Lack of Ptn promotes cell division recruitment. In the experiments presented here, LSK cells were first sorted from WT donors and then labeled with CFSE. Cells were cultured overnight in long-term culture medium. The next day, a tight peak of CFSE⁺ cells was sorted to facilitate detection of single divisions⁵¹ cocultured in the presence of primary stromal cells (A). After 3 days of coculture, cells were analyzed for the presence of Lin⁻, Kit⁺, and Ly6a⁺ cells (B). (C) Representative histogram of the cell division kinetics of Kit⁺ cells. The fraction of undivided and divided cells was then determined in cocultures on either Ptn^+/+ or Ptn^−/− stromal cells and was compared with the use of a paired Student t test. The ratio of these fractions (fraction of division n in Ptn^−/−)/(fraction of division n in Ptn^+/+) was then calculated. These fractions are shown per division (D). (E) The acquisition of lineage markers of the LSK cells per division and (F) the loss of Ly6a in those divisions in LSK cells cocultured with Ptn^+/+ and Ptn^−/− stromal cells. A total of 6 independent repeats of this experiment were performed (D) of which representative plots and histograms are shown. The experiment in panels E and F was performed independently 3 times; a representative example is shown.