The collagen fibrillar network in the human pial septa

The three-dimensional organization of the collagen fibrils in the human pial septa was studied by transmission electron microscopy and by scanning electron microscopy after cell maceration with sodium hydroxide. The collagen fibrils along the nerve fibers were loosely distributed thin filaments with a diameter ranging from 25 to 40 nm (mean, 32 +/- 6 nm). Most fibrils were oriented perpendicularly to the long axis of the nerve fiber. In the deeper stroma of the pial septa, collagen fibrils were well packed and the diameter was 40 +/- 6 nm (range, 35 to 50 nm). The scanning electron microscopic study demonstrated a smooth, fabric-like structure formed by wavy and loosely interwoven thin collagen fibrils. Under high magnification, multiple collagenous sheets were observed in the pial septa by scanning electron microscopy. In addition to the structure of loosely arranged thin fibrils on the surface layer adjacent to the nerve fibers, two distinct patterns of collagen fibril network were demonstrated in the deeper stroma. One exhibited fibrils with random distribution, and the other showed well-packed, undirectionally oriented collagen. Small openings corresponding to elastic tissues were also observed. This collagen architecture may contribute to the flexibility and tensile strength of the pial septa required in the intraorbital portion of the optic nerve.