EQUINE SYNOVIAL VILLI - DISTINCTIVE STRUCTURAL ORGANIZATION OF VASCULATURE AND NOVEL NERVE-ENDINGS

The structural arrangement and cellular distribution of endothelial an d lining cells of the synovial villi were studied in the equine palmar /plantar recess of the metacarpo- and metatarsophalangeal joints by li ght microscopy and electron microscopy. The extent and distribution of blood vessels varied with villous shape and length. The majority of v essels formed concentric circles in cross and longitudinal sections an d probably are arranged in a convoluted, spiral or helical pattern. Th e villi do not contain smooth muscle cells or typical capillaries as o bserved in other organs. Under the electron microscope, the endotheliu m is surrounded by connective tissue and discontinuous circular cells, presumably fibroblasts. The outermost layer was sometimes surrounded by type A and/or B synovial cells. The lumen of the blood vessels at t he top of villus appeared to be constricted in most cases, with a diam eter of about 12 +/- 3 mu m. Blood vessels formed by more than six end othelial cells in the middle portion of villus generally were not cons tricted. Well-developed cytoplasmic processes extended into the lumen of blood vessels. The constriction of blood vessels with no apparent s mooth muscle presence and the observation of numerous intermediate fil aments in the cytoplasm of the endothelial cells suggests that these v illous blood vessels constrict through contraction of their own endoth elial cells. Lining cells were distributed unevenly even within a sing le villus; the villous lining cells seemed to have directional prefere nces with domination of synovial type A cells. Surprisingly, structure s resembling myelinated nerve ends (similar to 0.2 mu m) were observed between juxtaposed endothelial cells as well as directly on an endoth elial cell, suggesting that these nerve endings may be a sensor detect or of either pressure or temperature or have a proprioceptive-like fun ction. Synovial villi have a distinctive structural arrangement of ves sels, lining cells, and nerve endings.