The Estrous Cycle and the Hemopoietic Stem Cell in the Mouse

A large variability in the stem cell content of the spleens of female mice by comparison with that of the bone marrow, or with that of the spleen and bone marrow of male mice, is demonstrated. This variability is shown to be associated with the estrous cycle. Stem cells are assayed by the spleen colony-forming test, either exogenously by transplantation to irradiated recipient mice, or endogenously by colony development in sublethally irradiated female mice. It is shown that, for mice in pre-estrus, the spleens contain up to three times as many hemopoietic stem cells (CFU) as during the estrus and post-estrus stages of the cycle. Although the association may be indirect, it is suggested that the changes in splenic stem cell numbers may be related to the hormonal milieu at the time of observation.