vitreous, resulting in visual loss; in addition, the fibrous tissue accompanying new vessels can cause traction on the retina leading to retinal detachment (26, 27).

METHODS TO MEASURE AND DETECT CAPILLARY DROPOUT

Histological assessment of capillary dropout can be performed in trypsin-digested retina preparations. Capillaries in retinal trypsin digests are stained with periodic acid Schiff-hematoxylin. Acellular capillaries are identified as capillary-sized vessel tubes having no nuclei anywhere along their length (presumably from degeneration of endothelial cells), leaving only a basement membrane tube (Fig. 4). Pericyte ghosts are detected as spaces or pockets in the capillary basement membrane from which pericytes have disappeared (28).

Clinically, the signs of diabetic retinopathy are routinely evaluated by ophthalmologists using fundus examination and photography. Fluorescein angiography helps show physicohemodynamic processes, and has allowed us to understand abnormalities of the capillary bed, including microaneurysms, dilated capillaries, capillary closure and abnormal permeability of the retinal vasculature (29). It can detect microaneurysms and other capillary changes with much greater sensitivity than routine ophthalmoscopy, including leaking vessels, capillary closure and even small new vessels (30). Further, fluorescein angiography has the capability to elucidate detailed microstructural vascular alterations that can escape fundus examination. Microaneurysms that appear as dots on fundus photograph are brightly fluorescent in fluorescein angiography (31, 32). Hemorrhages appear as areas of blocked fluorescence, while hard exudates usually show minimal blocking of fluorescence on fluorescein angiography. Cotton wool spots, which represent areas of ischemic infarction within the nerve fiber layer, appear as fluffy white lesions on fundus examination, and as an area of retinal haziness with blockage of underlying background fluorescence on angiography. Capillary closure is indicated on fluorescein angiography as black patches interrupting the normal capillary pattern (Figs. 5 and 6) (33).

There appears to be a good correlation between the results obtained from fluorescein angiography and trypsin-digested retinal vessels. Comparison of fluorescein angiography of an individual with a trypsin-digested retina preparation a few months afterward demonstrated that areas of capillary nonperfusion correspond to acellular capillary beds (34). In addition, India ink injection preparations fail to fill acellular capillaries, further suggesting their nonfunctional nature (35). These observations support the correspondence between acellular capillaries and capillary dropout.

MODELS TO STUDY RETINAL CAPILLARY DROPOUT IN DIABETES

In order to understand the mechanism of capillary dropout in diabetic retinopathy, suitable models (in vivo and in vitro) are vital. Various animal models have been used to investigate diabetes-induced capillary dropout in the retina. Animal models present an advantage because these models can be used to screen potential therapies for their ability to inhibit the development of retinopathy. In addition, these models have allowed dissection and isolation of various factors associated with the pathogenesis of diabetic retinopathy, and is helping the development of effective strategies to address the problem. For the