Electron microscopic studies on the lamprey meninges.

Lamprey meninges were revealed to consist of four layers. The I (outermost) layer was made up of densely packed collagen fibrils and microfibrils with fibrocytes interposed between them. It contained a vascular network (epidural vascular plexus). The II layer was a cellular layer, consisting of two parts: the outer, membranous and the inner, spongy part. The membranous part was composed of two sheets of type 1 cells which were held together by desmosomes, gap junctions and tight junctions. Continuous basal laminae were found on both the outer and the inner surface of the membranous part. The spongy part consisted of type 2 cells, collagen fibrils and microfibrils, which were loosely arranged. Type 2 cells were joined together with each other, or with type 1 cells, by gap junctions. These cells lacked a basal lamina covering. The III layer was a peculiar thick layer, being mostly occupied by fine filamentous matrix substance with scattered round cells, fibroblasts and microfibrils. The IV (innermost) layer was a thick fibrous one, consisting of two parts: the outer, vascular and the inner, loose part. The former consisted of a vascular network (perineural vascular plexus) and thick bundles of microfibrils which frequently contained a dense filamentous core. The latter was composed of loosely arranged collagen fibrils. The inner part of the I layer (inside the epidural vascular plexus) was considered to correspond to the true dura, and the II and IV layer to the arachnoid and pia in the meninges of higher animals, respectively. Characteristically the subarachnoid space was absent in lamprey meninges, and instead a peuliar "gelatinous" layer, III layer, was present between the II and IV layer.