The synovial surface in rabbit knees was examined by scanning electron microscopy (SEM) to define normal surface contour, cell shape, and interstitial exposure. Comparison was made between specimens excised before immersion fixation (I), fixed in situ by vascular perfusion (V) before excision, or fixed in situ under an effusion pressure of 5-25 cmH2O (E). The deeply convoluted appearance of rabbit areolar-muscular synovium fixed after excision (I) was found to be an artefact; areolar-muscular synovium fixed in situ (V) was much smoother. The well documented cobblestone contour of immersion fixed adipose synovium was present after fixation in situ, but may be exaggerated by the SEM preparative process. At higher magnification the synoviocytes showed evidence of considerable surface activity (smooth granules, larger cauliflower-like excrescences, thin lamelliform filopodia). Cell shape was variable but many synoviocytes extended long cytoplasmic processes along the surface, producing fibroblastoid and even stellate outlines. At an intra-articular pressure of 25 cmH2O (E) the cytoplasmic processes were elongated and branched, creating a highly dendritic outline. Also, the exposure of interstitium increased markedly at the higher pressure. It is concluded that extension of lengthy cytoplasmic processes is a feature of normal healthy synoviocytes, and that an increase in interstitial area with joint pressure contributes to the increased hydraulic permeability of synovium at raised pressure.
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