Stem Cell Derivation

These cells, derived from the blastocysts of embryos, have the potential to differentiate into any tissue in the body (and in the case of some species, into extra-embryonic tissue such as placenta). Yet other types of pluripotent stem cells offer some of the same hopes for regenerative medicine, drug discovery, and the basic life sciences, while possibly steering clear of many political, ethical, and even scientific quagmires associated with their embryonically derived cousins.

Many different “adult stem cell” lines have been reported in the past few years, each differing in how they were derived and cultured, the claims that were made about them (and the assays used to support these claims), and the cells’ ultimate applicability to human medicine. Yet even the possibility that such lines exist calls into question some fundamental tenets of developmental biology. Many bold claims have been made, and many counter-explanations have been asserted. And so the field of stem cell biology is moving very fast—but deliberately and cautiously—to understand (and possibly redefine) notions of differentiation, lineage, hierarchy, and especially “stemness.”

Krause doesn’t care much about the field’s semantic disputes over the terms ‘progenitor,’ ‘totipotent,’ ‘pluripotent,’ ‘multipotent,’ etcetera, she says, as long as it’s clear how the terms are being used. In deference to Krause, this article will use ‘pluripotent’ to mean capable of differentiating into cells of each of the three germ layers.

Such pluripotent stem cells were first cultured from early-stage blastocysts (embryonic stem cells), and later from embryonic germ cells (from the embryonic gonadal ridge), and perhaps from others as well. Conventional wisdom says that they have yet to undergo any developmental transformations that would commit them to a given lineage. This assumption, too, though, is not without controversy. There is a small group of scientists who maintain that some stem cells may have the ability to de-differentiate and then re-differentiate into another cell type; or, they postulate, perhaps there is a continuum along which cells reversibly gain and lose the potential to differentiate.

HSCs are multipotent, but they are not pluripotent in the sense that the term is most commonly used; they are restricted to making blood lineages. Similarly, neural stem cells (NSC) give rise to neurons, astrocytes, and oligodendrocytes, and so on. But these will not give rise to tissues of a different germ layer—that is, NSCs, from (ectodermal) nervous tissue, will not make (endodermal) pancreatic beta cells.

More potent stem cells can be derived from bone marrow, however. Perhaps the most celebrated of these were initially found in an attempt to develop what were at the time the most promising “adult stem cells,” the mesenchymal stem cells (MSC). After isolating a group of mononuclear cells from human bone marrow by magnetic sorting, Catherine Verfaillie’s group at the University of Minnesota cultured them on fibronectin-coated surfaces in the presence of serum and cytokines, obtaining clusters of small adherent cells that they termed multipotent adult lineage cells. Individual retrovirally marked MAPCs were subsequently shown to differentiate “into mesenchymal cells, but also cells with visceral mesoderm, neuroectoderm and endoderm characteristics in vitro,” and contributed to most cell types when injected into a developing blastocyst. Jiang, Nature 2002.

More recently, different manipulations have yielded other cell lines from which multiple germ lineages have been obtained. To obtain marrow-isolated adult multilineage inducible (MIAMI) cells, for example, Paul Schiller of the University of Miami School of Medicine and his colleagues co-cultured non-adherent and adherent bone marrow cells on fibronectin in serum, at low oxygen tension, to more closely mimic the microenvironment in which “more primitive” cells are expected to flourish. Clusters of small adherent cells were then isolated and plated at low density, in low oxygen tension, in the presence of serum, for an additional two weeks. Undifferentiated MIAMI cells express markers found in embryonic stem cells as well as mesodermal-, endodermal-, and ectodermal-derived lineages. D’Ippolito, J Cell Sci 2004

At about the same time, Douglas Losordo’s group at Tufts obtained human bone marrow-derived multipotent stem cells by growing hemolyzed marrow on fibronectin in culture medium with high serum concentration; hBMSC clones (from single cells) have been grown for more than 140 doublings without loss of multipotency, and can differentiate into cells of all three germ layers. Yoon, J Clin Invest 2005