Development of Retinal Astroglia

Abstract

Müller cells and retinal nerve fiber layer astrocytes are the major astroglia of the mammalian retina. They have numerous important functions in adulthood for maintaining neuronal homeostasis as well as in developing retina, where they facilitate key events in the assembly of the retinal tissue. Recent years have seen substantial progress in understanding how these astroglial cells develop and how their development shapes the cells around them. We review the mechanisms underlying the formation, maturation, and spatial patterning of Müller glia and retinal astrocytes, with an emphasis on how they acquire their functional properties. We focus on developmental events that have a major impact on overall retinal integrity, such as the formation of neuro-glial junctions at the outer limiting membrane and the patterning of retinal astrocytes into a template that guides angiogenesis. Finally, we discuss examples of retinal diseases that originate in developmental defects affecting Müller cells or retinal astrocytes. These include certain classes of inherited retinal degenerations, as well as retinopathy of prematurity.

Keywords: Müller glia; fovea; synapse; tiling; vasculature.

Figure 1:. The mammalian retina and its astroglial cell types

A) Drawing of a mature mammalian retina with all of the principal cell types represented. B) Individual astrocytes of adult mouse retina (adapted from Holden et al., 2023) demonstrating the variety of distinct astrocyte morphologies. Scale bars, 25 μm. C) Individual Müller cell of bovine retina, illustrated by Santiago Ramón y Cajal (1893). D) Drawing of embryonic mouse retina (Ramón y Cajal, 1893) illustrating the morphology of radial glial progenitor cells. The progenitors have attachments at the apical side (top), while newborn neurons are located basally. Abbreviations: Hz, horizontal cell; RGC, retinal ganglion cell; RNFL, retinal nerve fiber layer; OLM, outer limiting membrane; ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer.

Figure 2:. Colonization of the retina by astrocytes and vasculature

A) Astrocytes and vasculature colonize the mouse retina via sequential waves of migration outward from the optic nerve (asterisk). Illustrations show progression of astrocytes (top) and vasculature (bottom) across the retina at three different stages of mouse development. B) Cross-sectional view of an E12 mouse retina highlighting the fate-committed progenitor region that generates RNFL astrocytes. Astrocytes migrate out from this region to colonize the RNFL (A) C) Top: Morphology of the astrocyte honeycomb-like template that guides angiogenesis. Green, astrocyte cell surface marker (anti-PDGFRα); magenta, vascular cell surface marker (IB4 lectin). Arrows: Endothelial tip cells at the advancing vessel wavefront; arrowheads: tip cell filopodia. Bottom: Phenotypic differences between astrocyte precursor cells (APCs) located ahead of the vascular wavefront and mature astrocytes behind the wavefront. VEGF-A (green) is highly expressed by APCs. Red, pan-astrocyte marker (Sox9). Blue, vasculature (IB4 lectin). D) High-magnification view of endothelial tip cells (arrows) and tip cell filopodia (arrowheads). Scale bars: 50 μm (C); 25 μm (D).

Figure 3:. Müller cell tiling

Adult mouse retinas in which Müller cells express a combination of membrane-targeted fluorescent proteins through the Brainbow virus system. Left, cross-section view. The entire Müller cell is labelled from endfeet at RNFL to microvilli at OLM. Note lack of overlap between adjacent cells at all retinal layers. Right, en face view of a retinal wholemount, imaged at the IPL level to show synapse-associated Müller arbors. The lack of arbor overlap is best appreciated in high-magnification view (inset). Abbreviations as in Fig. 1. Scale bars, 10 μm.

Figure 4:. Anatomy of the outer limiting membrane

A) Representation of the mouse outer retina showing OLM (red arrow) in cross-section view. Schematic drawing (right) is aligned to the photomicrograph (left). OLM separates nuclear layer from inner segments. B) En face views of the OLM. Top: Brainbow-labeled Müller glia in a retinal wholemount (as in Fig. 3), imaged by confocal microscopy at the OLM level. Small unlabelled circles within Müller arbor territories correspond to photoreceptor inner segments. Middle: En face electron micrograph of adult mouse OLM. Bottom: schematic illustrating the cellular compartments within middle image. Müller cells tile the OLM, meeting at borders containing electron dense homotypic adherens junctions. Heterotypic adherens junctions surround inner segments. Numbers indicate territories belonging to individual Müller cells in the real image (middle) and schematic (bottom). Scale bars, 10 μm (top); 1 μm (middle).