Transient ultrastructural variations of pulmonary lymphatic capillaries during the respiratory cycle of the rat lung.

The effect of respiratory movements on the endothelial wall of lung lymphatic capillaries was investigated by a morphometric study of their ultrastructure using a rodent model. The juxta-alveolar lymphatic capillaries from lungs were fixed in the inflated phase by vascular perfusion with aldehydes and compared to those from lungs fixed in a deflated phase. Important quantitative differences in lymphatic endothelial wall architecture were observed between the two states. The most salient finding was a marked reduction of the mean endothelium thickness from 320 +/- 55 nm in deflated lungs to 210 +/- 36 nm (P less than .01) in inflated lungs. The endothelial attenuation observed with inflated lungs was accompanied by a reduced frequency of interdigitation, the intricate pattern of intercellular contact (P less than .02), and a decreased number of lumenal and ablumenal endothelial projections (P less than .05). Despite these important variations in the dilated lymphatic capillaries, no typical open junctions (greater than 30 nm) were found among 105 intercellular contacts. The vesicular system varied slightly after lung inflation. Dilated capillaries showed a significant reduction in cytoplasmic vesicles during inspiration (P less than .01); however, the observed increases in lumenal and ablumenal vesicles were not found to be significant. As for numerical density and vesicular diameter, the fraction of cytoplasm occupied by the vesicles remains unchanged. Contrary to previous assumptions, these results suggest that intercellular clefts are not widened to the extent of open junctions (greater than 30 nm) by the mechanical movement of inspiration in normal lungs. Moreover, transient changes to the endothelial architecture seem to preclude any excessive widening of the intercellular cleft.