Heterogeneous histochemical reaction pattern of the lectin Bandeiraea (Griffonia) simplicifolia with blood vessels of human full-term placenta

Perinatal Derivatives: Where Do We Stand? A Roadmap of the Human Placenta and Consensus for Tissue and Cell Nomenclature

Progress in the understanding of the biology of perinatal tissues has contributed to the breakthrough revelation of the therapeutic effects of perinatal derivatives (PnD), namely birth-associated tissues, cells, and secreted factors. The significant knowledge acquired in the past two decades, along with the increasing interest in perinatal derivatives, fuels an urgent need for the precise identification of PnD and the establishment of updated consensus criteria policies for their characterization. The aim of this review is not to go into detail on preclinical or clinical trials, but rather we address specific issues that are relevant for the definition/characterization of perinatal cells, starting from an understanding of the development of the human placenta, its structure, and the different cell populations that can be isolated from the different perinatal tissues. We describe where the cells are located within the placenta and their cell morphology and phenotype. We also propose nomenclature for the cell populations and derivatives discussed herein. This review is a joint effort from the COST SPRINT Action (CA17116), which broadly aims at approaching consensus for different aspects of PnD research, such as providing inputs for future standards for the processing and in vitro characterization and clinical application of PnD.

Conditional loss of ERK1 and ERK2 results in abnormal placentation and delayed parturition in the mouse

Extracellular-signal-regulated kinases (ERK) 1 and 2 regulate many aspects of the hypothalamic-pituitary-gonadal axis. We sought to understand the role of ERK1/2 signaling in cells expressing a Cre allele regulated by the endogenous GnRHR promoter (GRIC-ERKdko). Adult female GRIC-ERKdko mice were hypogonadotropic and anovulatory. Gonadotropin administration and mating led to pregnancy in one-third of the ERKdko females. Litters from ERKdko females and pup weights were reduced coincident with delayed parturition and 100% neonatal mortality. Based on this, we examined Cre expression in implantation sites as a potential mechanism. GnRHR mRNA levels at e10.5 and e12.5 were comparable to pituitary levels from adult female mice at proestrus and GnRHR mRNA in decidua was enriched compared to whole implantation site. In vivo studies confirmed recombination in decidua, and GRIC-ERKdko placentas showed reduced ERK2 expression. Histopathology revealed abnormalities in placental architecture in the GRIC-ERKdko animals. Regions of apoptosis at the decidual/uterine interface at e18.5 were observed in control animals but apoptotic tone in these regions was reduced in ERKdko animals. These studies support a potential model of ERK-dependent signaling within the implantation site leading to loss of placental architecture and mis-regulation of apoptotic events at parturition occurring coincident with prolonged gestation and neonatal mortality.

Placental formation in early pregnancy: how is the centre of the placenta made?

BACKGROUND

Correct development of the placenta is critical to establishing pregnancy and inadequate placentation leads to implantation failure and miscarriage, as well as later gestation pregnancy disorders. Much attention has been focused on the placental trophoblasts and it is clear that the trophoblast lineages arise from the trophectoderm of the blastocyst. In contrast, the cells of the placental mesenchyme are thought to arise from the inner cell mass, but the details of this process are limited. Due to ethical constraints and the inaccessibility of very early implantation tissues, our knowledge of early placentation has been largely based on historical histological sections. More recently, stem cell technologies have begun to shed important new light on the origins of the placental mesenchymal lineages.

OBJECTIVE AND RATIONALE

This review aims to amalgamate the older and more modern literature regarding the origins of the non-trophoblast lineages of the human placenta. We highlight ways in which rapidly developing stem cell technologies may shed new light on these crucial peri-implantation events.

SEARCH METHODS

Relevant articles were identified using the PubMed database and Google Scholar search engines. A pearl growing method was used to expand the scope of papers relevant to the cell differentiation events of non-trophoblast placental lineages.

OUTCOMES

At the start of pregnancy, cells of the extraembyronic mesoderm migrate to underlie the primitive trophoblast layers forming the first placental villi. The mesenchymal cells in the villus core most likely originate from the hypoblast of the embryo, but whether cells from the epiblast also contribute is yet to be determined. This is important because, following the formation of the villus core, vasculogenesis and haematopoiesis take place in the nascent placenta before it is connected to the embryonic circulation, making it likely that haematopoietic foci, placental macrophages, endothelial cells and vascular smooth muscle cells all arise in the placenta de novo. Evidence from the stem cell field indicates that these cells could directly differentiate from the extraembryonic mesoderm. However, the lineage hierarchy involved in cell fate decisions has not been well-established. Mesodermal progenitors capable of differentiating into both vascular and haematopoietic lineages can be derived from human embryonic stem cells, but the identification of such stem cells in the placenta is lacking. Future work profiling rare progenitor populations in early placentae will aid our understanding of early placentation.

WIDER IMPLICATIONS

Understanding the origins of the cell lineages of the normal placenta will help us understand why so many pregnancies fail and address mechanisms that may salvage some of these losses.

Hemangioblastic foci in human first trimester placenta: Distribution and gestational profile

INTRODUCTION

The human placenta is a site of both hematopoiesis and vasculogenesis. There are reports of hemangioblastic foci (HAF) in the first trimester placenta, but little published information about their spatiotemporal incidence.

METHODS

We have used semi-thin sections and whole mount staining techniques on archival early pregnancy hysterectomy material as well as freshly-collected termination tissue.

RESULTS

We report a description of the distribution of HAF, their gestational profile, and some characteristics of the constituent cells. We show crypt-shaped HAF are present in villi at different levels from 4 to 11 weeks and in the chorionic plate from 4 to 9 weeks. In the villous placenta, the foci often approach closely at one end to the trophoblast basement membrane. Morphologically they show remarkable similarity to those found in the yolk sac at similar stages. In some crypts, all cells are CD34+, but CD34 and nestin progressively segregate into the endothelial lineage. Brachyury is present in less differentiated cells. The erythroid lineage is dominant, as shown by the widespread expression of CD235a/glycophorin and characteristic erythroid morphologies, indicating various degrees of differentiation. However, CD41 is also present in non-endothelial cells. Initially a discontinuous UEA-1/CD31-positive endothelium forms at the periphery of the foci. These cells appear to become integrated into the developing vasculogenic/angiogenic vessel network. We also demonstrate that, independent of HAF, vasculogenesis occurs near the tips of growing villi during the first trimester.

DISCUSSION

We suggest HAF interface with the developing vascular network, producing communication channels that allow erythrocytes to enter the placental-embryonic circulation. We speculate that the erythroid cells act as oxygen reservoirs during the period before flow of maternal blood through the intervillous space of the placenta, allowing a slow feed of oxygen-rich cells to the developing embryo.

Human fetal placental endothelial cells have a mature arterial and a juvenile venous phenotype with adipogenic and osteogenic differentiation potential

Growing interest in the sources of origin of blood vessel related diseases has led to an increasing knowledge about the heterogeneity and plasticity of endothelial cells lining arteries and veins. So far, most of these studies were performed on animal models. Here, we hypothesized that the plasticity of human fetal endothelial cells depends on their vascular bed of origin i.e. vein or artery and further that the differences between arterial and venous endothelial cells would extend to phenotype and genotype. We established a method for the isolation of fetal arterial and venous endothelial cells from the human placenta and studied the characteristics of both cell types. Human placental arterial endothelial cells (HPAEC) and human placental venous endothelial cells (HPVEC) express classical endothelial markers and differ in their phenotypic, genotypic, and functional characteristics: HPAEC are polygonal cells with a smooth surface growing in loose arrangements and forming monolayers with classical endothelial cobblestone morphology. They express artery-related genes (hey-2, connexin 40, depp) and more endothelial-associated genes than HPVEC. Functional testing demonstrated that vascular endothelial growth factors (VEGFs) induce a higher proliferative response on HPAEC, whereas placental growth factors (PlGFs) are only effective on HPVEC. HPVEC are spindle-shaped cells with numerous microvilli at their surface. They grow closely apposed to each other, form fibroblastoid swirling patterns at confluence and have shorter generation and population doubling times than HPAEC. HPVEC overexpress development-associated genes (gremlin, mesenchyme homeobox 2, stem cell protein DSC54) and show an enhanced differentiation potential into adipocytes and osteoblasts in contrast to HPAEC. These data provide collective evidence for a juvenile venous and a more mature arterial phenotype of human fetal endothelial cells. The high plasticity of the fetal venous endothelial cells may reflect their role as tissue-resident endothelial progenitors during embryonic development with a possible benefit for regenerative cell therapy.

Fetomaternal glycosylation of early placentation events in the African elephant Loxodonta africana

During implantation in the African elephant (Loxodonta africana), fetal trophoblast displaces the surface uterine epithelium and superficially penetrates the uterine glands. This limited invasion is followed by the upgrowth of blunt fingers of endometrial stroma, covered with trophoblast and containing capillaries that subsequently vascularize the growing placenta. We have used lectin histochemistry to compare the glycosylation of maternal endothelial cells in the endometrium with those growing within the trophoblastic processes of a 2 g embryo (approximately 125 days' gestation), and also examine changes in the endometrial glands associated with trophoblastic invasion. Maternal vessels at the apices of the trophoblast-covered stromal upgrowths showed increased expression of terminal N-acetyl galactosamine, N-acetyl glucosamine oligomers, some sialic acids, and tri/tetra-antennate non-bisected complex N-linked glycan, as indicated by increased lectin staining. The areas of increased staining were also more resistant to neuraminidase digestion. Invaded glands had distended walls composed of flattened epithelial cells, some of which showed heavy lectin staining suggestive of intracellular glycan accumulation. The vascular changes suggest that new maternal capillary growth is accompanied by alterations in surface glycosylation. This may be the result of increased glycosyl transferase activity associated with cell proliferation and may also indicate the expression of significantly increased anti-adhesive molecules preventing blood stasis and egress of maternal immunocompetent cells into the fetal compartment.

Heterogeneity of microvascular endothelial cells isolated from human term placenta and macrovascular umbilical vein endothelial cells

The present study compares some phenotypic and physiologic characteristics of microvascular and macrovascular endothelial cells from within one human organ. To this end microvascular endothelial cells from human full-term placenta (PLEC) were isolated using a new method and compared with macrovascular human umbilical vein endothelial cells (HUVEC) and an SV40-transformed placental venous endothelial cell line (HPEC-A2). PLEC were isolated by enzymatic perfusion of small placental vessels, purified on a density gradient and cultured subsequently. Histological sections of the enzyme-treated vessels showed a selective removal of the endothelial lining in the perfused placental cotyledons. The endothelial identity of the cells was confirmed by staining with the endothelial markers anti-von Willebrand factor, Ulex europaeus lectin and anti-QBEND10. The cells internalized acetylated low-density lipoprotein and did not show immunoreactivity with markers for macrophages, smooth muscle cells and fibroblasts. The spindle-shaped PLEC grew in swirling patterns similar to that described for venous placental endothelial cells. However, scanning electron microscopic examination clearly showed that PLEC remained elongated at the confluent state, in contrast to the more polygonal phenotype of HPEC-A2 and HUVEC that were studied in parallel. The amount of vasoactive substances (endothelin-1,2, thromboxane, angiotensin II, prostacyclin) released into the culture medium and the proliferative response to cytokines was more similar to human dermal microvessels (MIEC) derived from non-fetal tissue than to HUVEC. Potent mitogens such as vascular endothelial growth factors (VEGF121, VEGF165) and basic fibroblast growth factor (FGF-2) induced proliferation of all endothelial cell types. Placental growth factors PIGF-1 and PIGF-2 effectively stimulated cell proliferation on PLEC (142 +/- 7% and 173 +/- 10%) and MIEC (160 +/- 20% and 143 +/- 28%) in contrast to HUVEC (9 +/- 8% and 15 +/- 20%) and HPEC-A2 (15 +/- 7% and 24 +/- 6%) after 48 h incubation time under serum-free conditions. These data support evidence for (1) the microvascular identity of the isolated PLEC described in this study, and (2) the phenotypic and physiologic heterogeneity of micro- and macrovascular endothelial cells within one human organ.

Differential mitogenic responses of human macrovascular and microvascular endothelial cells to cytokines underline their phenotypic heterogeneity

A variety of growth factors promote the complex multistep process of angiogenesis. The mitogenic activity of vascular endothelial growth factors (VEGFs) and placental growth factors (PlGFs), known as cytokines acting predominantly on endothelial cells, was tested on human umbilical vein endothelial cells (HUVEC) and microvascular endothelial cells (MIEC) and compared with the potency of the universally acting basic fibroblast growth factor (FGF-2). The cells were seeded at different cell numbers and incubated with various doses of growth factors for a period of 24-72 h in culture medium +/- serum. Proliferation was determined by measuring the optical density after staining the cells with the tetrazolium salt WST-1. VEGF121 and VEGF165 increased the number of HUVEC and MIEC at low and high seeding densities various doses and incubation times. The efficiency of FGF-2 was less pronounced at high seeding densities of the cells under serum-free conditions. PlGF-1 and PlGF-2 stimulated mitogenesis on HUVEC only at low cell numbers and after a short incubation time by 125 +/- 3% and 102 +/- 5% (P < 0.001), respectively. Longer incubation times with the lower seeding density in the absence of FCS did not induce a significant stimulatory effect of the PlGFs. MIEC responded stronger to all growth factors. In particular under serum free conditions, PlGF-1 and PlGF-2 effectively stimulated cell proliferation by 247 +/- 54% (P < 0.01) and 288 +/- 40% (P < 0.05) at low cell numbers, and by 81 +/- 13% (P < 0.05) and 49 +/- 13% (P < 0.01), respectively, at high cell numbers. The addition of fetal calf serum caused a reduced proliferative response of all growth factors on both cell types related to the controls. In conclusion, MIEC and HUVEC differ in their proliferative response to VEGFs, PlGFs and FGF-2.

Isolation of endothelial cells from human placental microvessels: effect of different proteolytic enzymes on releasing endothelial cells from villous tissue

Approaches for the isolation of human placental microvascular endothelial cells (HPMEC) using proteolytic enzymes have been described recently. However, the isolation procedure and enzyme composition most suitable for optimal disaggregation of placental tissue and isolation of HPMEC has not yet been established. We tested different proteolytic enzymes and enzyme mixtures for their capabilities of releasing endothelial cells from human term placental villous tissue. Best results were obtained with a mixture of collagenase/dispase/deoxyribonuclease I (0.28%/0.25%/0.01%). By adding a discontinuous Percoll gradient centrifugation step to the enzymatic dispersion, about 1 x 10(6) cells/g tissue with more than 30% von Willebrand factor (vWf)-positive cells were obtained. However, the total cell number and number of vWf-positive cells were highly dependent on the lot of collagenase used. A perfusion step prior to mincing of villous tissue did not increase the amount of vWf-positive cells. We conclude that the methods described in this study are suitable to isolate high yields of HPMEC and that the composition of the collagenase preparation is crucial to the successful release of endothelial cells from placental tissue. To obtain pure HPMEC, further separation steps, e.g., cell sorting with antibodies against endothelial specific cell surface antigens are necessary.

Establishment of a pure vascular endothelial cell line from human placenta

Endothelial cells (EC) from various sectors of the circulatory system have distinct characteristics, some of which have only been identified in cultures upon their isolation from specific organs or tissues. Cultured vascular EC, derived from the human placenta (HPEC), may be helpful for studying their specific function in the fetoplacental unit, such as in the control of maternofetal traffic. In this paper we report an improved method for isolation, purification and culture of HPEC, that implies an enzymatic perfusion of the term placenta, followed by separation of resulting cells on a Percoll density gradient. The inoculated starting suspension was purified by a two-step selection procedure, based on differential trypsinization, leading to a pure population of about 8x10(7)cells/placenta, with 2.7-3.4 population doublings. The average population doubling time during eight passages was 60-65 h and the life span of HPEC was approximately 45-50 population doublings. The cell morphology at optical and electron microscopical level revealed a good differentiation of HPEC, which were endowed with numerous plasmalemmal vesicles (caveolae) and Weibel-Palade bodies. The transendothelial electrical resistance of the HPEC monolayer varied between 22 and 52 Ohm/cm(2). The cultures were mycoplasma free, as revealed by fluorescence microscopy using DNA dyes and the polymerase chain reaction (PCR). The negative immunofluorescent reaction for keratin confirmed that the HPEC were not contaminated with either type of placenta cells, as syncytiotrophoblast. Cultured HPEC demonstrated a strong reaction for von Willebrand factor antigen (by fluorescence microscopy), took up AcLDL-DiI and expressed active angiotensin converting enzyme. These characteristics substantiate the endothelial nature of cultured cells. The interactions with different lectins (BS-I, SBA, RCA, UEA and WGA) assessed by fluorescence microscopy and blotting reveal a strong reaction of HPEC with UEA and a negligible reaction with BS-I lectin. WGA lectin displayed a marked fluorescence staining in subconfluent HPEC, and at the level of intracellular clefts in post-confluent cultures.

IN CONCLUSION

(i) we have obtained a pure line of cultured EC originating from the human placental venous side of the circulatory tree; (ii) the cells have the general characteristics and markers ascribed to EC; (iii) as opposed to large human placental vessels, HPEC do not react to BS-I lectin and, unlike human umbilical vein EC, have a much higher proliferation rate and a long lifespan; (iv) HPEC expressed a characteristic glycosylated coat particularly rich in alpha- L -fucose and beta-GlcNAc containing glycocompounds.