Transperineurial arterioles in human sural nerve.

The perineurial sheath of nerve fascicles is a protective cellular layer that separates the endoneurium from the epineurium. Transperineurial arterioles (TPA) connect the endoneurial capillary plexuses to the epineurial arterial nutrient supply. Transperineurial arterioles are defined as any arteriole that is confined to a perineurial cell compartment, which would include all arterioles within the perineurium proper or within perineurial sleeves in the epi- or endoneurium. In this study of biopsied human sural nerves, three-dimensional reconstruction of one micron sections and ultrastructural analysis of step serials, we find that TPA are confined in open-ended perineurial sleeves by which they pass from the epineurium through the perineurial sheath proper into the endoneurium. Most TPA are terminal arterioles as evidenced by size (10-25 microns), morphological characteristics, and the fact that they connect with capillaries. Transperineurial arterioles gradually lose their continuous muscle coat and become post-arteriolar capillaries (PAC). Vascular segments that emerge into the endoneurium from the perineurial sleeves are generally of the PAC variety. Transperineurial arterioles and post-arteriolar capillaries are often associated with a plexus of unmyelinated nerve fibers. Axon varicosities exhibit a variety of morphologically distinct vesicles including dense-cored and a diversity of agranular vesicles. These findings suggest that TPA play a role in the neurogenic control of endoneurial blood flow.