Vascular smooth muscle structure and juvenile growth in rat intestinal venules.

The morphological structure of individual vascular smooth muscle cells from intestinal venules was evaluated with a combination of quantitative scanning (SEM) and transmission (TEM) electron microscopy techniques. In addition, growth of individual venular smooth muscle cells and of the overall vessel wall was compared from measurements of these variables during the rapid juvenile growth spurt from ages 4 to 6 and 10 to 12 weeks in Wistar-Kyoto rats. SEM revealed that smooth muscle cells of intestinal venules in weanling rats are very long (379 +/- 91 [SD] microns) and wide (6.0 +/- 1.3 microns) and very little further cell enlargement occurs during rapid juvenile growth. TEM studies indicated that passive inner vessel diameter and total muscle layer cross-sectional area of both the largest and intermediate diameter venules of young rats, as well as the percentage of the total wall area as muscle tissue in each venule type, did not significantly increase during body growth. These observations indicate that both the intestinal venules and their smooth muscle cells reach mature dimensions at a very early stage of life. Comparison of intestinal vascular smooth muscle cell dimensions indicates that venular smooth muscle cells are much larger in both cell length and volume than comparable arteriolar smooth muscle cells.

Externally stented polytetrafluoroethylene valved conduits for right heart reconstruction. An experimental comparison with Dacron valved conduits.

Valve-containing conduits have made possible the repair of many congenital anomalies that involve right ventricular-pulmonary arterial discontinuity. The distressing problem of neointimal peel formation with eventual conduit obstruction in patients with Dacron valved conduits has led to the need for premature replacement in many patients. Externally stented polytetrafluorethylene has demonstrated superior patency in the venous system experimentally and clinically and was believed to have potential advantages over Dacron for conduit construction. This study compares the transconduit resistance and the thickness of the neointimal peel in right ventricular-pulmonary arterial conduits constructed of externally stented polytetrafluoroethylene with those of woven Dacron. The 19 mm externally stented polytetrafluoroethylene conduits (Impra, Inc.) containing a Hancock porcine valve (Extracorporeal Inc.) were implanted in six adult mongrel dogs followed by proximal occlusion of the pulmonary artery. In six additional animals, a Dacron valved conduit of similar size and length was inserted. Cardiac output, transconduit gradient, and resistance were measured at operation and at 3 months. All conduits were subsequently explanted, opened longitudinally, and the thickness of the neointimal peel (excluding suture lines) measured. No hemodynamic differences were noted during the 3 month follow-up. However, the thickness of the neointimal peel was fourfold greater in Dacron conduits (609 +/- 144 mu) than in the conduits constructed of externally stented polytetrafluoroethylene (156 +/- 50 mu) (p less than 0.01). The thick peel in Dacron conduits extended into the outflow portion of the porcine valve cusps and prevented their full excision. The neointima in externally stented polytetrafluoroethylene conduits was thin and uniform and did not extend onto the leaflets or limit their mobility. This study demonstrated that the early hemodynamic performance of externally stented polytetrafluoroethylene conduits was comparable to that of Dacron conduits; Dacron conduits were subject to an accelerated rate of peel formation that affected leaflet mobility and may be a factor in early valve degeneration; a thin neointima formed in externally stented polytetrafluoroethylene conduits and valve leaflet motion was preserved. This study showed that externally stented polytetrafluoroethylene conduits offer advantages over Dacron valved conduits and warrant clinical application.