Characterization of the bone marrow adipocyte niche with three-dimensional electron microscopy

Abstract

Unlike white and brown adipose tissues, the bone marrow adipocyte (BMA) exists in a microenvironment containing unique populations of hematopoietic and skeletal cells. To study this microenvironment at the sub-cellular level, we performed a three-dimensional analysis of the ultrastructure of the BMA niche with focused ion beam scanning electron microscopy (FIB-SEM). This revealed that BMAs display hallmarks of metabolically active cells including polarized lipid deposits, a dense mitochondrial network, and areas of endoplasmic reticulum. The distinct orientations of the triacylglycerol droplets suggest that fatty acids are taken up and/or released in three key areas - at the endothelial interface, into the hematopoietic milieu, and at the bone surface. Near the sinusoidal vasculature, endothelial cells send finger-like projections into the surface of the BMA which terminate near regions of lipid within the BMA cytoplasm. In some regions, perivascular cells encase the BMA with their flattened cellular projections, limiting contacts with other cells in the niche. In the hematopoietic milieu, BMAT adipocytes of the proximal tibia interact extensively with maturing cells of the myeloid/granulocyte lineage. Associations with erythroblast islands are also prominent. At the bone surface, the BMA extends organelle and lipid-rich cytoplasmic regions toward areas of active osteoblasts. This suggests that the BMA may serve to partition nutrient utilization between diverse cellular compartments, serving as an energy-rich hub of the stromal-reticular network. Lastly, though immuno-EM, we've identified a subset of bone marrow adipocytes that are innervated by the sympathetic nervous system, providing an additional mechanism for regulation of the BMA. In summary, this work reveals that the bone marrow adipocyte is a dynamic cell with substantial capacity for interactions with the diverse components of its surrounding microenvironment. These local interactions likely contribute to its unique regulation relative to peripheral adipose tissues.

Nerves in Bone: Evolving Concepts in Pain and Anabolism

“…Instead, nonsynaptic vesicular fusion in axon varicosities releases signaling chemicals into the extracellular space that diffuse to target‐cell receptors by volume transmission. Ultrastructural analysis has provided evidence of free varicose nerve fibers in the vicinity of putative skeletal target cells: peptidergic sensory fibers running along the periosteal cellular layer and at the osteochondral junction at sites of mineralization, contacting osteoblasts and their precursors, osteoclasts, hematopoietic cells, and endothelial cells of intramedullary blood vessels and sympathetic nerve fibers contacting osteoblasts, osteoclasts, and bone marrow adipocytes . Several lines of evidence have teased out the influence of key neurotransmitters—the neuropeptides CGRP, SP, and NPY and the biogenic amine NE.…”

“…Ultrastructural analysis has provided evidence of free varicose nerve fibers in the vicinity of putative skeletal target cells: peptidergic sensory fibers running along the periosteal cellular layer and at the osteochondral junction at sites of mineralization, contacting osteoblasts and their precursors, osteoclasts, hematopoietic cells, and endothelial cells of intramedullary blood vessels (17,75,76) and sympathetic nerve fibers contacting osteoblasts, osteoclasts, and bone marrow adipocytes. (27,75,77) Several lines of evidence have teased out the influence of key neurotransmitters-the neuropeptides CGRP, SP, and NPY and the biogenic amine NE. Throughout this discussion of the effects of neurotransmitters on bone, it is important to consider that many of these can be expressed by other, non-neural, cell types in bone.…”

Fat for fuel: lipid metabolism in haematopoiesis

“…Diacylglycerol (DAG) is an important lipid intermediate involved in many signalling processes . Within the BM niche, BM adipose tissue (BMAT) has been shown to influence haematopoiesis in addition to its canonical roles as an endocrine tissue and energy store …”

“…37 Within the BM niche, BM adipose tissue (BMAT) has been shown to influence haematopoiesis in addition to its canonical roles as an endocrine tissue and energy store. 38 Direct interactions between BMAT and haematopoietic cells have recently been visualised by the Scheller group, where they applied focused ion beam scanning electron microscopy to examine how these cells interact within the BM niche. 38 This study showed that budding lipid droplets from BMAT interacted with maturing myeloid cells and phagocytic reticular cells or erythroblast island macrophages in the BM, where they may act to support myeloid and erythroid maturation and aid in their release into sinusoids ( Figure 3bi).…”

“…38 Direct interactions between BMAT and haematopoietic cells have recently been visualised by the Scheller group, where they applied focused ion beam scanning electron microscopy to examine how these cells interact within the BM niche. 38 This study showed that budding lipid droplets from BMAT interacted with maturing myeloid cells and phagocytic reticular cells or erythroblast island macrophages in the BM, where they may act to support myeloid and erythroid maturation and aid in their release into sinusoids ( Figure 3bi). Furthermore, these reticular phagocytes were shown to have an extensive network of lipid droplets that associated with nucleated erythroblasts and, to a lesser extent, granulocytes.…”