Structural response of the rabbit thoracic aorta and inferior vena cava to in situ collapse.

When the transmural pressure of the rabbit aorta falls below 5 mmHg in situ, the vessel collapses to form regions of high curvature along both lateral margins which stain deeply with Trypan Blue dye. The ultrastructure of the collapsed vessel was examined in order to investigate its response to localized stresses. Lamellar convolution was characterized by a convolution index, C.I. (length of a segment of lamella divided by the length of the arc of best fit spanned). In high curvature regions, C.I.'s of the internal and outer lamellae were 1.64 SD 0.20 and 1.04 SD 0.04 respectively. Corresponding values in low curvature regions were 1.30 SD 0.08 and 1.31 SD 0.04. The mean wave numbers of internal lamellar convolutions in high and low curvature regions were 53 mm-1 and 63 mm-1 SD 7 mm-1. When the ratio of medial thickness to internal radius, t/ri exceeded 1.9, the internal lamella buckled and endothelial cells detached. The mode of cell deformation depended on the position of the cell junctions relative to the convolutions. A small series of studies showed the endothelial cells in the vena cava did not detach for t/ri less than 15.0.