Ession of KRT genes (SI Appendix, Fig. S6E), which are frequently used diagnostically to identify trophoblast in sections of pregnant endometrium (37, 38). Moreover, whereas the ESCd >70 m and PHTd would both appear to be engaged in synthesis of CG, the expression of CGB genes and its partner, CGA, was manyfold higher in the former than in the latter (Fig. 6). Fig. 7 illustrates the correlation between the relative expression values for eight trophoblast marker genes in the ESCd >70-m fraction and PHTd cells and their expression in placental villous samples obtained at different stages of pregnancy. In particular, GABRP, NTRK2, WFDC2, VTCN1, and even COL4A5 appear to be features of the early placental samples, whereas PSG4, CSH1, and ALPP (placental alkaline phosphatase) are associated most CEP-37440 chemical information strongly with term placentas. Together, these observations suggested that the >70-m fraction represents STB from early stage pregnancies. However, the situation may be more complicated than that, as the clustering analyses in Fig. 4 indicate major disparities in gene expression, even for genes encoding proteins engaged in anticipated placental functions, such as transportersPNAS | Published online April 5, 2016 | EDEVELOPMENTAL BIOLOGYSEE COMMENTARYPNAS PLUS(SI Appendix, Fig. S4) and steroid metabolism (SI Appendix, Fig. S5). Such data seem inconsistent with any perception that the two cell types have particularly close pedigrees, despite the fact that the culture systems used to generate the STB from hESCs and from term placental cytotrophoblast were dissimilar. If, as seems reasonable, PHTd represents the STB that overlays placental villi, what then is the in vivo equivalent of the STB formed from BAP-treated ESC colonies? As pointed out in the introduction, there is an earlier form of STB, a syncytium that emerges during the initial breaching of the uterine epithelium (6), but it is unlikely under present restrictions for conducting research on human subjects that such specimens will ever become available. This syncytial tissue then invades maternal stromal tissue, degrades the matrix with which it makes contact, hollows out lacunae, and colonizes blood capillaries and uterine glands and thereby gains access to solutes and macromolecules necessary to support the growing conceptus. Additionally, it also provides a base through which growing cytotrophoblast columns migrate outward to establish the rudiments of the placental villous tree. This early syncytium is also the major form of trophoblast present when the mother must respond to trophoblast signals to maintain her pregnancy, particularly the hormone hCG, which is required for continued corpus luteum support and avoidance of endometrial shedding and menstruation, which would normally occur after about 2 wk if pregnancy recognition had not ensued (39). Is it possible that the hESCderived STB represents this early syncytium? Its invasive features, particularly the high expression of MMP2, its potential for substantial hCG and extracellular matrix production, and its broad expression of transporter genes, including that for the iron-carrier SLC40A1, make this hypothesis a feasible one. Another alternative is that this STB is a homolog of villous STB from very early pregnancy. However, detailed transcript analysis of such cells has not yet been published; nor have there been experiments to determine whether purified cytotrophoblasts from the first trimester terminations can be purchase AZD3759 induced to fuse in vi.Ession of KRT genes (SI Appendix, Fig. S6E), which are frequently used diagnostically to identify trophoblast in sections of pregnant endometrium (37, 38). Moreover, whereas the ESCd >70 m and PHTd would both appear to be engaged in synthesis of CG, the expression of CGB genes and its partner, CGA, was manyfold higher in the former than in the latter (Fig. 6). Fig. 7 illustrates the correlation between the relative expression values for eight trophoblast marker genes in the ESCd >70-m fraction and PHTd cells and their expression in placental villous samples obtained at different stages of pregnancy. In particular, GABRP, NTRK2, WFDC2, VTCN1, and even COL4A5 appear to be features of the early placental samples, whereas PSG4, CSH1, and ALPP (placental alkaline phosphatase) are associated most strongly with term placentas. Together, these observations suggested that the >70-m fraction represents STB from early stage pregnancies. However, the situation may be more complicated than that, as the clustering analyses in Fig. 4 indicate major disparities in gene expression, even for genes encoding proteins engaged in anticipated placental functions, such as transportersPNAS | Published online April 5, 2016 | EDEVELOPMENTAL BIOLOGYSEE COMMENTARYPNAS PLUS(SI Appendix, Fig. S4) and steroid metabolism (SI Appendix, Fig. S5). Such data seem inconsistent with any perception that the two cell types have particularly close pedigrees, despite the fact that the culture systems used to generate the STB from hESCs and from term placental cytotrophoblast were dissimilar. If, as seems reasonable, PHTd represents the STB that overlays placental villi, what then is the in vivo equivalent of the STB formed from BAP-treated ESC colonies? As pointed out in the introduction, there is an earlier form of STB, a syncytium that emerges during the initial breaching of the uterine epithelium (6), but it is unlikely under present restrictions for conducting research on human subjects that such specimens will ever become available. This syncytial tissue then invades maternal stromal tissue, degrades the matrix with which it makes contact, hollows out lacunae, and colonizes blood capillaries and uterine glands and thereby gains access to solutes and macromolecules necessary to support the growing conceptus. Additionally, it also provides a base through which growing cytotrophoblast columns migrate outward to establish the rudiments of the placental villous tree. This early syncytium is also the major form of trophoblast present when the mother must respond to trophoblast signals to maintain her pregnancy, particularly the hormone hCG, which is required for continued corpus luteum support and avoidance of endometrial shedding and menstruation, which would normally occur after about 2 wk if pregnancy recognition had not ensued (39). Is it possible that the hESCderived STB represents this early syncytium? Its invasive features, particularly the high expression of MMP2, its potential for substantial hCG and extracellular matrix production, and its broad expression of transporter genes, including that for the iron-carrier SLC40A1, make this hypothesis a feasible one. Another alternative is that this STB is a homolog of villous STB from very early pregnancy. However, detailed transcript analysis of such cells has not yet been published; nor have there been experiments to determine whether purified cytotrophoblasts from the first trimester terminations can be induced to fuse in vi.
