectable reaction of HMM with the thick filaments. After incubation with EDTA, the thin filaments were labile, and the thick filaments became the major filamentous component in the endothelial cells. In smooth muscle cells, the actin myofilaments were also labile while the 100 A filaments were stable. These observations support the hypothesis that the actin-like thin endothelial lymphatic filaments form part of a contractile system, while the thick filaments constitute a plastic cell skeleton. The significance of the contractile system in lymphatic endothelial cells might lie in a mechanism for the active regulation of the endothelial intercellular junction and gaps and hence the permeability of the lymphatic endothelial cell lining.