Endometrial lymphangiogensis

Abstract

This article briefly summarises some of the more important recent advances in understanding of lymphangiogenesis, and then reviews current knowledge of the lymphatics and lymphangiogenesis in the endometrium. The recent identification of vascular endothelial growth factor-C (VEGF-C) and VEGF-D, as well as specific lymphatic endothelial cell (LEC) markers such as vascular endothelial growth factor receptor-3 (VEGF-R3), lymphatic endothelial hyaluronan receptor-1 (LYVE-1), podoplanin, and prospero-related homeobox-1 (PROX1), has provided the tools to characterize and investigate lymphatic development and function in a wide range of tissues. There are conflicting reports on the distribution of endometrial lymphatics, with some studies reporting lymphatics in the functional zone of human endometrium, others only in the endometrial basalis, and some reporting none at all. Using immunohistochemical methods we have shown that lymphatic vessels of the functionalis were small and sparsely distributed whereas the basalis lymphatics are larger, more frequent and often closely associated with spiral arterioles. Based on comparisons of serial sections, the majority of lymphatic vessels are positive for CD31 but not FVIII or CD34. By comparing CD31 with D2-40 (labels lymphatic endothelial cells) vessel immunostaining, it was estimated that 13% of the vessel profiles in the functionalis, 43% in the basalis and 28% in the myometrium were lymphatics. The lymphangiogenic growth factor VEGF-C is immunolocalized most prominently in the glandular cells, vascular endothelium and some stromal cells in normal cycling endometrium. There is no difference in staining intensity observed between the basalis and functionalis. VEGF-D is immunolocalized throughout the endometrial and myometrial tissues, with no difference in intensity between endometrial glands and stroma or between the basalis and functionalis across the normal cycle. In conclusion, despite an apparently similar distribution of VEGF-C, VEGF-D and VEGF-R3 in endometrial functionalis and basalis, the lymphatic vascular density is 4-5 times higher in the basalis compared to the functionalis. There is also a close association between some lymphatics in the basalis and the spiral arterioles, thus identifying a potential mechanism for a vascular control feedback loop.