The microscopic transitional zone between elastic and muscular arteries.

The microscopic transitional zone (TZ) is a segment of the arterial tree, where elastic-type wall architecture is relayed by one of muscular type. Since arterial TZ's are often sites of atherosclerotic and non-atherosclerotic lesions, this histologic anatomic substrate should be considered in the etiology and pathogenesis of vascular diseases. Herein, the topography and fine structure of TZ in renal arteries and carotid bifurcation (tripod) is described, based on randomly collected human autopsy specimens. The average length of the TZ was found to be 10 mm in the renal arteries. The TZ In the carotid tripod was localised only in the postbifurcational segments, where the length varied between 5 and more than 20 mm. In accordance with previous data from the literature, our results confirm a different ratio of scleroprotein components (elastin, collagen) and vascular smooth muscle cells (VSMC) in the microscopic TZ of arteries. Immunohistochemistry for lymphomonocytic antigens revealed no underlying inflammatory condition, especially no evidence of active cellular scavenging and an increased apoptotic rate of VSMC. We perceive the TZ as an arterial segment of reduced architectural compliance and, therefore, as a potential site of mechanotransductional failure leading to vascular diseases.