Astrocytes in the lamina cribrosa of the rat optic nerve: are their morphological peculiarities involved in an altered blood-brain barrier?

The lamina cribrosa of the mammalian optic nerve is thought to build a barrier for oligodendroglia progenitor cells migrating from the optic nerve towards the retina. One of the best-known properties of this region is that all optic axons are unmyelinated. Additionally, the blood-brain barrier appears to be interrupted by free access of blood-borne substances from leaky choroid vessels at the surface of the nerve. Several authors suggested the astrocytes to be responsible for these unusual features of this part of the central nervous system. Therefore, we decided to study the astrocytes morphologically by means of ultrathin section and freeze-fracture replica investigation. The main results are the followings: 1. In 12, 23 and 40 days old rats, axonal spheroids with multiple dense bodies were observed within the lamina cribrosa, but not in the optic nerve proper. Large vacuoles with similar inclusions were found in cells of the lamina cribrosa of adult rats which were identified as astrocytes by the occurrence of gap junctions and intermediate filaments. We assume that lamina cribrosa astrocytes have phagocytozed degenerating axonal spheroids. Microglial cells appear to be not involved in this process. 2. Freeze-fracture replicas allow to study the distribution of orthogonal arrays of particles (OAPs) in astrocytic membranes. The OAP polarity index (ratio of OAP-densities or OAP-areas in endfoot versus non-endfoot membranes) is reduced in the lamina cribrosa, as was shown previously by Rohlmann et al. (1992) in the myelin-deficient (md) rat mutant, too. We conclude that non-myelination reduces the polarity of astrocytes and discuss the possibility of an interrelationship between myelination, axonal growth, astrocytic properties and quality of the blood-brain barrier.