Dynamic regulation of Ghr on HSCs induces age-dependent effects upon ex vivo rGH stimulation. (A) Expression of Ghr in hematopoiesis in the indicated populations as revealed by microarray analysis. (B) Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) of Ghr expression in HSCs (LSKCD34⁻Flk2⁻), multipotent progenitor-1 (MPP1; LSKCD34⁺Flk2⁻) and myeloid progenitors (MP) (Lin-Sca1⁻cKit⁺) young (4-month-old) mice. (C) Expression of Ghr in young, middle age, and old HSC (LSKFlk2⁻CD34⁻) and MPP1 (LSKCD34⁺Flk2^–) populations. (D) The qRT-PCR of Ghr expression in young and old HSCs. (E) Ghr expression in young and old HSCs over 24 hours of ex vivo culture. (F) Experimental design of ex vivo rGh treatment of isolated young and old HSCs (LSKCD34⁻Flk2⁻CD150⁺) followed by in vivo functional analysis. (G-I) PB analysis after transplantation of rGh-treated or control-treated young and old HSCs showing (G) donor engraftment at 4 and 17 weeks posttransplant. (H) Fold-change in PB engraftment between by young and old untreated and rGh-treated HSCs at week 17 posttransplant. (I) Lineage reconstitution at 17 weeks posttransplant. Unpaired Student t test: *P < .05; **P < .01; ***P < .001. ns, not significant; WBM, whole BM.