Table 1. Characteristics differentiating the canopies from the rest of the bone marrow envelope22.

	Bone marrow envelope
		Canopy
Activity of neighboring bone	Quiescent	Remodeling
Physical separation from bone matrix	Bone-lining cells	Osteoclasts, Rv.Cs, osteoblasts and sometimes a lumen
Physiological status	Resting	Exposed to osteoclast and capillary osteogenic products
Visibility through light microscopy (in human biopsies)	Poor	Satisfying, good if immunostained

Abbreviation: Rv.Cs, reversal cells.

The bone marrow envelope refers to a continuous layer of elongated cells lining the bone marrow. The term canopy refers to the zone of the bone marrow envelope covering bone-remodeling sites (that is, about 30% of the cancellous bone surface in human adult iliac crest22). Hypertrophy and lifting may contribute to the better visibility of the canopy. Additional discriminating characteristics are under investigation.