Abstract
Purpose :
Unlike peripheral nerve, the ON is a central nervous system (CNS) tract. Unlike most of the CNS, the ON undergoes relentless mechanical loading and deformation during eye movements. We performed histological analysis of human orbits to quantify variations in the proportion of neural and connective tissue along the intraorbital ON.
Methods :
We studied orbits from 5 adults 44–93 (mean 64) yrs, children aged 17 mos and 4 yrs, and a stillborn fetus at 33 wks gestation. Whole orbits were exenterated en bloc, embedded in paraffin, and serially sectioned to the orbital apex at 10 mm thickness in the coronal plane perpendicular to the long orbital axis. Sections were stained with Masson’s trichrome and digital micrographs were obtained at 200mm intervals throughout the orbit. The ON was analyzed within the surrounding adherent pia mater. Using digital color thresholding, the blue-staining collagen of connective tissue and the red-staining neural tissue were independently selected. Percentage areas of each component were calculated relative to total area of the ON in each section. Linear regression was performed to relate the contribution of each tissue type to the distance posterior to the globe.
Results :
In adults, neural tissue contributed 71.5±8.2% to ON cross section anterior to the annulus of Zinn (AZ), and 64.5±12.4% posterior to it. Connective tissue contributed 28.5±8.2% and 35.5±12.4%, respectively. Linear regression of connective tissue percentage showed that it declines with distance from the globe with mean slope of -0.08±0.75% per mm anterior to AZ and increases 2.08±3.2% per mm posterior. Analysis of variance showed that connective tissue proportions varied significantly among the three pediatric specimens (p<0.0001). Fetal specimen had significantly more connective tissue (62.6±8.7%) than the 17 mos (46.5±9.3%) and 4 yr (23.7±5.6%) specimens. Values in the youngest specimens were significantly greater than in adults (p<0.0001).
Conclusions :
The intraorbital adult ON contains substantial internal connective tissue in the range of 20-30% of total cross section, and tends to increase posterior to AZ. This distinguishes it from other CNS white matter tracts, and is probably related to the unique mechanical loading of the ON during eye movements. Connective tissue in the ON may account for physiologic and pathologic changes associated in conditions such as glaucoma and ischemic optic neuropathies.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.