Structural remodeling of fibrous astrocytes after axonal injury.

Reactive astrocytes are a pathological hallmark of many CNS injuries and neurodegenerations. They are characterized by hypertrophy of the soma and processes and an increase in the expression of glial fibrillary acidic protein. Because the cells obscure each other in immunostaining, little is known about the behavior of a single reactive astrocyte, nor how single astrocytes combine to form the glial scar. We have investigated the reaction of fibrous astrocytes to axonal degeneration using a transgenic mouse strain expressing enhanced green fluorescent protein in small subsets of astrocytes. Fibrous astrocytes in the optic nerve and corpus callosum initially react to injury by hypertrophy of the soma and processes. They retract their primary processes, simplifying their shape and dramatically reducing their spatial coverage. At 3 d after crush, quantitative analysis revealed nearly a twofold increase in the thickness of the primary processes, a halving of the number of primary processes leaving the soma and an eightfold reduction in the spatial coverage. In the subsequent week, they partially reextend long processes, returning to a near-normal morphology and an extensive spatial overlap. The resulting glial scar consists of an irregular array of astrocyte processes, contrasting with their original orderly arrangement. These changes are in distinct contrast to those reported for reactive protoplasmic astrocytes of the gray matter, in which the number of processes and branchings increase, but the cells continue to maintain nonoverlapping individual territories throughout their response to injury.

Morphology of astrocytes in a glaucomatous optic nerve.

PURPOSE: To establish the morphologic changes of astrocytes in the glial lamina of glaucomatous mice. METHODS: A strain of mice that expresses GFP in individual astrocytes (hGFAPpr-GFP) was crossed into the DBA/2J strain that develops glaucoma. In the resulting strain (D2.hGFAPpr-GFP) we assessed the severity of glaucoma by staining the retina for neurofilaments and counting the neurons of the retinal ganglion cell layer. We observed the morphology of astrocytes in the glial lamina of the optic nerves. RESULTS: D2.hGFAPpr-GFP mice developed glaucoma in an age-dependent manner. Astrocytes in the glial lamina showed morphologic changes that correlated with the severity of glaucoma. The cells showed thickening of processes from 1.3 ± 0.28 µm in nondiseased animals to 1.71 ± 0.46 µm in eyes with moderate glaucoma and 2.1 ± 0.42 µm in those with severe glaucoma. Their spatial coverage, as determined by their convex polygon area, was reduced in eyes with severe glaucoma. The astrocytes in severely glaucomatous optic nerves also showed simplification of their processes. In 6-month-old mice with no obvious signs of degeneration in the retina, we found astrocytes with appendages growing out of primary astrocyte processes into the axon bundles. This localized hypertrophy of processes was never observed in the hGFAPpr-GFP strain. CONCLUSIONS: Confirming results after optic nerve crush, astrocytes in glaucomatous optic nerves had thickened and simplified processes, and reduced spatial coverage. We also found evidence of localized sprouting of new processes in early stages of the disease, before detectable changes in ganglion cell number.

The morphology and spatial arrangement of astrocytes in the optic nerve head of the mouse.

We evaluated the shapes, numbers, and spatial distribution of astrocytes within the glial lamina, an astrocyte-rich region at the junction of the retina and optic nerve. A primary aim was to determine how the population of astrocytes, collectively, partitions the axonal space in this region. Astrocyte processes labeled with glial fibrillary acidic protein (GFAP) compartmentalize ganglion cell axons into bundles, forming "glial tubes," and giving the glial architecture of the optic nerve head in transverse section a honeycomb appearance. The shapes of individual astrocytes were studied by using transgenic mice that express enhanced green fluorescent protein in isolated astrocytes (hGFAPpr-EGFP). Within the glial lamina the astrocytes were transverse in orientation, with thick, smooth primary processes emanating from a cytoplasmic expansion of the soma. Spaces between the processes of neighboring astrocytes were spatially aligned, to form the apertures through which the bundles of optic axons pass. The processes of individual astrocytes were far-reaching-they could span most of the width of the nerve-and overlapped the anatomical domains of other near and distant astrocytes. Thus, astrocytes in the glial lamina do not tile: each astrocyte participates in ensheathing approximately one-quarter of all of the axon bundles in the nerve, and each glial tube contains the processes of about nine astrocytes. This raises the mechanistic question of how, in glaucoma or other cases of nerve damage, the glial response can be confined to a circumscribed region where damage to axons has occurred.