Abstract
Purpose: :
Elevated intraocular pressure (IOP) is one of the most important risk factors for the development of glaucoma. A number of studies have suggested that morphological changes of retinal ganglion cell (RGC) dendrites occur prior to RGC apoptosis. Here we sought to characterize the timecourse and morphological features of RGC changes in response to elevated IOP in a mouse model of glaucoma.
Methods: :
Elevated IOP was induced in one eye of transgenic mice expressing cyan fluorescent protein (CFP) under the control of the Thy1 promoter through injections of a hyaluronic acid solution into the anterior chamber. The contralateral eye served as a control. Animals were monitored daily and injections were repeated as needed to maintain elevated IOP. After four and nine days of elevated IOP mice were euthanized, enucleated and their retinas were flat mounted for confocal microscopy evaluation. Confocal image stacks of individual retinal ganglion cells were collected at 200x, 400x and 600x, and analyzed using ImageJ software. Parameters evaluated included cell body size, dendritic field area, number of primary dendrites, and degree of dendrite branching.
Results: :
Hyaluronic acid injections increased the IOP on average to 16.2 mmHg (control eyes = 9.4 mmHg). In this strain of mice approximately 0.07% of all RGC express CFP allowing morphometric analyses of the RGC soma, axon, and dendrites, typically without interference from neighboring cells. Our findings demonstrate that the degree of dendrite branching increases in response to elevated IOP. The changes reach statistical significance as early as four days after induction of elevated IOP (p= 0.01 and 0.005 at 4 and 9 days, respectively). We did not observe significant changes in RGC soma size (p= 0.60 and 0.52 at 4 and 9 days, respectively), dendritic field size (p= 0.43 and 0.35 at 4 and 9 days, respectively), or the number of primary dendrites (p= 0.74 and 0.11 at 4 and 9 days, respectively). The number of RGC was not reduced after four and nine days of elevated IOP.
Conclusions: :
The Thy1-CFP mouse is an excellent model to evaluate morphological changes of individual RGC. Elevated IOP leads to rapid changes in RGC morphology that precede significant RGC loss. In particular we observed an increase in the degree of dendrite branching. These findings suggest that aberrant dendrite structure may be an early indicator of IOP induced retinal damage.
Keywords: ganglion cells • microscopy: confocal/tunneling • intraocular pressure