Unique Retinal Microvasculature Architecture in Tree Shrews Visualized by Visible-Light Optical Coherence Tomography Angiography

Abstract

Purpose : The tree shrew has become a popular animal model for studying retinal pathology as it shares greater similarities to the human retina compared to rodent models, including a structured lamina cribrosa and higher cone photoreceptor density. However, the microvasculature structure of the tree shrew retina remains unexplored. Thus, we applied visible-light optical coherence tomography angiography (vis-OCTA) to visualize the microvasculature of the tree shrew retina with high resolution.

Methods : We imaged healthy 5-36 month-old tree shrews using vis-OCTA. Before imaging, tree shrews were anesthetized with 3% isoflurane with supplemental oxygen. Tropicamide and phenylephrine drops were applied to both eyes to dilate the pupils and induce cycloplegia. Tree shrews were kept warm during imaging using a near-infrared heat lamp, and artificial tears were applied to prevent corneal dehydration. Temporal speckle averaged (TSA) datasets were acquired as five repeated vis-OCTA volumes consisting of 512 A-lines/B-scan × 512 B-scans, with each B-scan repeated twice. An A-line rate of 75 kHz was used, resulting in a total acquisition time of ~35 seconds. Repeated vis-OCTA volumes were registered and averaged after processing to reduce speckle noise and increase the signal-to-noise ratio. After processing, we segmented the vascular layers from vis-OCTA images using the structural landmarks in the vis-OCT image for reference.

Results : Figure 1a shows an example angiogram revealing the dense microvascular network in the tree shrew retina. We color-coded the same angiogram in depth (Fig. 1b), with red representing the superficial vascular plexus (SVP), green the intermediate capillary plexus (ICP), and blue the deep capillary plexus (DCP). We identified a possible fourth vascular plexus located within the inner plexiform layer (IPL) that can be found throughout the retina, colored yellow in Figure 1b. Further study is needed to validate this unique plexus.

Conclusions : The visualization by vis-OCTA reveals the dense microvasculature of the tree shrew retina, enabling a better understanding of the tree shrew’s retinal anatomy and function as an attractive animal model.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

Example tree shrew vis-OCT angiogram (a), depth encoded angiogram (b), OCT B-scan (c), and vis-OCTA B-scan (d) reconstructed along orange dashed line.