NATURE BIOTECHNOLOGY VOL 18 AUGUST 2000 http://biotech.nature.com 827

ANALYSIS

A recent paper in Science by Clarke et al.1

adds another piece to the puzzle that is the“stem cell.” It joins a series of other reports2–8

from the past two years suggesting that mam-malian cells with exceptional and unantici-pated capacities to mature into a broad rangeof phenotypes may be isolated from a varietyof organs at multiple ages, grown to abun-dance in culture, and reimplanted into otherregions with apparent accommodation tothese new environments. Nevertheless, ques-tions are emerging at a far greater rate thananswers.

Primordial cells with this degree of self-renewal and potential have come to betermed stem cells (SCs). Because their“potential” has typically been revealed byforcing the cells to mature in altered envi-ronments or in regions and developmentalstages other than those of their origin, theimplications of these findings for under-standing normal development remainareas of intense speculation. That suchcells, however, may be the repository ofunimagined plasticity provides an irre-sistible invitation to translational biolo-gists and entrepreneurs to exploit suchphenomena.

Following fertilization of the egg andformation of the blastocyst, “totipotent”SCs are generated within the inner cell mass(ICM). With the onset of gastrulation,these “embryonic stem (ES) cells” arethought to segregate into three major lin-eages on the basis of their incorporationwithin three primary germ layers (ecto-derm, mesoderm, and endoderm) where, ithas been assumed, their capacity to gener-ate all cell types becomes narrowed as theymature into tissue- and organ-specific/committed SCs. These latter SCsare assumed to be responsible for the subse-quent growth of tissues and organs duringdevelopment, and for tissue homeostasisand repair throughout life. Although thereis a growing sense that SCs across organsmay share characteristics and gene expres-sion patterns conferring “stem-ness,” it hasbeen assumed that, as plastic as it might be,

a given SC’s repertoire is restricted to gen-erating cell types of the somatictissue/organ in which it resides.

Perhaps it should not be surprising,however, given how radically the stem cellconcept has altered our formerly determin-istic view of such organs as the nervous sys-tem8–15, that the conventional view of therigid segregation of embryonic lineagesshould also begin to crack in the face ofsome unexpected, and admittedly poorlyunderstood, observations. It appears thatostensibly organ/tissue-committed SCs,even SCs from adult tissue, when placed invarious tissues and organs outside their nor-mal residence, can yield cells characteristicof those environments. Intra-germal plas-ticity, in which SCs give rise to derivativesfrom the same germ layer (e.g., mesenchy-mal SCs yielding cartilage, bone, andadipocytes6; bone marrow (BM) cellsundergoing myogenic differentiation4, andvice versa3) is certainly important. Possiblymore striking, however, is the suggestion ofinter-germ layer transdifferentiation (e.g.,ectoderm-derived neural SCs yieldingmesoderm-derived hematopoietic cells2;mesoderm-derived BM stroma producingendoderm-derived hepatocytes and neu-roectoderm-derived glia5).

Now, Clark et al. report that adult neuralSCs, when injected into the blastocyst, mayeven contribute to embryonic tissues ofendodermal, ectodermal, and mesodermalorigin (in a manner similar to ES cells), andthat some of these cells express markersappropriate to that layer’s derivatives (e.g.,albumin in liver, PAX2 in kidney). (It isunclear if these neural-derived cells will dif-ferentiate sufficiently to integrate functional-ly into these tissues.) Whether these apparenttransgermal differentiations reflect truedevelopmental programs or simply our abili-ty to “de-differentiate” cells and push themby extraordinary means in given directionsremains to be determined. Does any organ-specific SC have the potential to be “totipo-tent” (analogous to ES cells) under the rightcircumstances? Are our traditional markersfor discerning tissue-specific cell types spe-cific enough?

Although such questions may seemdevelopmental biology arcana, they willimpact practical concerns in stem cell-mediated tissue engineering becauseorgans may, in essence, be “re-created” byinvoking developmental processes. Whatare some of the emerging quandaries incontemplating the use of SCs in transla-tional biology, particularly when envision-

The possibilities/perplexities of stem cells

Evan Y. Snyder and Angelo L. Vescovi

Figure 1. Speculating on stem cells. The left hand portion suggests that stem-like cells frommultiple structures and organs at a range of developmental stages may derive from, or beforced, under certain (often unusual) conditions to give rise to each other. This possibilitysuggests the panoply of transplantation strategies (both as allografts and autografts)illustrated on the right. At present, all are merely speculative.

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ADULTSkeletalmuscle

Brain

Vasculartissue

Blastocyst

Liver

Bone(mesenchymal/hemopoietic)

Evan Y. Snyder is an investigator in theDepartments of Pediatrics, Neurosurgery, &Neurology, Children’s Hospital, Boston,Harvard Medical School, Boston, MA 02115(snyder@a1.tch.harvard.edu), and Angelo L.Vescovi is co-director of the Institute For StemCell Research, DBIT, Hospital San Raffaele,Milan Italy I-20132 (vescovi.angelo@hsr.it).

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The cross-fertilization of biology, chem-istry, and materials science is opening uptremendous opportunities for innovationin previously unrelated disciplines such aselectronics and information processing.The extraordinary recognition capability ofbiomolecules has suggested their use as ameans for direct recognition and binding ofinorganic materials—in other words actingas a sort of natural software for program-ming the formation of matter. A recentpaper in Nature by Whaley et al.1, whichdescribes an approach for generating pep-tides suitable for use in semiconductors andmetal binding, is the latest of a series ofpapers2–8 that demonstrate the potential ofthese biological approaches in materialsengineering.

Researchers working in fields as dis-parate as molecular electronics and the biol-ogy of bone formation are becoming

increasingly aware of the use of self–assem-bling biomolecules as tools for studying andcontrolling the formation of complex struc-tures, both organic and inorganic (metalsand semiconductors). The main idea is toexploit the highly specific binding proper-ties characteristic of naturally formedorganisms to drive the formation of hybridinorganic structures containing semicon-ductors or metals.

By way of example, Whaley et al. havescreened a combinatorial phage library ofmillions of peptides with unknown bindingproperties against inorganic molecules andsucceeded in selecting specific peptidesequences that can distinguish different crys-tallographic planes of the most importantsemiconductors (gallium arsenide and sili-con). These peptides could therefore be usedto control the positioning and the assemblyof materials at the nanoscale.

A somewhat similar approach has beenpursued by Brown and coworkers2, but in atotally different context. In this case, theauthors were interested in investigatingpeptide–driven formation of gold crystals,as a prototype mechanism for the forma-tion of natural solids like bones and teeth in

828 NATURE BIOTECHNOLOGY VOL 18 AUGUST 2000 http://biotech.nature.com

ANALYSIS

Biological software for materialsengineering

Roberto Cingolani

Roberto Cingolani is professor, INFMResearch Unit, Department of InnovationEngineering, University of Lecce, ViaArnesano 73100 Lecce, Italy(roberto.cingolani@unile.it).

ing extension to human systems in clinicalsettings?

Probably foremost among them iswhether one can, or should, use adult tissueas a source of SCs in autologous graft para-digms. The alternative to this approach, intransplantation-based strategies, is the use ofstable SC lines as “universal donor cells.” Thelatter has the appeal of being an “off-the-shelf” reagent, prepared and/or additionallyengineered under good manufacturing prac-tices readily available in limitless quantitiesfor the acute phases of an injury or disease.Its downside is the possibility of immuneincompatability. An additional concern iswhether the starting material for such linesmight initially need to come (albeit just ini-tially) from a developmental stage (embryo,fetus, adult) that empirically proves optimalfor yielding potent SCs. (Prenatal sources—which may prove biologically optimal—arecolored by an ethical dimension not typicallyborne by adult tissues.)

While grafting a patient’s own SCs mightcircumvent these problems, the prospectiveisolation, expansion, characterization, anddirected differentiation of cellular reagentsfor each new patient—with its attendantcosts in time, resources, manpower, andpotential inter-preparation variability—rep-resents a considerable challenge. This alsopresumes that we know how to translateobservations like those of Clarke et al. forpractical use, which, in turn, presupposes aknowledge of the signals involved and anability to provide them controllably—farfrom realized.

Is it necessary to expose adult SCs to theprimitive blastula environment for expres-sion of a more generalized lineage poten-tial? An embryonic milieu may reactivate asubset of options (e.g., kidney, heart), butnot others (e.g., certain mesodermal-derived cells1), whereas exposure to anadult milieu may be necessary to promotethese other repertoires (e.g., skeletal mus-cle, blood)2,3. One must also create safe-guards such that cells with theoretical“totipotency” do not give rise to inappro-priate cells (e.g., muscle in brain), trans-form to teratocarcinomas, or createautonomous organs within the larger organ(e.g., neural tubes within the heart).Although pre-differentiation ex vivo mightpreclude this, the comfort of invariantcommitment must be weighed against theloss of plasticity and multipotency, whereinthe environment directs SCs toward neededphenotypes with potential reconstitution ofdegenerated regions with multiple celltypes8. (Interestingly, despite the extensivepluripotency suggested for neural SCs byrecent papers1,2, undesired cells have neverbeen observed in intracranial transplantparadigms8–13.) An additional consideration

with the autograft strategy is more funda-mental than technical: though possibly use-ful for trauma-based deficits, it wouldprobably not be optimal for geneticallybased diseases (i.e., reimplanting cells already harboring a defect or predispo-sition).

Human versions of organ-specific6,9–15

and embryonic SCs16–18 are available for labo-ratory experimentation. They emulate manyof the characteristics of their rodent counter-parts, suggesting great promise for their ulti-mate use in clinical situations. Each of thequestions posed above, therefore, will be car-ried forward to the conference rooms of realhospitals: totipotent vs. organ-specific cells;embryonic- vs. fetal- vs. adult-derived cells.

A paper, such as that by Clarke et al., sug-gesting that adult organ-specific cells mayfulfill the role of ES cells, simply makes thedebate—particularly the ethical dimen-sions—all the more intense and interesting.At present, the idea that adult SCs are equiva-lent to, and may replace, embryonic SCs inbasic and therapeutic investigations is notsolidly founded. However, the recent spate of

studies suggesting unexpected degrees ofplasticity are certain to stimulate experi-ments that, three years ago, may have beenunimaginable.

1. Clarke, D.L., et al. Science 288, 1660–1663(2000).

2. Bjornson, C.R., et al., Science 283, 534–537 (1999).3. Gussoni, E., et al., Nature 401, 390–394 (1999).4. Jackson, K.A., Mi, T. & Goodell, M.A., Proc. Natl.

Acad. Sci. USA 96, 14482–14486 (1999).5. Kopen, G.C., Prockop, D.J. & Phinney, D.G., Proc.

Natl. Acad. Sci. USA 96, 10711–10716 (1999).6. Pittinger, M.F Science 284, 143–147 (1999).7. Vijayakumar, K.R. et al. Nat. Med. 6, 278–282

(2000).8. Yandava, B.D., Billinghurst, L.L. & Snyder, E.Y. Proc.

Natl. Acad. Sci. USA, 96, 7029–7034 (1999).9. Svendsen, C.N. et al. Exp. Neurol. 148, 135–146

(1997).10. Vescovi, A.L. et al. Exp. Neurol. 156, 71–83 (1999).11. Fricker, R.A. et al. J. Neurosci. 19, 5990–6005

(1999).12. Brüstle, O. et al. Nat. Biotechnol. 11, 1040–1049

(1998).13. Flax, J.D. et al. Nat. Biotechnol. 16, 1033–1039

(1998).14. Roy, N.S. et al. Nat. Med. 6, 271–277 (2000).15. Villa, A. et al. Exp. Neurol. 161, 67–84 (2000).16. Thompson, J. et al. Science 282, 1145 (1998).17. Shamblott, M.J. et al. Proc. Natl. Acad. Sci. USA 95,

13726–13731 (1998).18. Reubnoff, B.E. et al. Nat. Biotechnol. 18, 399–404

(2000).

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