Ultrastructure of presumptive hematopoietic stem cells.

The repopulating potential of bone marrow cells that have been subjected either to cryoprotective or to cryodestructive treatments were assayed by the spleen colony technique. There were four treatments. For each treatment group the ultrastructure of aliquots of the cell suspensions was studied, and an attempt was made to identify the cells responsible for hematopoietic repopulation. The repopulating potential of fresh, glycerolized cells was greater than that of slowly cooled glycerolized cells, and both repopulating potentials were far greater than that of rapidly cooled glycerolized cells. With the exception of one cell type that was constantly well preserved, fresh glycerolized cells and cells frozen at controlled rates showed morphologic disruption. This type of cell was not found in suspensions of bone marrow cells that had been frozen rapidly, and it is proposed that it is the hematopoietic stem cell (presumptive stem cell or PSC). Two observations support the hypothesis that these cells are responsible for effecting hematopoietic repopulation of lethally irradiated mice injected with suspensions of marrow cells frozen to preserve their repopulating potential: (1) They are the only intact cells found in cryopreserved suspensions of marrow cells that effect hematopoietic repopulation. (2) They are not present in suspensions of marrow cells frozen so as to destroy the hematopoietic-repopulating potential.