Collagen Microstructure In The Glaucomatous Human Optic Nerve Head: Distribution And Fiber Orientation

Abstract

Purpose: To analyse fibrillar collagen distribution and connective tissue fiber orientation within the glaucomatous human optic nerve head (ONH).

Methods: Human optic nerve heads (ONHs, 3 glaucoma and 3 age-matched controls) were cryosectioned transversely (10-15 100μm-thick sections from each ONH) from pre- to post-laminar. Using second harmonic generation (SHG) microscopy and small angle light scattering (SALS), the distribution and orientation of fibrillar collagen was visualized and preferred fiber orientation and degree of fiber alignment (DOFA) was calculated in each section. The lamina cribrosa (LC) was subdivided into 12 regions for analysis: 2 regions in the superior (S1, S2), inferior (I1, I2), temporal (T1,T2), nasal (N1,N2), and SN, IN, IT and ST.

Results: In normal human ONHs, SHG scatter identified fibrillar collagen oriented radially in the LC, surrounding central retinal vessels and circumferentially in the peripapillary sclera. In all ONHs, the DOFA was high, average: 0.64 (0 is random and 1 is perfect alignment) in the peripapillary sclera. This can be attributed to highly aligned fibrillar collagen, identified in both SHG and SALS datasets, circumferentially orientated around the ONH. SHG scatter patterns were distinct in areas of glaucomatous LCs; observations that were supported in fiber orientation plots. In glaucomatous LC, quantitative measurements of DOFA showed greater inter-regional variation compared to that determined in a normal LC. In addition, glaucomatous ONH DOFA plots indicated sub-regions of higher alignment within the LC. Interestingly, significant differences in DOFA were observed in all regions when compared between normal and glaucoma ONHs (p<0.05), with the exclusion of the temporal LC region, and in one case the inferonasal region.

Conclusions: Fibrillar collagen distribution and fiber orientation vary with location in the human ONH and appear to be affected in glaucomatous optic neuropathy. Our findings of altered collagen alignment within the glaucomatous LC reflect a change in LC biomechanics, likely a consequence of connective tissue disorganization, remodelling and/or loss. The presence of greater disorganization suggests that homeostatic mechanisms for aligning connective tissue elements in the direction of load may be impaired in the glaucomatous LC.

Keywords: 577 lamina cribrosa • 519 extracellular matrix • 551 imaging/image analysis: non-clinical