Within the body, stem cell pools serve as a regenerative reservoir that activate and differentiate into any cell type, such as muscle cells, skin cells, blood cells and bone cells for the purposes of growth and repair. Importantly, stem cells have the ability to self-renew to avoid exhaustion. However, with aging and certain disorders, these pools experience a quantitative and qualitative decline, diminishing the body’s regenerative capacity that lead to age-associated phenotypes such as muscle loss, bone frailty, and immunosuppression. It is therefore crucial, for the purposes of combating age-related decline and maintaining tissue homeostasis, to understand the dynamics and behavior of the stem-cell niche. This understanding can lead to better cultivation and maintenance of stem cells, both in vivo and in vitro, that can enhance their use in the development of treatment strategies.

Microgravity provides unique advantages to study stem cells, particularly in the context of understanding the aging process as well as offering an environment that optimizes in vitro cell culture conditions for stem cell renewal and expansion. Every part of the body, down to single cells, is shaped by mechano-stimulatory forces – compression or stretching of cell boundaries brought on by gravity. It is therefore not surprising that microgravity imposes many effects on cell behavior, including stem cells, both in vivo and in vitro. Three benefits of stem cells in microgravity are highlighted here.

First, microgravity induces an aging phenotype in many in vivo physiological systems on an accelerated timescale, which can be modeled in stem cells to study their behavior under the same conditions. Detectable measures of tissue degeneration are seen within the first few days of microgravity exposure. Degeneration of tissues in microgravity is attributed, in part, to changes in stem cell behavior and is conjectured to occur as follows: (1) decreased progenitor cell populations contributing to tissue dysfunction through inhibition of cell proliferation and induction of cell death pathways; (2) disruption of progenitor cell structure (particularly in bone stem cells), through a loss of cellular cytoskeleton and contractility caused by the unloading effect of microgravity; (3) lastly, delayed differentiation of stem cells into specialized cells as a result of microgravity, further disrupting the ability of tissues to repair and regenerate. Microgravity, therefore, provides a platform to explore failure of stem cell regeneration and tissue repair and leverage these insights for pharmaceutical drug discovery. 

Second, microgravity provides a superior environment for cultivating stem cells in vitro. On Earth, it is difficult to maintain primary cultures of stem cells, as these cells are driven to differentiate or quickly reach senescence. This is a problem considering the push for stem cell use in regenerative medicine. Therefore, methods and protocols to produce large numbers of clinical-grade stem cells need to be developed. 

Finally, space provides an environment that could potentially address the need to maintain and expand stem cells more efficiently. In space, while differentiation of cultured progenitor cells remained inhibited, the proliferative capacity of progenitor cells was enhanced. The cells maintained or overexpressed many gene markers of self-renewal, early development, and immortality. Upon return to Earth, these space-flown cells retained their ability to self-renew while regaining and even enhancing their ability to differentiate, indicating that gravity is necessary for progenitor transition to differentiated cells. Importantly, these results suggest that space-flown cells could be a source of self-renewing stem cells that could then be returned to Earth and induced to differentiate into desired cell types for research or clinical applications. 

A variety of hardware equipment is available for the study of cells in space. Some of the advanced systems allow for automated control of physiological conditions as well as manipulation and evaluation of cells.