Ratajczak group [1] and related Pomeranian University group [2] isolated the cells from umbilical cord blood (UCB) that are small and embryonary-like, and called them “stem cells”. This term is highly questionable in context of umbilical cord blood “VSEL”. The only proof that in this population of so called “VSEL” exist some cells of stem cell character is the paper of the same group showing that CD45- GlyA-, CD133+, ALDH low and, especially ALDH high, cells could differentiate over Op9 cell layer toward hematopoietic stem cells (coblestone area-like colonies) and acquire a capacity to establish human lympho-hematopoietic chimerism in lethally irradiated immuno-deficient mice [3]. As Danova-Alt et al discussed in their paper, these data could be questioned due to the possibility of contamination with hematopoietic stem cells. But, one could not conclude upon this question without a confirmation of principal results by an independent research group. So, in absence of an independent confirmation, the fact that Danova-Alt et al. did not reproduce results of Ratajczak group does not mean that their approach is wrong.

Danova-Alt et al. honestly described their strategy to isolate Lin-CD45-CXCR+ umbilical cord blood (UCB) cells, based on the original Ratajczak group approach [1], These cells were further analyzed on so called “stem cells markers”. The cells belonging to this population are indeed very small. They have a high nucleus to cytoplasm ratio, but did not exhibit any functional property to be called “stem cells”. Furthermore, this cell population did not express CD34, CD133 and SSEA4.

The manuscript of Danova-Alt et al. presents the methods in a precise and logic manner, and their conclusions are based on the results presented. Using the metaphors of the Dr Ratajczack’s letter, Danova-Alt et al. were searching for oranges and they found apples. Are they wrong? The results of an independent group are necessary to judge.

1. Kucia M, Halasa M, Wysoczynski M, Baskiewicz-Masiuk M, Moldenhawer S, et al. (2007) Morphological and molecular characterization of novel population of CXCR4+ SSEA-4+ Oct-4+ very small embryonic-like cells purified from human cord blood: preliminary report. Leukemia 21: 297-303.
2. Halasa M, Baskiewicz-Masiuk M, Dabkowska E, Machalinski B. An efficient two-step method to purify very small embryonic-like (VSEL) stem cells from umbilical cord blood (UCB). (2008) Folia Histochem Cytobiol 46: 239-43.
3. Ratajczak J, Zuba-Surma E, Klich I, Liu R, Wysoczynski M, et al. Hematopoietic differentiation of umbilical cord blood-derived very small embryonicepiblast-like stem cells. (2011) Leukemia 25: 1278-1285.