527. EXTENSIVE LYMPHATIC REMODELLING OCCURS IN HUMAN PLACENTAL BED DURING THE FIRST 20 WEEKS OF PREGNANCY

Abstract

The aim of this study was to describe the distribution of lymphatics in human placental bed from 6–20 weeks of pregnancy, with emphasis on the lymphatics surrounding the spiral arterioles as they undergo remodelling associated with trophoblast invasion. Placental bed biopsies were obtained following elective termination (n=41, 6–18 weeks gestation) and at term (n=5). Samples were routinely processed for serial sectioning at 3um to allow structural relationships to be compared in consecutive sections. Double immunohistochemistry protocols were used to identify lymphatic vessels in conjunction with one of: blood vessels, smooth muscle, epithelium and trophoblast, or proliferating cells. Immunostaining demonstrated that lymphatic vessels were present in abundance in the hypersecretory endometrium throughout all stages of gestation, and were prominent encircling spiral arterioles and adjacent to the endometrial glands. In contrast, the decidua was nearly always devoid of lymphatics, and in particular they were absent from the vicinity of spiral arterioles that were surrounded by decidual cells. In hypersecretory endometrium lymphatic vessel profiles varied greatly in architecture and size, and included focal accumulations of endothelial cells, through to fully formed lymphatic vessels. There was evidence of lymphatic vessel endothelial cell proliferation in all the different vessel types. The trophoblast appeared to have no overt influence on the structure or appearance of the lymphatic endothelium, with no evidence of incorporation into, or distortion of, the lymphatic vessel walls. Obvious increases were noted in the size of the lymphatic vessel profiles in hypersecretory endometrium as gestation progressed. The study provides the first detailed description of the placental bed lymphatics and their relationship to other endometrial structures during human pregnancy. We have shown that lymphatic vessels are absent from the decidua, apparently regressing as surrounding stromal cells decidualise. In contrast, the underlying hypersecretory endometrium contains abundant lymphatics which increase in size as pregnancy progresses.