Bone Marrow

Bone marrow is the specialized connective tissue and stem cell tissue located in the center of the large bones and used, among other things, for the formation of blood cells. The red bone marrow in the finely woven ball structure (Kortikalis und Spongiosa/en|Cancellous_bone) of the bones is the formation site of the erythrocytes, the yellow bone marrow of the large cavities of the tubular bones consists of fatty tissue and can be reconverted into red bone marrow in case of severe blood loss.

https://en.wikipedia.org/wiki/Bone#Cancellous_bone _([1])