Building bones for blood and beyond: the growing field of bone marrow niche model development.

The bone marrow (BM) niche is a complex microenvironment that provides the signals required for regulation of hematopoietic stem cells (HSCs) and the process of hematopoiesis they are responsible for. Bioengineered models of the BM niche incorporate various elements of the in vivo BM microenvironment, including cellular components, soluble factors, a three-dimensional environment, mechanical stimulation of included cells, and perfusion. Recent advances in the bioengineering field have resulted in a spate of new models that shed light on BM function and are approaching precise imitation of the BM niche. These models promise to improve our understanding of the in vivo microenvironment in health and disease. They also aim to serve as platforms for HSC manipulation or as preclinical models for screening novel therapies for BM-associated disorders and diseases.

Bioengineered niches that recreate physiological extracellular matrix organisation to support long-term haematopoietic stem cells.

Long-term reconstituting haematopoietic stem cells (LT-HSCs) are used to treat blood disorders via stem cell transplantation. The very low abundance of LT-HSCs and their rapid differentiation during in vitro culture hinders their clinical utility. Previous developments using stromal feeder layers, defined media cocktails, and bioengineering have enabled HSC expansion in culture, but of mostly short-term HSCs and progenitor populations at the expense of naive LT-HSCs. Here, we report the creation of a bioengineered LT-HSC maintenance niche that recreates physiological extracellular matrix organisation, using soft collagen type-I hydrogels to drive nestin expression in perivascular stromal cells (PerSCs). We demonstrate that nestin, which is expressed by HSC-supportive bone marrow stromal cells, is cytoprotective and, via regulation of metabolism, is important for HIF-1α expression in PerSCs. When CD34+ve HSCs were added to the bioengineered niches comprising nestin/HIF-1α expressing PerSCs, LT-HSC numbers were maintained with normal clonal and in vivo reconstitution potential, without media supplementation. We provide proof-of-concept that our bioengineered niches can support the survival of CRISPR edited HSCs. Successful editing of LT-HSCs ex vivo can have potential impact on the treatment of blood disorders.

Current insights into the bone marrow niche: From biology in vivo to bioengineering ex vivo.

Hematopoietic stem cells (HSCs) are fundamental to the generation of the body's blood and immune cells. They reside primarily within the bone marrow (BM) niche microenvironment, which provides signals responsible for the regulation of HSC activities. While our understanding of these signalling mechanisms continues to improve, our ability to recapitulate them in vitro to harness the clinical potential of the HSC populations is still lacking. Recent studies have applied novel engineering techniques combined with traditional in vitro work to establish ex vivo BM niche models. These models exhibit promising potential for research and clinical applications. In this review, BM niche factors that regulate the HSCs in vivo are discussed and their applications in the engineering of BM biomaterial-based platforms are considered. Many questions remain regarding the heterogeneity of niche components and the interactions of HSCs with their microenvironment. A greater understanding of the niche would help to elucidate these remaining questions, leading to the development of novel therapeutic tools.