Immunohistochemical expression of vascular endothelial growth factor receptors Flt1 and KDR in the endometrium during the estrus cycle in albino rats

Background Angiogenesis is an important process in endometrial development and embryonic implantation and is regulated through vascular endothelial growth factor (VEGF); its receptors Flt1 and KDR. Aim This work aimed to study the immunoexpression of VEGF receptors (VEGF-Rs) in the endometrium at different ages and reproductive phases and correlate them with the histological profiles in these phases. Materials and methods Seventy female albino rats were included in this study. They were divided into seven groups of 10 rats each: one group consisted of rats in the prepubertal period at age 4–6 weeks; five groups consisted of rats in the reproductive period at age 6–10 months, which were divided according to estrus cycle phases into proestrus, estrus, metestrus, diestrus, and pregnant groups; and the sixth group consisted of rats in the postmenopausal period at age 15–18 months. The uteri of all rats were removed and processed for staining with H&E and were subjected to immunohistochemical staining for Flt1 and KDR. For morphometric measurements, uterine wall thickness and Flt1 and KDR optical density in the endometrial surface epithelium, glandular epithelium, stromal cells, and endometrial endothelial cells were measured using image analysis. Results were statistically compared. Results The expression of VEGF-Rs was highest in the pubertal age group with marked expression of these receptors in the proestrus phase followed by the estrus phase. This supports the role of sex hormones, especially the estrogen hormone, in regulating VEGF-R expression. The Flt1 receptor was predominantly expressed in endometrial and stromal cells as well as in blastocysts, whereas the KDR receptor was predominantly expressed in endometrial endothelial cells. Comparison among all groups and then between each two groups revealed statistically significant differences in the measured morphometric parameters. Conclusion The upregulation of Flt1 and KDR could be involved in the regulation of endometrial endothelial cell proliferation and in increase in endometrial vascular permeability, especially at implantation sites.