Nerve fiber layer splaying at vascular crossings

To examine the anatomic relationships of retinal blood vessels to the nerve fiber layer.

Retinal flat preparations from monkey and cat were stained with methylene blue dye and examined by light microscopy.

A focal transition in nerve fiber bundle patterns occurs just above major retinal blood vessels. Bundles abandon a compact form and splay open just as they reach a major blood vessel. Maximum separation of axon fascicles occurs immediately above the blood vessel and the bundles reform after the crossing of the blood vessel is complete. In so doing, nerve fiber bundles temporarily thin and broaden, assuming the shape of an inverted hull. In contrast, at the retinal periphery, minor blood vessels and axon fascicles have no special relationship to each other.

At major blood vessel crossings, a focal alteration in nerve fiber bundle anatomy takes place under which local deformability may be enhanced. This adaptation probably lessens the risk of injury to ganglion cell axons from vascular compression. In addition, deflection of the nerve fiber pathway obviates the need for major blood vessels to bend or kink to achieve a crossing, thus avoiding turbulent flow.