Meningeal sheaths are made up of meningothelial cells—flattened cells resembling fibroblasts with tonofilaments—and connective tissue, which is mainly collagenous, but also contains elastic fibers (especially in the dura mater) and microfilaments (near fibroblasts and beneath mesothelium).
Pia mater is mostly connective tissue, lined by a thin layer of meningothelial cells, the mesothelium. Arachnoid trabeculae are bundles of collagen that connect pia mater to the arachnoid. Pial mesothelium is reflected onto the surface of each trabecula and continues onto the inner surface of the arachnoid. This provides a complete lining for the subarachnoid space, which contains cerebrospinal fluid. The arachnoid consists of several layers of meningothelial cells, held together by desmosomes, and a network of collagen bundles. The dural sheath is composed of dense connective tissue with mesothelium on its inner surface.
Anderson DR. Ultrastructure of Meningeal SheathsNormal Human and Monkey Optic Nerves. Arch Ophthalmol. 1969;82(5):659-674. doi:10.1001/archopht.1969.00990020653015