Morphology and origin of arachnoid cysts. Scanning and transmission electron microscopy of three cases.

Three surgically removed primary arachnoidal cysts were studied with scanning electron microscopy (SEM) and two of the cases with transmission electron microscopy (TEM). The cells lining the cyst cavity had microvilli at the surface, true cilia were absent. In the cytoplasm multivesicular bodies, many pinocytotic vesicles, some large vacuoles and strands of tonofilaments were prominent features. The cells were interconnected by desmosome-like junctions and were separated from the surrounding tissue by a distinct but sometimes incomplete basal membrane. Based on these findings it is concluded that arachnoid cysts are derived from the outer arachnoid cells (subdural neurothelium), the formation of the cysts being attributabel to secretory capacity of the subdural neurothelium.