In vivo, the absence of Opn in the microenvironment results in a significant increase in HSCs and consequently total BM cellularity. (A) Compared with wt controls Opn^–/– mice had a significant increase in total marrow cellularity. *P = .003. (B) In addition, there is also a significant increase in the number of LSK cells present in the BM. *P < .001. Data are the mean ± SEM of femurs, tibias, and iliac crests from 5 mice.