Morphological and quantitative analysis of the fascicular pattern of monkey optic nerve

Fascicular formation by processes of astrocytes was investigated immunohistochemically with an antibody against glial fibrillary acidic protein in the monkey optic nerve. Fascicular patterns were analyzed using an image analyzer. A steep decrease in total fascicular number, following "fusion" of small fascicles, was seen in the rostral part of the optic nerve; this became more gradual in the caudal part of the optic nerve. Just behind the eyeball, most of the fascicles were small (75.4% of all fascicles, n=958); these decreased sharply in number (16.7% of all fascicles) in the rostral part of the optic nerve. In contrast, the numbers of the middle-sized, large, and largest fascicles increased gradually in the rostral part of the optic nerve. The fascicular number in each fascicular group was constant at the caudal part of the optic nerve, except for the largest fascicles, which varied in number throughout the optic nerve. The change in fascicular size along the optic nerve was strikingly similar to that of the fiber dispersion. Distribution patterns of the small and middle-sized fascicles were different from that of the largest fascicles, which were preferentially distributed at the periphery of the optic nerve. The pattern of fascicular distribution according to size seemed to be related to the centroperipheral retinotopic fiber order.