Elevated blood/lymphatic vessel ratio in pterygium and its relationship with vascular endothelial growth factor (VEGF) distribution.

Abstract

Pterygium is a conjunctival fibrovascular tissue growth on the cornea. The pathogenesis of pterygium involves several factors such as the presence of active angiogenic factors. Expansion of the lymphatic microvasculature has also been hypothesized. This study examines the activity of the angiogenic/lymphangiogenic factor VEGF and the expression of vascular and lymphatic endothelial proteins in pterygia and normal conjunctival tissues. Primary grade 2 pterygium (n=20) and normal conjunctiva (n=20) biopsies were obtained during surgery after written informed consent. mRNA expression for CD31, podoplanin, and VEGF (isoforms VEGF-A and VEGF-165) were determined by qRT-PCR. Tissue samples were also processed for immunohistochemical techniques to examine the lymphatic and vascular endothelium (anti-D2-40, anti-CD31 respectively) and VEGF-A and VEGF-C levels and distribution. VEGF-A gene expression levels failed to differ between the healthy and pterygium tissues. However, expression of its more angiogenic isoform, VEGF-165, was significantly higher in the pterygia. Immunohistochemistry revealed the greater presence of VEGF-A, compared to VEGF-C, in pterygium than conjunctiva, both in blood vessels and extracellular matrix. In addition, pterygia showed higher expression levels of the endothelial junction protein CD31. Lymphatic marker D2-40 expression was slightly augmented in this pathological tissue. The ratio between blood and lymphatic vessel counts was 1.05 in the normal conjunctiva and 3-fold this value in pterygium. In pterygium, while both lymphangiogenesis and angiogenesis take place, the formation of new blood vessels is the most relevant event, correlating with the increased expression of vascular endothelial CD31 and an elevated blood/lymphatic vessel ratio. The presence of high levels of VEGF-A in both vessel networks and extracellular matrix in human pterygium tissue may have a major impact on angiogenesis in this pathological tissue.