目前的上皮生物学理论认为干细胞具有较高的克隆形成能力。然而血液学的研究表明某种细胞短期的体外克隆试验结果并不能反映其在体内的长期增殖能力。大量研究表明利用胶原的快速贴壁法可以可以获得具有高克隆形成率的细胞，但这些细胞是否富含干细胞且具有体内长期倍增能力并不清楚。我们发现5分钟内快速贴壁于胶原的细胞具有较强的短期体外克隆形成率，但并不具备体外长期扩增的能力或9周的体内倍增能力。相反，20分钟后仍未贴壁的缺乏短期克隆形成能力，但体外长期增殖力相同，且体内倍增能力较贴壁细胞高。快速贴壁和非快速贴壁细胞都含有形态较小的非复层基底样细胞，表达整合素alpha2（一种IV型胶原受体），以及假定的表皮干细胞表型整合素alpha6（较高）和CD71（较低）。此结果表明胶原粘附细胞较高的短期克隆形成能力并不与体内或体外的长期倍增能力相关，以干细胞体外的增殖能力反映其体内的真实行为并不可靠。

Stem Cells. 2007 Oct 18; [Epub ahead of print] Related Articles, Links
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    Rapid adhesion to collagen isolates murine keratinocytes with limited long-term repopulating ability in vivo despite high clonogenicity in vitro.

    Strachan LR, Scalapino KL, Lawrence HJ, Ghadially R.

    Department of Dermatology, University of California San Francisco and San Francisco Veterans Affairs Medical Center, San Francisco, CA 94121.

    A prevalent belief in epidermal biology is that stem cells are highly clonogenic, i.e. have the ability to produce many large colonies in vitro. However, it has been well established in hematology, and recently suggested in epithelial biology, that short-term in vitro clonogenic assays may not be reliable predictors of long-term in vivo repopulating ability. Numerous groups have shown that rapid adhesion to collagen selects for highly clonogenic keratinocytes, but it has not been demonstrated whether this subpopulation is enriched in stem cells as defined by long-term repopulating ability in vivo. We found that while rapid adhesion to collagen (within 5 minutes) selected for cells with increased short-term colony forming ability in vitro, these cells were not enriched in long-term proliferative ability in vitro, or repopulating ability in vivo after nine weeks. Conversely, keratinocytes that did not adhere to collagen (after 20 minutes) were less clonogenic in short-term assays, but possessed equivalent long-term proliferative ability in vitro, and superior long-term repopulating ability in vivo. Both the rapidly adherent cell (RAC) and not rapidly adherent cell (NAC) populations contained small, non-complex basaloid cells, expressed integrin alpha2 (a collagen IV receptor), and expressed the putative epidermal stem cell phenotype, integrin alpha6(hi)CD71(lo). Our results indicate that the superior short-term colony forming ability of collagen-adherent murine keratinocytes does not correlate with long-term repopulating ability in vitro or in vivo, and that proliferation in vitro is not a reliable surrogate for stem cell behavior in vivo.