Engineering the haematopoietic stem cell niche in vitro

In vitro models recapitulate tissue-specific microenvironments of the human body in an experimental setting to study cell functions and responses in a holistic approach and to develop improved therapeutics. The hematopoietic stem cell (HSC) niche is located in the bone marrow in two main compartments: the endosteal niche, next to the inner face of the bone, and the perivascular niche, next to bone marrow sinusoids. Current in vitro models of the HSC niche introduce elements of only one of these two compartments. Tissue engineering strategies can advance our understanding of the HSC niche and current treatments of blood-related diseases. Herein, we describe a novel multiphasic tissue-engineered construct that incorporates elements of both compartments. To recreate the endosteal niche, primary human osteoblasts are grown on medical grade polycaprolactone scaffolds produced by melt electrospinning under osteogenic conditions to allow the deposition of a dense bone matrix. To recreate the perivascular niche, human umbilical vein endothelial cells and placenta-derived mesenchymal stem cells are embedded in star-shaped polyethylene glycol-heparin-based hydrogels to allow the formation of capillary-like networks. The multiphasic tissue-engineering construct is built by aligning the bone matrix with the capillary-like networks to enable the short-term culture and expansion of CD34+ cells. To our knowledge, this is the first in vitro model of the HSC niche that aligns elements of both the endosteal and the perivascular niche in a single culture system.