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

Abstract

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. J. Comp. Neurol. 516:1–19, 2009. © 2009 Wiley‐Liss, Inc.

Structural Remodeling of Astrocytes in the Injured CNS

“…These processes have none of the numerous fine processes making up the spongiform appearance characteristic of protoplasmic astrocytes. The primary processes can be smooth, rough, straight, or undulating (Sun and others 2009). They vary in the degree of higher order branching, with some showing many short fine offshoots (Fig.…”

“…Panels A and H adapted from Sun and others (2010). Panels B, D, and E adapted from Sun and others (2009). Panels F and G adapted from Oberheim and others (2009).…”

“…Fibrous astrocytes show much greater variation in morphology both between and within the same CNS structures than protoplasmic astrocytes. For example, fibrous astrocytes in the rodent optic nerve have three dominant forms based on the orientation of their primary processes with respect to the long axis of the nerve: random, transverse, and longitudinal forms (Butt and others 1994a, 1994b; Sun and others 2009) (Fig. 3A-3D).…”

“…3E). The transversely and longitudinally oriented astrocytes may represent extreme variants of the random form (Butt and others 1994a, 1994b; Sun and others 2009). It is unknown whether these morphologically distinct forms have different functional/molecular differences, such as reported for gray matter astrocytes in the hippocampus and oligodendrocyte precursor glial cells (Steinhaüser and others 1992; Zhou and Kimelberg 2001; Bachoo and others 2004; Wallraff and others 2004; Karadottir and others 2008).…”

“…Visualizing EGFP-expressing reactive astrocytes in the crushed optic nerve reveals a dramatic reorganization of the normal astrocytic architecture (Sun and others 2010). This is particularly apparent in the optic nerve head because in normal animals, there is a distinct arrangement of the astrocytes (Sun and others 2009).…”