Table 1 Previous characters used to identify medullary bone (MB) fail to distinguish this female-specific tissue from other medullary bone like tissues (MBL)

From: Medullary bone in an Early Cretaceous enantiornithine bird and discussion regarding its identification in fossils

Character Description
1 Occur in the medullary cavity and other cancellous spaces throughout the appendicular and axial skeletons
2 Be of endosteal origin
3 Primarily have a woven arrangement of the collagen fibers indicative of rapid formation (may also be partially parallel-fibered or lamellar in some instances)
4 Co-occur with a periosteal surface free from pathological indicators
5 Line a majority of the medullary cavity (including trabecular surfaces)
6 Be clearly demarcated from the cortical tissue without a graded transition
7 Occur in multiple elements including the tibiotarsus
8 Coincide with reduced growth rates indicative of sexual maturity
9 Often have vascular canals with a doublet or triplet pattern within osteon-like structures (“vascular sinuses”)
10 Have a histochemistry comparable to that of extant avian medullary bone (e.g., shows higher amounts of glycosaminoglycans than that of cortical bone) (unique histochemical signature, yet to be determined)
11 Have a mineral to collagen ratio significantly greater than that of cortical bone
  1. To be strongly considered MB, the bone tissue must fulfill these criteria (characters 10 and 11 cannot be utilized at this time but may prove useful in the future with better understanding of bone histochemistry and the development of new protocols)