English: Fossil - Avimaia Schweitzerae - Histology - Medullary Bone ^{[1] [2] [3]}

https://www.nature.com/articles/s41467-019-09259-x

Histology reveals the presence of small amounts of medullary bone (MB) in the femur of Avimaia. Natural light (a, c, d—left image) and polarized light microscopy (b, d—right image) show slow growing, parallel-fibered and almost avascular cortical bone (CB), and small, endosteal trabeculae of lamellar MB (which are much darker than the CB and the endosteal bone, EB) extending into the medullary cavity. Other abbreviations: IPS intact periosteal surface, LAG line of arrested growth, LB lamellar bone, LMI layer of microbial invasion, LVC longitudinal vascular canal, PFB parallel-fibered bone, RL resorption line. Scale bar is 500 μm in a, 200 μm in b, 100 μm in c, and 50 μm in d