Fluid in the tissue channels of vascular adventitia investigated by AFM and TEM.

BACKGROUND: Tissue channels as a part of microcirculation system have been proposed over three decades, playing an important role in fluid transportation as reported. Adventitia of inferior vena cava (IVC) is a typical hierarchical porous media with abundant tissue channels. Its fluid transportation behaviors attract massive research interest. However, the mechanism of the driving force and microstructure was lack of deep research. OBJECTIVE: This study was to investigate the microstructural basis of fluid transportation within inferior vena cava (IVC). METHODS: Rat IVC samples were extracted and fixed on a gelatin substrate. Four samples were randomly used as 4 cases: Case 1 with AFM loading and the fluorescent tracer adding; Case 2 with fluorescent tracer adding only; Case 3 with AFM loading only as the control group; Case 4 with no treatment. The movement of fluorescent tracer was observed by two-photon fluorescent microscope and analyzed by self-made Matlab program. The microscopic structure was characterized by high resolution TEM. RESULTS: The fluorescent tracer in Case 1 exhibited faster and longer transportation comparing to other cases, while in Case 2 diffused normally following Fick's law. Case 3 with only AFM loading demonstrated that collagen bundles twisting along the fluid orientation, while the bundles in Case 4 with no treatment were straggling. The brush-like macromolecule structure of collagen microfibril was found on the bundle surfaces under TEM. CONCLUSIONS: Transportation within loose connective tissues is observed ex vivo. AFM loading, as the mechanical stimulation resemblance to muscle constrictions and blood pulsations, can facilitate the transportation as the driving force. The brush-like glycosaminoglycan macromolecules on the surfaces of the collagen bundles can be considered as a type of hierarchical porous media, which might form the transport pathway for fluids. The possible mechanism was conducted regarding the conformation of the superficial macromolecule brushes.