The density and distribution of capillaries around a synovial cavity

The interpretation of trans-synovial fluxes requires a knowledge of the number of capillaries present around the joint cavity, and their depths below the surface of the joint lining. Measurements of capillary numbers, depths and dimensions were therefore carried out by light microscopy on synovial tissue sections from five rabbit knees. In addition the surface area of synovial tissue lining the joint was estimated, and the thickness of the synovium measured. Synovial capillary density was found to be uneven, with respect to both anatomical location in the joint cavity and depth below the synovial surface. The large area of synovium over areolar or adipose tissue was highly vascular (67000-83000 capillaries X cm-2 section; mean intercapillary distance 35-55 micron; 228-329 cm2 endothelium X cm-3 synovium). The small area of synovium over tendons was of very low vascularity (2000 capillaries X cm-2 section). From these data and a previous estimate of net filtration capacity, mean synovial endothelial conductivity was estimated as approximately 16 X 10(-7) cm X s-1 X mmHg-1. The depth distribution of the capillaries was extremely skewed, with a sharp peak at 6-11 micron depth. The rapid decay in capillary density at depths greater than 6-11 micron was partly due to an uneven density of capillaries within the synovium itself; and partly due to a lower density in subsynovial tissues. It was concluded that most synovium has a rich capillary bed which is very superficially located and that the depth of periarticular tissue which is effective in exchange processes with the joint cavity is less than 25 microns.