Table 1 The heterogeneity of hard tissue-resorbing cells. Diversity in hard tissue-resorbing cells at different sites and biological settings

Calvarial osteoclasts

Larger in size and utilize distinct proteases from long bone osteoclasts

[43,44,45]

Odontoclasts

Resorb dental tissues, but differences from osteoclasts are not clear

[46]

Vascular-associated osteoclasts (VAOs)

Closely associated with type H vessels to regulate blood vessel growth

[47]

Septoclasts

Cartilage-resorbing mesenchymal cells characterized by expression of FABP5 and MMPs

[48]

Type H endothelial cells

Produce MMP9 to degrade cartilage

[47]

Arthritis-associated osteoclastogenic macrophages (AtoMs)

Arthritis-associated osteoclast precursors controlled by transcription factor FoxM1

[49]

Osteoclast precursors with myeloid suppressor function

Expand in the bone marrow of arthritic mice and inhibit T-cell proliferation

[50]

Osteoclasts associated with bone loss induced by colitis and estrogen deficiency

Containing heterogeneous population with distinct immune regulatory functions

[51, 52]

Fracture-associated osteoclasts

Derived from yolk-sac macrophage descendants residing in the adult spleen

[53]

Fracture-associated circulating CX3CR1+ precursors

Migrate to the fracture sites and differentiate into osteoclasts

[54]

Obesity-associated osteoclast precursors

High-fat diet-induced monocytic MDSCs capable of differentiating into osteoclasts

[55]

Osteomorphs

Daughter cells produced by osteoclast fission capable of fusing back into osteoclasts

[56]