Bone marrow

Bone marrow is a tissue found inside the bones. In adult humans, marrow in some bones such as the pelvis, sternum and femur produces new blood cells.
There are two types of bone marrow: red and yellow. Red bone marrow is mainly consisted of myeloid tissue, while the yellow marrow is consisted of fat cells. Of course, when someone is born, all marrow is red. Eventually, some of it gets converted into yellow marrow. Red and most of the white blood cells are produced in red marrow, while a part of the while blood cells is produced in yellow marrow. In the case of need, yellow marrow can be converted into red marrow in order to produce new blood cells.

This is the structure of bone marrow

The stroma of bone marrow is actually all tissue that is not involved in the process of producing new blood cells (hematopoiesis). Cell types that make bone marrow are blood vessels, osteoblasts, adipocytes, macrophages and fibroblasts. Macrophages are very important since they transfer the iron for production of hemoglobin.
The stroma of bone marrow contains stem cells (marrow stromal cells).

Bone marrow has a barrier which basically stops the cells that haven’t matured from leaving. The immature cells do not have the membrane proteins which are needed for attaching/passing and leaving.

Bone marrow can be „attacked“ by various infections such as tuberculosis, which stops the production of blood cells. The cancer of cells inside the bone marrow is called leukemia.
Of course, to make the right diagnosis about a disease of bone marrow, it’s examination is neccessary. It is done by a needle and under local anesthesia.