The pig synovium. I. The intact synovium in vivo and in organ culture.

1. The normal synovium of the metacarpophalangeal joints of young pigs was examined by light and electron microscopy with special reference to the superficial layer (intima). 2. Cells of the macrophage-like or A-type (Barland et al. 1962) constituted only a small proportion of the intimal synoviocytes; the majority were of the intermediate and B-types. 3. Synovial villi were explanted on Millipore filters and maintained as organ cultures. The intimal cells in contact with the Millipore formed long branched processes which penetrated deeply into the substrate; these cells, which had a very well-developed endoplasmic reticulum, resembled those of the B-type. The synoviocytes at the upper (free) surface of the villus withdrew their long processes, acquired lamelliform pseudopodia, and their endoplasmic reticulum regressed; they were similar in appearance to the A-type. 4. In the organ cultures the highly branched cells (B-type) next to the Millipore were less phagocytic than the rounded cells (A-type) at the free surface of the villus.