Ultrastructural features of retinal capillary basement membrane thickening in diabetic swine

Abstract

Purpose: To assess retinal capillary basement membrane thickening (BMT) in a swine model of type 1 diabetes.

Materials and methods: Yorkshire pigs were rendered diabetic with streptozotocin and dyslipidemic with a high fat and cholesterol diet. At 18, 26, and 32 weeks of diabetes, the retina sections within 3 disc diameters from the optic disc were examined under transmission electron microscopy to evaluate the ultrastructural features of the capillary BM. Digital morphometric analysis was performed to measure BMT.

Results: Diabetic swine had significantly thicker retinal capillary BMs compared to controls. Pigs that sustained diabetes for longer periods or experienced severe diabetes tended to have more BMT. Those pigs that did not sustain glucose levels above 200 mg/dL did not demonstrate thicker retinal capillary BMs. Characteristic ultrastructural features of diabetic vasculopathy observed included rarefaction as an early stage of Swiss cheese cavitation, lamellation with multiplication of electron dense layers, and fibrillar materials within capillary BM.

Conclusions: Diabetic Yorkshire pigs develop characteristic features of an early retinal microvasculopathy fairly rapidly and may serve as a higher-order animal model for studies of type 1 diabetes.

Figure 1. Digital morphometric analysis of basement membrane thickness

For quantitative measurement of BM thickness, the inner and outer boundary of the BM were drawn (A) and the area of outer BM between the glial cells and pericyte or endothelium (gray color) was measured digitally (B). The average outer BM thickness was calculated by dividing the area by the length of the outer BM.

Figure 2. Fasting glucose level and body weight change of diabetic pigs

(A) While the fasting glucose of pig A, B, and D were well maintained at high level after STZ injection, the glucose levels of pig C and E were fluctuating below 200 mg/dL at least one time during the follow-up. (B) The diabetic pigs B and D which experienced the ketoacidosis gained the smallest body weight during the follow-up, while the moderate diabetic pigs C and E gained body weight more than twice that of the severe diabetic pigs B and D.

Figure 3. Thickening of retinal capillary basement membrane in diabetic pigs

The mean retinal capillary BM widths of pigs A, B and D were significantly increased compared to those of controls after 32, 26, and 18 weeks of hyperglycemia, respectively. Pigs C and E that had variable hyperglycemia did not demonstrate a statistically significant change in thickness of capillary BM.

Figure 4. The ultrastructural features of retinal capillary basement membrane in diabetic pigs

The EM of capillary from diabetic pig A reveals rarefaction with irregular electron dense materials splitting the BM, which appears to be an early stage of Swiss cheese vacuolization (white arrows in A, magnified in B). BM thickening with fibrillar materials extending outwards to adjacent cells was also observed (black arrows in A and magnified in C). The capillary from diabetic pig D demonstrated lamellation, a multiplication of electron dense layers leading thickening of BM (arrows in D and magnified in E).