Angiogenesis is an essential component of endometrial renewal during the menstrual cycle and placenta formation during
pregnancy. Vascular endothelial growth factor (VEGF) and its specific receptors Flt-1 (VEGFR-1), flk1/KDR (VEGFR-2) flt4 (VEGFR-3)
play an important role in vasculogenesis, and both physiological and pathological angiogenesis. However, it is not known whether
angiogenic growth factors play a role in the regulation of angiogenic processes in the postpartum uterus. To determine the temporal
and spatial expression of vascular angiogenic factor and its receptors in postpartum uterine involution, we analyzed by immunohistochemistry
the expression levels of VEGF, Flt-1, flk1/KDR, flt4, and VEGI in the rat uterus 1,3, 5,10 and 15 days after parturition.
In the current study, the immunoreactivity of VEGF and its receptors were assessed separately in stromal, vascular smooth
muscle, endothelial and glandular epithelial cells. Throughout the postpartum involution period, cytoplasmic and membrane
staining for VEGI, VEGF and its receptors was observed in the luminal, crypt and glandular epithelial cells, as well as in the
connective tissue and both vascular endothelial and smooth muscle cells in the endometrium. However, the results demonstrated
the intensity of immunoreactions varied in the endometrial components of the uterus. VEGI, VEGF and its receptor immunoreactivities
in the stromal cells and blood vessels were stronger throughout the involution period, supporting the hypothesis that these peptides most probably contribute to the regulation of angiogenesis and blood vessel function in the involuting uterus of the
postpartum rat and that the involuting endometrium is a highly angiogenic tissue.