EM findings on arachnoid and fibrous cysts. TEM of dural aspects of arachnoid cyst (a) and fibrous cyst (b); SEM of luminal surface of arachnoid cyst (c) and of fibrous cyst (d). A typical arachnoid organisation is seen in (a) with highly complex cellular extensions in whorls encircling wide intercellular spaces. In (b) only a single extremely attenuated cell covers a matrix rich in collagen and ground substance. In (c) and (d) scattered microvilli are seen as bright spots. The luminal surface epithelium is smooth in (c), more irregular with some overlapping or desquamating cells in (d).