Aged individuals and astronauts experience bone loss despite rigorous physical activity. Bone mechanoresponse is in-part regulated by mesenchymal stem cells (MSCs) that respond to mechanical stimuli. Direct delivery of low intensity vibration (LIV) recovers MSC proliferation in senescence and simulated microgravity models, indicating that age-related reductions in mechanical signal delivery within bone marrow may contribute to declining bone mechanoresponse. To answer this question, we developed a 3D bone marrow analog that controls trabecular geometry, marrow mechanics and external stimuli. Validated finite element (FE) models were developed to quantify strain environment within hydrogels during LIV. Bone marrow analogs with gyroid-based trabeculae of scaffold volume fractions (SV/TV) corresponding to adult (25 ​%) and aged (13 ​%) mice were printed using polylactic acid (PLA). MSCs encapsulated in migration-permissive hydrogels within printed trabeculae showed robust cell populations on both PLA surface and hydrogel within a week. Following 14 days of LIV treatment (1 ​g, 100 ​Hz, 1 ​h/day), cell proliferation, type-I collagen (Collagen-I) and filamentous actin (F-actin) were quantified for the cells in the hydrogel fraction. While LIV increased all measured outcomes, FE models predicted higher von Mises strains for the 13 ​% SV/TV groups (0.2 ​%) when compared to the 25 ​% SV/TV group (0.1 ​%). While LIV increased collagen-I volume 34 ​% more in 13 ​% SV/TV groups when compared to 25 ​% SV/TV groups, collagen-I and F-actin measures remained lower in the 13 ​% SV/TV groups when compared to 25 ​% SV/TV counterparts, indicating that both LIV-induced strains and scaffold volume fraction (i.e. available scaffold surface) affect cell behavior in the hydrogel phase. Overall, bone marrow analogs offer a robust and repeatable platform to study bone mechanobiology.

A study appearing Monday, March 31 in Nature Physics presents a striking example of cooperative organization among cells as a potential force in the evolution of multicellular life. Based on the fluid dynamics of cooperative feeding by Stentor, a relatively giant unicellular organism, the study originated from the Marine Biological Laboratory (MBL), Woods Hole, Mass.