REVIEW Stem Cell Spheroids and Ex Vivo Niche Modeling: Rationalization and Scaling-Up Isotta Chimenti 1 & Diana Massai 2 & Umberto Morbiducci 3 & Antonio Paolo Beltrami 4 & Maurizio Pesce 5 & Elisa Messina 6 Received: 27 October 2016 /Accepted: 27 February 2017 # Springer Science+Business Media New York 2017 Abstract Improved protocols/devices for in vitro culture of 3D cell spheroids may provide essential cues for proper growth and differentiation of stem/progenitor cells (S/PCs) in their niche, allowing preservation of specific features, such as multi-lineage potential and paracrine activity. Several plat- forms have been employed to replicate these conditions and to generate S/PC spheroids for therapeutic applications. However, they incompletely reproduce the niche environ- ment, with partial loss of its highly regulated network, with additional hurdles in the field of cardiac biology, due to de- bated resident S/PCs therapeutic potential and clinical trans- lation. In this contribution, the essential niche conditions (met- abolic, geometric, mechanical) that allow S/PCs maintenance/ commitment will be discussed. In particular, we will focus on both existing bioreactor-based platforms for the culture of S/ PC as spheroids, and on possible criteria for the scaling-up of niche-like spheroids, which could be envisaged as promising tools for personalized cardiac regenerative medicine, as well as for high-throughput drug screening. Keywords Stem cell niche . Cell spheroids . Bioreactors . Cardiospheres . Scaling-up Introduction Blike an eagle that stirs up its nest and hovers over its young, that spreads its wings to catch them and carries them aloft.^ (Deuteronomy, 32:11) The biblical metaphor reflects the scientific and biotechno- logical bases underlying the development of protocols and systems to expand therapeutic numbers of stem/progenitor cells (S/PCs) for pre-clinical and clinical applications. In fact, similarly to the little eaglets, S/PCs nested in different organs require a proper microenvironment, i.e., the niche (composed of feeder/supporting cells, specialized vascular cells, and a specific extracellular matrix (ECM)). S/PCs, while maintain- ing an active cross talk with the external environment, modu- late in turn tissue renewal, homeostasis, and the function of the organ where they reside [1]. Unlike the Eagle’ s nest, however, the niche is never devoid of S/PCs, as this would cause exhaustion of the tissue’ s ability to renew. A similar circumstance could be observed in differ- ent tissues in the elderly, due to the impact that risk conditions and aging have on the self-renewing and differentiation capa- bilities of S/PCs. This condition could ultimately lead to the accumulation of senescent and dysfunctional S/PCs into the niches, driving them to compete for trophic factors with the remaining functional ones [2]. Associate Editor Adrian Chester oversaw the review of this article. * Elisa Messina elisa.messina@uniroma1.it 1 Department of Medical Surgical Sciences and Biotechnology, BLa Sapienza^ University of Rome, Rome, Italy 2 Leibniz Research Laboratories for Biotechnology and Artificial Organs (LEBAO), Department of Cardiac, Thoracic-, Transplantation and Vascular Surgery, Hannover Medical School, Hannover, Germany 3 Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Turin, Italy 4 Department of Medical and Biological Sciences, University of Udine, Udine, Italy 5 Tissue Engineering Research Unit, BCentro Cardiologico Monzino^, IRCCS, Milan, Italy 6 Department of Pediatrics and Infant Neuropsychiatry, BUmberto I^ Hospital, BLa Sapienza^ University, Viale Regina Elena 324, 00161 Rome, Italy J. of Cardiovasc. Trans. Res. DOI 10.1007/s12265-017-9741-5