HISTOMORPHOMETRY OF THE OPTIC NERVES OF NORMAL DOGS AND DOGS WITH HEREDITARY GLAUCOMA

The beagle dog with hereditary primary open-angle glaucoma, unlike oth er animal models of human glaucoma, possesses a slowly progressive, su stained elevation of intraocular pressure. The effects of this insidio us elevation in intraocular pressure on the axons of the optic nerves of three beagles at early stages of glaucoma and two beagles with adva nced signs of glaucoma were compared to the optic nerves of four age-m atched normal dogs. Plastic embedded optic nerve cross-sections (1 mu m) 1 mm posterior to the lamina cribrosa were osmicated and stained wi th Toluidine Blue. Axons from 0.2 to 2.0 mu m in diameter were counted and measured in 16 cross-sectional regions of equal size within the w hole optic nerve using a computerized image analysis system. The mean optic nerve axon diameters in the normal, early glaucomatous, and adva nced glaucomatous dogs were 1.53, 1.25 and 1.13 mu m respectively. The average total optic nerve axon count in the normal dogs was 148 303. Approximately 16% of the total axonal fibers were counted in each nerv e. The counts of optic nerve axons 2.0 mu m or greater in diameter wer e reduced by up to 60% in the central regions of the optic nerves of a ffected beagles. The large diameter axons of the peripheral optic nerv e of the beagle dogs with glaucoma were more resistant to the elevated intraocular pressure. The counts of axons > 0.6 to 0.8 mu m in diamet er were significantly increased in glaucomatous beagles.