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PLACENTAL STEM CELLS
                                                               


A Scientific Summary

 

Introduction:
The field of stem cell research is filled with a multitude of cells with multilineage potential and self-renewal ability. Progress in science has uncovered stem cells from almost every part of the body, such as the intestines, gum, eyes, and even the most transient and primitive organ- the placenta. The placenta is a temporary organ in the development of the embryo, existing only to provide the developing baby with the required nutrients, gas exchange, excretion, ion reabsorption, and bone development 1.

Since the development of the fetus is so robust, the placenta can have many different kinds of stem cells present in its microenvironment. Placental stem cells are derived from the mesodermal layer of the developing zygote 10. Labyrinthine trophoblast stem cells are a type of stem cells found in the placenta 2. Some studies have also produced Unrestricted Somatic Stem Cells (USSC) from placental cord blood 5. Placental-derived multipotent cells (PDMC) have been found that mimic characteristics of mesenchymal stem cells (MSC), embryonic stem cells (ESC), and embryonic germ cells (EGC) 10.

Basic Biology:
The placenta physically connects the mother to her developing fetus. It is responsible for providing the fetus with nutrients, oxygen, excretion, as well as contributing to the bone development through Ca2+ ion reabsorption 1. The tropholast can differentiate into three lineages: trophoblast giant cells, spongiotrophoblasts, and the labyrinthine trophoblasts. Figure 1 illustrates the organization of the placenta.   Trophoblast giant cells are present at the fetal-maternal connection and regulate maternal tolerance to the fetus through active blood flow 3. The spongiotrophoblast cells separate the trophoblast giant cells from the labyrinthine trophoblasts, and are also known as the ectoplacental cone at E8.5 2. The labyrinthine trophoblasts facilitate in the transport of ions and nutrients across the placenta 2. Chorioallantoic fusion occurs where the chorion and the allantois meet 3.

Human placental stem cells are also known as labyrinthine trophoblast stem cells. Since the placental-derived stem cells are transient in nature, their in vitro equivalent is used in culture- primary villous cytotrophoblasts that differentiate into synctiotrophoblasts 2. Synctiotrophoblasts have no proliferative capacity, limiting their use for genetic manipulations.
           
Unrestricted somatic stem cells (USSC) from placental cord blood are pluripotent and CD45- and CD34+. They are spindle-shaped and measure about 20-25 μm. These cells can be expanded up to 1015 in vitro without losing pluripotency, and can differentiate into osteoblasts, chondroblasts, adipocytes, hematopoietic cells, and even neural cells. These cells do not express MHC Class II proteins, and possess immunosuppressive properties. Thus, transplantation of USSCs in sheep did not produce tumors in target sites 5.
Placental-derived multipotent cells (PDMC) are another type of cells derived from the placenta. They are CD34-, and CD45-, possess immunosuppressive qualities due to presence MHC Class II protein. They express CD105/endoglin/ and SH-3, Sh3, Sh-4 and many other markers found on mesenchymal stem cells (MSC). PDMCs have a fibroblastoid morphology and plastic-adherence nature further resembling MSCs 12. They also express embryonic stem cell (ESC) specific and embryonic germ cell (EGC) specific markers, such as SSEA-4, TRA-1-60, TRA-1-81 12. This suggests that PDMCs not only have properties of MSCs, but also ESCs and EGCs.
 
Development:
Fertilization of the ovum by the sperm is followed by the development of the zygote. Embryonic development involves multiple proliferative stages; the blastocyst stage leads to the formation of the placenta. The trophoectoderm layer and the inner cell mass arise in the blastocyst stage. Pluripotent cells are present in the trophoectoderm layer of the zygote 2.

Studies of cytotrophoblasts have revealed multiple genes involved in development. Id2 and Cdx2 are involved in cell proliferation. Transcription factors, such as Gcm1, Esx1, Dlx3, Tfeb, and Tec regulate labyrinthine cell development, as observed in human placental syncytiotrophoblasts. Tpbp, a spongiotrophoblast marker, directs maintenance of a proper placental microenvironment 2. Trophoblast giant cells are regulated by a host of transcription factors including, Hand 1 and Mash2 that mediate giant cell differentiation, while Gata2/Gata3 maintain optimal levels of giant cell hormones, such as placental lactogen I and proliferin. The gene Eomes has been conserved throughout the evolution of placenta, and is involved in the maintenance of trophoblast stem cells 3. Figure 2 summarizes the genes involved in the development of placental stem cells.

Unrestricted somatic stem cells can produce a variety of different lineages. Epidermal growth factor, platelet-derived growth factor, insulin-like growth factor, and runt-related transcription factor (Runx1) all regulate the development of USSCs. USSCs are able to differentiate into neural precursor cells, but they cannot function as mature neural cells. Dexamethasone, ascorbic acid, and DAG induced USSC differentiation into bone, cartilage, and adipocytes. Hemapoietic cells, hepatic cells, and myocardial cells were also produced from the placental cord-blood derived USSCs 5.

Placental tissues have high expression of stem cell factor (Scf) and its ligand c-kit. Scf promotes placental stem cell proliferation though intracellular and paracrine actions 8.
Other cytokines secreted by placental stem cells are Il-1, Il-6, macrophage-colony stimulating factor (M-CSF), and granulocyte-macrophage-colony stimulating factor (GM-CSF) 8.

Methods of Investigation:
Human trophoblasts have a short passaging time in culture. Therefore, model systems are used to study the properties of placental stem cells. Tumorigenic (choriocarcinoma) and non-tumorigenic cell lines have been used in vitro since they can be infinitely passaged in lab without loss of inherent characteristics 4. Isolation and incubation of cytotrophoblasts leads to colonies of trophoblast stem cells grown on serum-free RPMI medium with hydrocortisone, insulin, transferrin, and selenate. In some cultures, 3H-thymidine could be added to tag cells for specific cytokine uptake 8. PDMCs do not require feeder layers in culture, unlike MSCs, allowing for numerous passages and simple maintenance of cells.
           
Telomere length measurements, Real Time-PCR analysis, and Immunophenotyping of placental stem cells are common investigational techniques. Flow cytometric analysis is a standard technique used in most studies to analyze and identify populations of cells9.

Implications for Disease:
Placental stem cells disorders can be caused by abnormal proliferation of cells or inappropriate concentrations of cytokines and chemokines in the microenvironment. Intrauterine growth restriction (IUGR) and spontaneous abortion in pregnant women is caused by abnormal labyrinthine trophoblast cell formation 2. Since there is not sufficient exchange of nutrients and ions for the baby, growth is limited or even ceased. Some genes that regulate placental development also regulate the development of the heart. Therefore, it is possible that placental defects could lead to congenital heart defects in the fetus 3.
           
Tumors and skeletal malformations are a result of improper Cdx2 gene expression. Cdx2-/- showed kinky tails, shifts of vertebrae, malformation on ribs, and tumors in small intestine and other endothelial linings 5. Refractory placental trophoblastic tumor is caused by hyperproliferation of the trophoblasts at the maternal-fetal interface. Usually, the tumor is benign, but in cases of metastasis, it can lead to pregnancy complications, and in advanced cases the result could be miscarriage.

Implications for Medicine:
 Placental-derived multipotent cells can be used to treat multiple disorders. The use of MSCs is limiting in medicine because rates of infection are high during cell aspiration from the bone marrow, the procedure is invasive, and the there is a lower number of available MSCs in elderly patients 10. ESCs have thus far been considered the best source for pluripotent cells; they can form cells of every lineage and can be considered for cell-based therapy. However, ethical concerns regarding the destruction of the embryo form the basis for non-approval if federal funds are used for the research. ESCs are also known to cause teratomas and can also be rejected by unmatched host. PDMCs, on the other hand, are from primitive sources with respect to age; are derived from the placenta, and is obtained by non-invasive methods. The sources of PDMC are unlimited, and based on the information presented, they might have lower risk of rejection across allogeneic barriers.
           

PDMCs have been reported to suppress the functions of the proinflammatory cytokines, IFN-γ and TGF-β action, suggesting a partial mechanism for their value in clinical application. PDMCs have been show to produce cells that are cytokeratin-18 positive and produce albumin. This indicates that these cells could have repair potential for injured livers11. PDMCs have applications for stroke and cardiac damage by stimulating angiogenesis. Another major advantage of PDMC is reduced probability for malignancy.

 

Figure 2. Click to Maximize.

References:

  1. Jaffredo T, Bollerot K, Sugiyama D, Gautier R, Drevon C. Tracing the hemangioblast during embryogenesis: developmental relationships between endothelial and hematopoietic cells. Int J Dev Biol 2005;49:269-77.
  2. Selesniemi K, Reedy M, Gultice A, Brown T. Identification of Committed Placental Stem Cell Lines for Studies of Differentiation. Stem Cells and Dev 2005;14:535-47
  3. Hemberger M, Cross JC. Trends in Endocrinology and Metabolism. 2001;12:162-8
  4. Pollheimer J, Knöfler M. Signalling Pathways regulating the Invasive Differentiation of Human Trophoblasts: A Review. Placenta 2005;26A:S21-30.
  5. Chawengsaksophak K, James R, Hammond VE, Kontgen F, Beck F. Homeosis and intestinal tumours in Cdx2 mutant mice. Nature 1997;386:84-7.
  6. Kogler G, Sensken S, Airey JA, Trapp T, Muschen M, Feldhahn N, Liedtke S, Sorg RV, Fischer J, Rosenbaum C, Greschat S, Knipper A, Bender J, Degistirici O, Gao J, Caplan AI, Colletti EJ, Almeida-Porada G, Muller HW, Zanjani E, Wernet P. A new human somatic stem cell from placental cord blood with intrinsic pluripotent differentiation potential. J Exp Med 2004;200:123-35.
  7. Aoki Y, Kodama S, Kurata H, Kase H, Tanaka K. Failure of high-dose chemotherapy with peripheral blood stem cell support for refractory placental site trophoblastic tumor. Gynecol Obstet Invest 1999;47:214-6.
  8. Saito S, Enomoto M, Sakakura S, Ishii Y, Sudo T, Ichijo M. Localization of stem cell factor (SCF) and c-kit mRNA in human placental tissue and biological effects of SCF on DNA synthesis in primary cultured cytotrophoblasts. Biochem Biophys Res Commun 1994;205:1762-9.
  9. Berger M, Fagioli F, Piacibello W, Sanavio F, Mareschi K, Biasin E, Bruno S, Gammaitoni L, Gunetti M, Nesi F, Madon E, Aglietta M. Role of different medium and growth factors on placental blood stem cell expansion: an in vitro and in vivo study. Bone Marrow Transpl 2002;29:443-8.
  10. Chien CC, Yen BL, Lee FK, Lai TH, Chen YC, Chan SH, Huang HI. In vitro differentiation of human placenta-derived multipotent cells into hepatocyte-like cells. Stem Cells 2006;24:1759-68.
  11. Chang CJ, Yen ML, Chen YC, Chien CC, Huang HI, Bai CH, Yen BL. Placenta-derived multipotent cells exhibit immunosuppressive properties that are enhanced in the presence of interferon-gamma. Stem Cells 2006;24:2466-77.
  12. Yen BL, Huang HI, Chien CC, Jui HY, Ko BS, Yao M, Shun CT, Yen ML, Lee MC, Chen YC. Isolation of multipotent cells from human term placenta. Stem Cells 2005;23:3-9.

Summarized by: Nidhi Shah and Shweta Rane (in alphabetical order), Graduate Stem Cell Course, Fall 2006.
Teaching Assistant: Katherine Liu