The surface ultrastructure of ganglia has been studied using the scanning electron microscope. This study showed that the ganglion wall consists of multidirectional strata of collagen fibres and has no cellular lining. The wall has a sponge-like appearance and does not appear degenerate or necrotic. Comparison with synovial membrane and adventitious bursa confirmed that these are distinct structures which have a cellular lining. Ganglia probably arise from the multifunctional mesenchymal cells which are found within their walls. The ganglion fluid may also originate from these cells.