The human optic nerve: fascicular organisation and connective tissue types along the extra-fascicular matrix

Fibres in the mammalian optic nerve are arranged into fascicles between which there is an extra-fascicular matrix containing connective tissue, a feature similar to that found in association with fibres in peripheral nerves, but not otherwise found in the CNS. The relationship between these major features of the nerve architecture are not known. We have addressed this question by examining the fascicular organisation of the optic nerve and the distribution of connective tissue and specific collagen types in the human. We have also examined the spatial development of connective tissue in the human nerve to determine when and from where it originates. Fibres are grouped into fascicles at all locations along the nerve, except intracranially, close to the chiasm where this pattern is lost. Relatively large fascicular numbers are found directly behind the eye and in the region of the optic canal, but decline in the mid-orbital segment of the nerve. Connective tissue is present in the extra-fascicular matrix throughout the fasciculated segment, but in many cases it does not fully encircle fascicles. The proportion of matrix occupied by connective tissue is similar along the length of the nerve (approximately 60%). Within the matrix, collagen types I, III, IV, V and VI are present throughout fasciculated regions. Staining for types V and VI appeared relatively weak compared with that for the other types. Although the collagen types in the nerve are similar to those at the lamina cribrosa and in peripheral nerves, they did not appear to be differentially distributed as in regions of the PNS. Connective tissue enters the nerve at a number of wide-spread locations early in development, consistent with the notion that it enters the nerve with the blood supply. It is present within the matrix before it is established at the lamina cribrosa.