Fenestrated connective tissue sheets, derived from the sclera as meridional extensions, form the main support for the posterior portion of the lamina cribrosa. The sheets consist of a dense collagenous tissue, often with elastic fibers, and in monkeys with pigment cells. Astrocytes fill the spaces between successive sheets and line the rim of each fenestration.
Anteriorly, at the level of the choroid, the connective tissue elements disappear and the lamina cribrosa becomes entirely glial. The anterior portion of the lamina cribrosa is an extensive structure that occupies virtually the entire optic nerve head and accounts for much of its volume. The astrocytes are modified in arrangement and internal structure, reflecting their function of supporting the unmyelinated nerve fibers as they make a 90° turn. The disc capillaries are enclosed within the glial framework, but whether the astrocytes are an important part of the nutritional pathway to visual axons is not known.