(registered average of 30 B scans) using custom software,8 and the ... All distances in microns (mean ± SD). ... well to the distance between OCT b1 and b4 in the euthermic 13-LGS, but do not correlate as well in the torpid 13-LGS retina. Inner.
#4142-B0261
Retinal Remodeling Throughout Hibernation in the 13-Lined Ground Squirrel B. Sajdak, B.A. Bell, A.E. Salmon, W. Li, D.K. Merriman, and J. Carroll 1
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Results
Ground squirrels are diurnal, cone dominant mammals (~85% cones) with large brain regions dedicated to processing visual information.1 Seasonal retinal remodeling occurs throughout the ground squirrel’s obligatory hibernation cycle.2 Shown below is an example of a 13-lined ground squirrel’s (13-LGS) core body temperature (solid line) compared to the ambient temperature (dotted line) throughout hibernation.3 This circannual cycle includes torpor bouts with brief periods (~12 hr) of inter-bout euthermia (IBE).4 13-LGS photoreceptors are affected by hibernation with shortened/absent outer segments (OS) and depleted inner segment mitochondria.5,6 Hibernation provides a unique natural model to examine the cellular basis of photoreceptor signals in OCT and AOSLO retinal images.
Longitudinal Imaging Suggests Limited Photoreceptor Structural Changes Throughout Hibernation
Inter-Bout Euthermia
24 hrs Post-Hibernation 72 hrs Post-Hibernation
CC
b1 b3 b4
b2
b1
b2 & b3 CC
b4 & CC
Choroid
b1 b2 b3 b4
CC
Choroid
Choroid
b2 b3 b4
Choroid
Feb
Mar
Apr
Methods 13-LGS were obtained from the captive breeding colony at the University of Wisconsin, Oshkosh. A FLIR E60 thermal imaging camera (FLIR Systems, Inc. Wilsonville, OR) was used to verify the 35°C 35°C physiological state (right). Under isoflurane anesthesia, 13-LGS were imaged with a Bioptigen Envisu R2200 SD-OCT (Leica Microsystems, 4°C 4°C Wetzlar, Germany), and an AOSLO as previously described.7 Ten 5-pixel-wide longitudinal reflectivity profiles (LRPs) were collected from OCT line scans (registered average of 30 B scans) using custom software,8 and the LRPs were averaged for peak to peak thickness measurements. Shown below is a comparison of SD-OCT and AOSLO images from the cone-rich regions of 13-LGS and human retinae; the visual streak and fovea, respectively. Scale bars = 50 µm. Thermal Photo
Color Photo
Euthermic 13-LGS
13-LGS Visual Streak
Human Fovea
Torpid 13-LGS
Superior
Inferior
ONH
Superior Retina
Inferior Retina
CC
b3 b4
Choroid
b1
10 20 30
0
b1
b2
ISm
ISm
ISe
ISe
b3
OS
b4
RPE
b2 b3
Superior
Superior Retina
RPE
b4
b1
b1 ISm
10
OS
40
20
b2
ISe
b3
OS
b2 b3
ISe OS RPE
RPE
30 40
ISm
b4
b4
Cone photoreceptors (n=10) were measured and compared to LRP data. ELM to Bruch’s membrane distances measured in the light micrographs compare well to the distance between OCT b1 and b4 in the euthermic 13-LGS, but do not correlate as well in the torpid 13-LGS retina. Inner segment myoid (ISm) measurements and b1 to b2 distances were similar in the euthermic and torpid retina. This alignment places b2 at the inner segment ellipsoid (ISe), which supports the “ellipsoid zone” hypothesis and nomenclature.11,12 The ISe distance was significantly smaller, and OS were fragmented throughout the torpid 13-LGS retina; consistent with previous findings.6 Euthermic and torpid retinal pigment epithelium (RPE) were similar in size, and melanosomes were concentrated in the RPE microvilli that extend alongside the OS.
ex vivo Light Microscopy
20 µm
Reduced Image Quality During Torpor Retinal imaging during torpor is reduced, in part, due to stromal edema. This phenomenon is missed with H&E histology.10 Image quality improves over time at room temperature, and arousal to euthermia can be prevented by maintaining isoflurane anesthesia. 300
Euthermic (n=8)
p
