Comparison of syncytiotrophoblast generated from human embryonic stem cells and from term placentas

Abstract

Human embryonic stem cells (ESCs) readily commit to the trophoblast lineage after exposure to bone morphogenetic protein-4 (BMP-4) and two small compounds, an activin A signaling inhibitor and a FGF2 signaling inhibitor (BMP4/A83-01/PD173074; BAP treatment). During differentiation, areas emerge within the colonies with the biochemical and morphological features of syncytiotrophoblast (STB). Relatively pure fractions of mononucleated cytotrophoblast (CTB) and larger syncytial sheets displaying the expected markers of STB can be obtained by differential filtration of dispersed colonies through nylon strainers. RNA-seq analysis of these fractions has allowed them to be compared with cytotrophoblasts isolated from term placentas before and after such cells had formed syncytia. Although it is clear from extensive gene marker analysis that both ESC-and placenta-derived syncytial cells are trophoblast, each with the potential to transport a wide range of solutes and synthesize placental hormones, their transcriptome profiles are sufficiently dissimilar to suggest that the two cell types have distinct pedigrees and represent functionally different kinds of STB. We propose that the STB generated from human ESCs represents the primitive syncytium encountered in early pregnancy soon after the human trophoblast invades into the uterine wall.is encountered during at least two stages of human placental development (1-3). The first coincides with early implantation when a multinucleated syncytium forms, presumably by cell fusion events, ahead of proliferating, mononucleated, cytotrophoblast (CTB) cells originating from polar trophectoderm (3,4). This invasive syncytium emerges either during or soon after the trophoblast passes through the breached uterine epithelium and into the decidualized stromal layer beneath and appears to be responsible for hollowing out regions within the stroma to form lacunae (5), which become filled with fluid and cells from maternal blood and uterine glands and presumably provide a source of nutrients for the conceptus (3, 6). By about 12 d of gestation, soon after the blastocyst has sunk below the endometrial surface, strands of cytotrophoblast begin to form and penetrate through the primitive syncytium to form primary chorionic villi, which are subsequently invaded by extraembryonic mesoderm to form secondary and tertiary villi (villous trees) (2-4). The cytotrophoblast cells associated with the villi continue to divide and provide a progenitor cell population for the villous STB, which is the cell layer that covers the outer surface of the villi and forms the definitive interface involved in exchange of gases, nutrients, and excretory materials between the fetal placenta and maternal blood. Villous STB is also the major site for production of placental hormones. Cytotrophoblast cells at the tips of the anchoring villi proliferate and colonize the endometrium, thus expanding the placental bed and simultaneously remodeling maternal spiral arteries. The extent to which extravillous trophoblast becom...

Notch1 controls development of the extravillous trophoblast lineage in the human placenta

“…As a consequence, alternative models, such as bone morphogenetic protein (BMP)-treated human ESCs (hESCs), have been developed, allowing in vitro formation of the trophoblast lineage (37,38). Despite concerns regarding specificity of trophectoderm induction with BMP (37,39,40), recent investigations using optimized culture conditions suggested that early stages of human trophoblast development could be mimicked in these cells (41,42).…”

Tipping the balance toward trophoblast development

“…As these protocols have been elaborated, concerns have been noted about the identity of the cell derivatives and the physiological relevance of the BMP-directed process (7,8). A recent study reported in PNAS using a simple yet elegant strategy for capturing subpopulations of cells from BAP-treated human ES cells provides further evidence of the trophoblastic potency of pluripotent stem cells (9).…”

“…Single-cell analyses of differentiated cells have also become a popular approach (11). Yabe et al (9) took advantage of a specialized property of one type of differentiated trophoblast cell that arises through the fusion of progenitors generating multinucleated cells referred to as syncytiotrophoblasts. These cells are known to possess a range of specific functions, including hormonogenesis and bidirectional solute transport, features vital to the placenta and essential for in utero growth and survival of the fetus (12).…”

“…In the PNAS report, human ES cells were treated with the BAP mixture for 8 d, dissociated, and then passed through nylon cell strainers of different pore sizes to fractionate cells on the basis of size. BAP-treated cells exhibited size heterogeneity (9). Transcript profiles of different cell size fractions were generated and each exhibited a trophoblast signature.…”

“…Some differences were also evident and may reflect gestation stage-dependent trophoblast behavior. Yabe et al (9) assert that human ES cell-derived multinucleated cells bear greater similarity to early-gestation syncytiotrophoblast, especially syncytiotrophoblast arising immediately postimplantation. This represents an attractive hypothesis and if further supported would make their model especially advantageous for investigating early events in human trophoblast lineage development.…”