Microwells as Minimalistic Niches to Study Heterotypic Interactions of Stromal and Hematopoietic Stem Cells.

Hematopoietic stem and progenitor cells (HSPCs) can migrate and reside within the bone marrow in distinct microenvironments or niches. The niches organize around specific stromal cells, such as endothelial cells at the capillary or sinusoid walls, and osteoblasts along the bone matrix. Within each niche, a specific combination of external cues, including secreted and diffusible factors, cell-matrix, and cell-cell interactions, controls HSPCs behavior and fate. Deciphering the interplay between HSPCs and stromal cells of the niches is challenging: in vivo, it is hindered by the opacity of the bone matrix; in vitro, classical co-culture models only poorly recapitulate essential features of the physiological niches. The difficulty is moreover amplified by the exceptional migration capacity of HSPCs.In this chapter, we present a method to overcome these limitations by producing arrays of microwells designed to mimic bone marrow niches in a functional manner. These "microniches" promote a long-term interaction between the HSPC and a stromal cell of interest. We describe their microfabrication based on a maskless photolithography method allowing the production of arrays of microwells with reproducible volume and geometry, and the iterative improvement of the geometric design of the wells. We describe the loading and culture of stromal cells with HSPCs. We discuss the potentiality of microwells, in basic and applied research, as a platform to investigate molecular mechanisms involved in direct cell-cell interactions and local effects of diffusible factors, for any adherent and non-adherent cells of interest.