Equine synovial villi: distinctive structural organization of vasculature and novel nerve endings

The structural arrangement and cellular distribution of endothelial and lining cells of the synovial villi were studied in the equine palmar/plantar recess of the metacarpo- and metatarsophalangeal joints by light microscopy and electron microscopy. The extent and distribution of blood vessels varied with villous shape and length. The majority of vessels formed concentric circles in cross and longitudinal sections and probably are arranged in a convoluted, spiral or helical pattern. The villi do not contain smooth muscle cells or typical capillaries as observed in other organs. Under the electron microscope, the endothelium 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 the top of villus appeared to be constricted in most cases, with a diameter of about 12 +/- 3 microns. Blood vessels formed by more than six endothelial cells in the middle portion of villus generally were not constricted. Well-developed cytoplasmic processes extended into the lumen of blood vessels. The constriction of blood vessels with no apparent smooth muscle presence and the observation of numerous intermediate filaments in the cytoplasm of the endothelial cells suggests that these villous blood vessels constrict through contraction of their own endothelial cells. Lining cells were distributed unevenly even within a single villus; the villous lining cells seemed to have directional preferences with domination of synovial type A cells. Surprisingly, structures resembling myelinated nerve ends (approximately 0.2 microns) were observed between juxtaposed endothelial cells as well as directly on an endothelial cell, suggesting that these nerve endings may be a sensor detector of either pressure or temperature or have a proprioceptive-like function. Synovial villi have a distinctive structural arrangement of vessels, lining cells, and nerve endings.