Because GFAP does not seem to be uniformly expressed by the entire
retinal astrocyte population, an RNA probe against PDGFRα was used as
an alternative marker for retinal astrocytes. PDGFRα is expressed by
retinal astrocytes
29 and is involved in the mitogen-driven
expansion of the retinal astrocyte population.
23 Unlike
GFAP mRNA, PDGFRα mRNA was detected strongly in all retinal
astrocytes, including those that were not in contact with the vascular
network
(Fig. 6) . In P0 mice, retinal astrocytes covering more than half of the retinal
surface were detected far beyond the central vascularized area
(Fig. 6A) . This distribution correlates with the distribution of spindle
cells stained with cresyl violet or Hoechst 33342
(Figs. 1D 1E) . At
the leading edge of the PDGFRα-positive network, cord-like structures
were observed
(Figs. 6B 6C) . These cords are formed by spindle-shaped
cells
(Fig. 6B) and have previously been described.
4 7 30 Double labeling with Hoechst demonstrated that all cells labeled with
the PDGFRα probe had a spindle-shaped nucleus
(Figs. 6C 6D 6E) . It is
obvious from
Figures 6D and 6E that these cells not only possessed
spindle-shaped nuclei (
Fig. 1D ; arrows) but also have a spindle-shaped
morphology, suggesting that PDGFRα-positive cells are the same as the
spindle-shaped cells revealed by cresyl violet. At P5 PDGFRα-positive
cells (retinal astrocytes) were seen clearly labeled up to the
periphery of the retina
(Fig. 6F) but their bipolar morphology,
apparent at P0, had changed to a more star-shaped morphology
(Figs. 6G 6H) . Retinal astrocytes beyond the collagen type IV–positive vessel
network were clearly labeled by the PDGFRα probe
(Fig. 6H) . At this
age, the nucleus of retinal astrocytes was not as elongated as at P0,
but was more ellipsoid (
Fig. 6I , arrows).