Intrinsic and Pial Connective Tissue Content of Human Retrobulbar Optic Nerve (ON)

Abstract

Purpose : Both prevalence of glaucoma and mechanical stiffness of the ON increase with age (Shin et al., ARVO 2017). We sought to clarify the structural basis of age-related ON stiffening by evaluating the connective tissue content of the ON and its ensheathing pia.

Methods : Seven human ONs aged 1.4, 4, 26, 62, 67, and 80 yrs were embedded in paraffin, sectioned transversely in the immediately retrobulbar region at 10 microns thickness, and stained with Masson trichrome for light microscopy using a Nikon E800 microscope. Digital colorimetry was performed in 3 sections of each specimen to quantify relative areas of pia connective tissue, intrinsic connective tissue within the ON, and ON axons.

Results : All ONs were ensheathed in a dense collagenous pial layer that was directly contiguous with a matrix of dense, interconnected collagenous septae intercalating intrinsically among the ON axon bundles. The intercalated relationship of this connective tissue to ON axon bundles differs markedly from the tenuous relationship of the diaphanous pial layer of the cerebral cortex to underlying brain tissue from which it is readily dissected. There was section to section variability in connective tissue quantity in each ON. Overall, however, ONs from ages 4-66 yrs had similar intrinsic connective tissue ranging 13-19%. The 1.4 yr old ON, however, contained 31±4% (SD) intrinsic connective tissue. There was a trend to increasing ON connective tissue beginning in the late 7^th decade of life, comprising 34±8% of 67 yr and 41±12% of 80 yr old ON area. A similar trend was evident in the pia, which comprised about 10% of total ON cross section from ages 4-66 yrs. The older ON’s exhibited greater relative pial cross section: 22±2% in the 67 yr old, and 25±4% in the 80 year old specimen.

Conclusions : Unlike brain, the human retrobulbar ON contains a substantial intrinsic matrix of interconnected collagenous septae connective tissue contiguous with its dense pial sheath. This structure is consistent with biomechanical modeling of the ON and its sheath as a composite matrix of both connective tissue and axons. The proportion of connective tissue varies among subjects, but may increase with age in the elderly.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.