In the central region hollow cavity of long bones, bone marrow is present. It is of two types, i.e., red bone marrow and yellow bone marrow.
The red bone marrow in mammals is the main site of hematopoiesis. It is present in the region of epiphysis or spongy bone. It is mainly composed of hematopoietic stem cells and supportive areolar tissue.
The yellow bone marrow is present in the region of diaphysis or compact bone. It is composed of areolar connective tissue, adipose tissue (white fat) and blood vessels. This white bone marrow is responsible for storage of WBCs and fat.
Inner surface of marrow cavity is lined by Endosteum.