Bone marrow is the tissue comprising the center of large bones. It is the place where new blood cells are produced.
Bone marrow contains two types of stem cells: hemopoietic (which can produce blood cells) and stromal (which can produce fat, cartilage and bone).
There are two types of bone marrow
Red marrow (also known as myeloid tissue) and yellow marrow.
Red, or hematopoietic Marrow
- Produces red blood cells, white blood cells and platelets
- Gets its red color from the hemoglobin in the erythroid cells
- Hematopoietic cells mature and migrate into sinusoids to enter the circulation when they are formed.
- Highly vascular
Yellow, or stromal Marrow
- Produces fat, cartilage, and bone
- Gets its yellow color from the carotenoids in the fat droplets in the high number of fat cells
- Paucity of vasculature
Red blood cells, platelets and most white blood cells arise in red marrow; some white blood cells develop in yellow marrow.
The color of yellow marrow is due to the much higher number of fat cells.
Both types of bone marrow contain numerous blood vessels and capillaries.
At birth, all bone marrow is red.
With age, more and more of it is converted to the yellow type.
Adults have on average about 2.6kg (5.7lbs) of bone marrow, with about half of it being red.
Red marrow is found mainly in the flat bones such as hip bone, breast bone, skull, ribs, vertebrae and shoulder blades, and in the cancellous (“spongy”) material at the proximal ends of the long bones femur and humerus.
Pink Marrow is found in the hollow interior of the middle portion of long bones.
There are several serious diseases involving bone marrow.
In cases of severe blood loss, the body can convert yellow marrow back to red marrow in order to increase blood cell production.
The normal bone marrow architecture can be displaced by malignancies or infections such as tuberculosis, leading to a decrease in the production of blood cells and blood platelets.
In addition, cancers of the hematologic progenitor cells in the bone marrow can arise. these are the leukemias.
Bone marrow timeline
Bone marrow first develops in the clavicle near the end of fetal development and becomes active about 3 weeks later. Bone marrow supersedes the liver as the major hematopoietic organ at 32-36 weeks’ gestation.
Bone marrow remains red until around the age of seven as the need for new continuous blood formation is high. As the body ages, the red marrow is gradually replaced by yellow fat tissue. Adults have an average of about 2.6 kg (5.7 lbs) of bone marrow, about half of which is red.3
In adults, the highest concentration of red marrow is in the bones of the vertebrae, hips (ilium), breastbone (sternum), ribs, skull and at the metaphyseal and epiphyseal ends of the long bones of the arm (humerus) and leg (femur and tibia). All other cancellous, or spongy, bones and central cavities of the long bones are filled with yellow marrow.
What does bone marrow do for your body?
The majority of red blood cells, platelets, and most of the white blood cells are formed in the red marrow. Yellow bone marrow produce fat, cartilage and bone.
White blood cells survive anywhere from a few hours to a few days, platelets for about 10 days, and red blood cells for about 120 days. These cells must be constantly replaced by the bone marrow as each blood cell has a set life expectancy.
Certain conditions may trigger additional production of blood cells, such as when the oxygen content of body tissues is low, if there is loss of blood or anemia, or if the number of red blood cells decreases. In such cases, the kidneys produce and release erythropoietin, a hormone that stimulates the bone marrow to produce more red blood cells.
Similarly, the bone marrow produces and releases more white blood cells in response to infections, and more platelets in response to bleeding. If a person experiences serious blood loss, yellow bone marrow can be activated and transformed into red bone marrow.