Source : Webster's Revised Unabridged Dictionary (1913)

Endosteum \En*dos"te*um\, n. [NL., fr. Gr. ? + ? a bone.]
   (Anat.)
   The layer of vascular connective tissue lining the medullary
   cavities of bone.

Source : WordNet®

endosteum
     n : vascular membrane that lines the inner surface of long bones
     [also: {endostea} (pl)]