Neither normal nor diseased placentas contain lymphatic vessels

Absence of lymphatic vessels in term placenta

Background

There has been debate about the existence of lymphatic vessels in placenta. Lymphatic endothelial cell (LEC) markers such as LYVE-1 and podoplanin/D2–40 have been found, although PROX1 has not been detected. The most reliable marker for LECs is the double staining for CD31 and PROX1, which has not been performed yet.

Methods

We studied three term placentas and dissected them into three areas: i.) basal plate area, ii.) intermediate area, and iii.) chorionic plate area. We used immunofluorescence single and double staining with antibodies against CD31, PROX1, LYVE-1, VEGFR-3, D2–40/PDPN, CD34, CCBE-1, and vimentin, as well as nested PCR, qPCR, Western blot and transmission electron microscopy (TEM).

Results

At TEM level we observed structures that have previously mistakenly been interpreted as lymphatics, however, we did not find any CD31/PROX1 double-positive cells in placenta. Absence of PROX1 was also noted by nested PCR, qPCR and Western blot. Also, LEC marker VEGFR-3 was expressed only in a small number of scattered leukocytes but was absent from vessels. The LEC marker D2–40/PDPN was expressed in most stromal cells, and the LEC marker LYVE-1 was found in a considerable number of stromal cells, but not in endothelial cells, which were positive for CD31, CD34, CCBE-1 and vimentin. Additionally, vimentin was found in stromal cells.

Conclusions

Our studies clearly show absence of lymphatics in term placenta. We also show that the functional area of the mother’s endometrium is not penetrated by lymphatics in term pregnancy.

Lymphatic Programing and Specialization in Hybrid Vessels

Building on a large body of existing blood vascular research, advances in lymphatic research have helped kindle broader investigations into vascular diversity and endothelial plasticity. While the endothelium of blood and lymphatic vessels can be distinguished by a variety of molecular markers, the endothelia of uniquely diverse vascular beds can possess distinctly heterogeneous or hybrid expression patterns. These expression patterns can then provide further insight on the development of these vessels and how they perform their specialized function. In this review we examine five highly specialized hybrid vessel beds that adopt partial lymphatic programing for their specialized vascular functions: the high endothelial venules of secondary lymphoid organs, the liver sinusoid, the Schlemm’s canal of the eye, the renal ascending vasa recta, and the remodeled placental spiral artery. We summarize the morphology and endothelial expression pattern of these vessels, compare them to each other, and interrogate their specialized functions within the broader blood and lymphatic vascular systems.

Biological importance of podoplanin expression in chorionic villous stromal cells and its relationship to placental pathologies

Podoplanin, a reliable marker of lymphatic endothelium, is a mucin-type transmembrane protein. Although the human placenta is devoid of a lymphatic system, chorionic villous stromal (CVS) cells express podoplanin. In this study, the pattern of podoplanin expression in normal and pathological placental tissues and the biological role of podoplanin were investigated. In total, 198 placental tissues belonging to 184 patients, seen at the Department of Pathology of Bulent Ecevit University Education and Research Hospital, Zonguldak, Turkey, were evaluated histopathologically and determined to meet the study criteria. The tissues were assigned to control, cisternal placental disorders, inflammation and hypoxic-ischemic pathology groups. Podoplanin expression in CVS cells was graded from 0 to 3 depending on the staining intensity, as determined by an immunohistochemical evaluation of chorionic villi in the most intensively stained tissue region. Podoplanin levels in control CVS cells increased in parallel with placental maturation, whereas in molar pregnancies podoplanin expression was lower than in control tissues. In the acute placental inflammation group, podoplanin immunoreactivity was similar to that in the control group, whereas in the preeclampsia group, podoplanin expression was higher than in all other groups. Our study showed an increase in podoplanin expression in CVS cells during pregnancy. In preeclamptic patients, the increase in podoplanin expression may be a response to hypoxic-ischemic conditions, whereas in molar pregnancies the decrease in podoplanin levels may cause villous swelling by disrupting intercellular fluid homeostasis.

Abnormal lymphatic vessel development is associated with decreased decidual regulatory T cells in severe preeclampsia

PROBLEM

The lymphatic vasculature controls leukocytes trafficking and limits the adaptive immune response. In previous models of preeclampsia (PE), defective immune function caused by disruption of lymphangiogenesis was shown to be involved in the disease pathophysiology. Especially, the dysfunction of regulatory T cells (Treg) at the maternal-fetal interface may be one of the causes of severe PE. In particular, activation of Tregs to obtain immune tolerance requires adequate antigen presentation through the lymphatic system. We hypothesized that impaired lymphangiogenesis and imbalanced Tregs at the maternal-fetal interface are associated with the pathophysiology of severe PE. However, the current research addressing this hypothesis is limited. Therefore, to compare differences in lymphangiogenesis in severe PE and normal conditions, we aimed to examine the location of lymphatics at the maternal-fetal interface and to investigate the association between lymphangiogenesis and Tregs in severe PE.

METHOD OF STUDY

We obtained entire uterus from normal pregnant mice. Placental and fetal membranes, including decidua, were obtained from 10 pregnant women with severe PE and 10 gestational age-matched controls. Immunohistochemistry for LYVE1 was used to localize the distribution of lymphatic vessels and CD4, CD25, and FOXP3 for Treg.

RESULTS

LYVE1-positive vessels were present in the uterine wall of mice. LYVE1-positive lymphatic vessels were localized on the human decidua. Tubular lymphatics were abundant in the control decidua, but significantly reduced in severe PE. Furthermore, lymphatic vessel density correlated with the number of decidual Tregs.

CONCLUSION

Abnormal decidual lymphangiogenesis is associated with reduced numbers of decidual Tregs in severe PE.

Lymphatic vascular endothelial hyaluronan receptor-1 immunoexpression in placenta of HIV infected pre-eclamptic women

INTRODUCTION

Lymphangiogenesis is the formation of new vessels from pre-existing lymphatic vessels. Data on lymphangiogenesis in the placenta of HIV-infected pre-eclamptics are sparse and the findings are conflicting. The aim of this novel study was to evaluate LYVE-1 immunoexpression in the placenta of HIV infected normotensive versus pre-eclamptic women.

METHODS

Placental tissue was obtained from normotensive and pre-eclamptic women stratified according to their HIV status. The pre-eclamptic group was divided into early (<34 weeks) and late (>34 weeks) onset. Immunohistochemistry utilized mouse anti-human LYVE-1 antibody and was morphometrically evaluated.

RESULTS

LYVE-1 immunostaining was localized within endothelium of the arterial supply and venous drainage of both conducting and exchange villi as well as within medial cells of arteries. LYVE-1 immunostained macrophage-like cells were observed within the fetal and maternal circulation. LYVE-1 immunoexpression was higher (p=0.0001) in HIV positive cohort, regardless of pregnancy and villous type. Irrespective of HIV status and pregnancy type, LYVE-1 immunoexpression was significantly elevated in the conducting compared to the exchange villi (p=0.01). LYVE-1 immunoexpression was higher in N and LOPE compared to EOPE groups for both conducting and exchange villi types respectively (p=0.0001 and p=0.006). There is a decrease of LYVE-1 expression in EOPE+ (conducting villi) and EOPE- (exchange villi) compared to N and LOPE subgroups.

CONCLUSION

This study provides a novel insight into an up-regulation of LYVE-1 expression in the fetal circulation of conducting and exchange villi of HIV-infected pre-eclamptics.

The defect of both angiogenesis and lymphangiogenesis is involved in preeclampsia

INTRODUCTION

Pre-eclampsia (PE) is characterized by failed remodeling of maternal vessels perfusing the placenta. Blood vessels and lymphatic system are involved in vessel remodeling and flow homeostasis in the uterus during pregnancy. This study aims to investigate the involvement of angiogenesis and lymphangiogenesis in PE.

METHODS

Placental and decidual tissues were obtained from pregnancies with PE (n = 90), including PE cases with decidual vasculopathy (DV) (n = 52) and without DV (n = 38), and healthy pregnancies (control, n = 20). The clinical characteristics of these groups were analyzed. The expression levels of VEGF1, CD34, PROX-1, VEGFR3, and CD31 in the placenta and decidua were detected through immunohistochemistry, reverse-transcription polymerase chain reaction, and Western blot.

RESULTS

The lymphangiogenic markers PROX-1 and VEGFR3 were negatively expressed in the placenta but positively expressed in the decidua. The expression levels of the angiogenic markers VEGF1 and CD34 and the panendothelial marker CD31 were significantly lower in the placenta and decidua of the PE group than in those of the control group. The expression levels of VEGF1, CD34, and CD31 were significantly lower in the placenta and decidua with DV than in those without DV. Furthermore, the expression trends of PROX-1 and VEGFR3 was similar to those of VEGF1, CD34, and CD31 among the groups.

CONCLUSIONS

Lymphangiogenesis occurred in the decidua but not in the placenta. Impaired angiogenesis and lymphangiogenesis were associated with PE, particularly in the presence of DV.

VEGFR-3 neutralization inhibits ovarian lymphangiogenesis, follicle maturation, and murine pregnancy

Lymphatic vessels surround follicles within the ovary, but their roles in folliculogenesis and pregnancy, as well as the necessity of lymphangiogenesis in follicle maturation and health, are undefined. We used systemic delivery of mF4-31C1, a specific antagonist vascular endothelial growth factor receptor 3 (VEGFR-3) antibody to block lymphangiogenesis in mice. VEGFR-3 neutralization for 2 weeks before mating blocked ovarian lymphangiogenesis at all stages of follicle maturation, most notably around corpora lutea, without significantly affecting follicular blood angiogenesis. The numbers of oocytes ovulated, fertilized, and implanted in the uterus were normal in these mice; however, pregnancies were unsuccessful because of retarded fetal growth and miscarriage. Fewer patent secondary follicles were isolated from treated ovaries, and isolated blastocysts exhibited reduced cell densities. Embryos from VEGFR-3-neutralized dams developed normally when transferred to untreated surrogates. Conversely, normal embryos transferred into mF4-31C1-treated dams led to the same fetal deficiencies observed with in situ gestation. Although no significant changes were measured in uterine blood or lymphatic vascular densities, VEGFR-3 neutralization reduced serum and ovarian estradiol concentrations during gestation. VEGFR-3-mediated lymphangiogenesis thus appears to modulate the folliculogenic microenvironment and may be necessary for maintenance of hormone levels during pregnancy; both of these are novel roles for the lymphatic vasculature.