Interactions between hematopoietic stem cells and their niche

Hematopoietic stem cells (HSCs) provide all blood cells throughout our lives. HSCs reside in a specialised microenvironment in the bone marrow (BM) referred to as the niche, where they are kept in a quiescent state for the majority of time. In recent years considerable research into the composition of and signalling in the niche has been performed, yielding an increased understanding of niche regulation and HSC homeostasis. Many non-hematopoietic niche cells, such as: mesenchymal stem cells, endothelial cells and HSCs descendants, such as: megakaryocytes, regulatory T-cells and macrophages support niche regulation. Other niche constituents, being the extracellular matrix and the sympathetic nervous system also assist in HSC regulation. The niche is a highly vascularised complex micro-environment and scientist hypothesize that there are many perivascular niches, each with distinct functions. Even though many regulatory aspects are elucidated, plenty of niche cells and factors remain undiscovered. In this essay, the existing knowledge regarding niche composition and regulation during homeostasis will be outlined and future directions will be discussed.