Concentration of astrocytic filaments at the retinal optic nerve junction is coincident with the absence of intra-retinal myelination: comparative and developmental evidence

The structure of the lamina cribrosa (LC) and astrocytic density were exami ned in various species with and without intra-retinal myelination. Sections of optic nerve from various species were stained with Milligan's trichrome or antibodies to glial fibrillary acidic protein, myelin basic protein (MB P) and antibody O4. Marmoset, flying fox, cat, and sheep, which lack intrar etinal myelination, were shown to possess a well-developed LC as well as a marked concentration of astrocytic filaments distal to the LC. Rat and mous e, which lack intraretinal myelination, lacked a well-developed LC but exhi bited a marked concentration of astrocytic filaments in this region. Rabbit and chicken, which exhibit intraretinal myelination, lacked both a well-de veloped LC and a concentration of astrocytes at the retinal optic nerve jun ction (ROJ). A marked concentration of astrocytes at the ROJ of human fetus es was also apparent at 13 weeks of gestation, prior to myelination of the optic nerve; in contrast, the LC was not fully developed even at birth. Thi s concentration of astrocytes was located distal to O4 and MBP immunoreacti vity in human optic nerve, and coincided with the site of initial myelinati on of ganglion cell axons in marmoset and rat. Myelination proceeded from t he chiasm towards the retinal end of the human optic nerve. Moreover, the o uter limit of oligodendrocyte precursor cells (OPC) migration into the rabb it retina was restricted by the outer limit of astrocyte spread. These obse rvations indicate that a concentration of astrocytic filaments at the ROJ i s coincident with the absence of intraretinal myelination. Differential exp ression of tenascin-C by astrocytes at the ROJ appears to contribute to the molecular barrier to OPC migration (see Bartsch et al., 1994), while expre ssion of the homedomain protein Vax 1 by glial cells at the optic nerve hea d appears to inhibit migration of retinal pigment epithelial cells into the optic nerve (see Bertuzzi et al., 1999). These observations combined with our present comparative and developmental data lead us to suggest that the astrocytes at the ROJ serve to regulate cellular traffic into and out of th e retina.