Hemmule: A Novel Structure with the Properties of the Stem Cell Niche

Abstract

Stem cells are nurtured and regulated by a specialized microenvironment known as stem cell niche. While the functions of the niches are well defined, their structure and location remain unclear. We have identified, in rat bone marrow, the seat of hematopoietic stem cells—extensively vascularized node-like compartments that fit the requirements for stem cell niche and that we called hemmules. Hemmules are round or oval structures of about one millimeter in diameter that are surrounded by a fine capsule, have afferent and efferent vessels, are filled with the extracellular matrix and mesenchymal, hematopoietic, endothelial stem cells, and contain cells of the megakaryocyte family, which are known for homeostatic quiescence and contribution to the bone marrow environment. We propose that hemmules are the long sought hematopoietic stem cell niches and that they are prototypical of stem cell niches in other organs.

Precise tissue bioengineering and niches of mesenchymal stem cells: Their size and hierarchy matter

“…It is interesting to note that various authors refer to specialized microterritories as niches of a wide range of sizes. For instance, in the case of HSCs, niche diameter can vary widely from 1000 microns (Vodyanoy et al, 2020) and 200-300 microns (Ahmadbeigi et al, 2013) to 20-25 microns (Crocker and Gordon, 1985;Chasis, 2006;Yu and Scadden, 2016), and niches can include numerous cells. Therefore, we attempted to find scientific references addressing the in vitro design of artificial microterritories for single stem cells similar to the natural units listed in Table 2.…”