Introduction
Bone marrow refers to the semi-solid tissue found in cancellous or spongy parts of the bones. In humans, the bone marrow is the main site where hematopoiesis or production of blood cells occurs. The bone marrow consists of marrow adipose tissue, hematopoietic cells, and supportive stromal cells. In adults, the bone marrow can mainly be found in the pelvic bones, sternum, vertebrae, and ribs. On average, bone marrow comprises about 4 percent of the body mass. This means in an average adult who weighs 65 kilograms (143lbs), the bone marrow is about 2.6 kilograms (5.7lbs). The bone marrow is estimated to produce about 500 billion blood cells daily. These blood cells then join the circulation through permeable vasculature sinusoids that can be found in the medullary cavity. The hematopoietic stem cells in the bone marrow produce all three classes of blood cells: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).
Structure
In humans, the bone marrow can be divided into the red and yellow bone marrow. This depends on the prevalence of hematopoietic cells and fat cells. A newborn baby usually exclusively has red bone marrow. With age, there is a progressive conversion towards yellow marrow. In adults, red bone marrow can be found in the pelvic bones, cranium, sternum, ribs, scapulae, vertebrae, and proximal epiphyseal ends of long bones. In dire situations where there is chronic hypoxia, yellow marrow can be converted to red marrow to help increase the production of blood cells. The tissue in the bone marrow that is not directly involved in hematopoiesis is also known as the stroma. The stromal cells function to provide an environment that helps the functioning and differentiation of hematopoietic cells. Some of the cell types found in the bone marrow stroma include:
• Macrophages
• Adipocytes
• Fibroblasts
• Osteoblasts
• Endothelial cells
• Osteoclasts
The mesenchymal stem cells in the stroma are multipotent stem cells that can differentiate into chondrocytes, osteoblasts, marrow adipocytes, myocytes, and beta-pancreatic islet cells.
Red vs Yellow Bone Marrow
Red bone marrow refers to the red colored tissue where there are reticular networks that are critical in the production and development of blood cells. Red bone marrow is situated in the shoulder blades, long bones, and skull. With age, it can be predominantly found in flat and long bones such as the hip bones, skull, ribs, and vertebrae. Yellow bone marrow can be found in the hollow regions of compact bones of the axial skeleton. It is called the yellow bone marrow as it refers to the yellow-colored tissue that functions to produce blood cells and store fats during dire circumstances.
Generally, as the number of red bone marrow increases, the yellow marrow decreases. This also means that when the number of red bone marrow decreases, the yellow marrow increases. During fetal development until the point of childbirth, there is only red bone marrow found in the bone cavities. Once the individual reaches approximately 5 years old, the red bone marrow present in the long bones of the body is gradually replaced with yellow bone marrow. While red bone marrow is rich with hematopoietic cells that function to produce red blood cells, white blood cells, and platelets, the yellow bone marrow instead stores the adipocytes that function as fat storage. In emergency situations where there is blood loss, the yellow bone marrow can also be converted into the red bone marrow to help with the production of blood cells. The fats stored in the yellow marrow can also be used as the body’s last reserve for energy when there is extreme hunger. It can also convert into cartilage and bone as needed.
The red bone marrow can attribute its red color to the presence of hemoglobin while the yellow bone marrow has a distinct yellowish color due to the presence of carotenoid in the tissue’s fat droplets. The red bone marrow contains active cells that constantly multiplies to continuously produce blood cells while the yellow bone marrow contains inactive cells.
Clinical Significance
The architecture in the bone marrow can be distorted by various conditions such as multiple myeloma, aplastic anemia, tuberculosis, leukemia, etcetera. Besides diseases, therapy such as radiation and chemotherapy can also affect cell division in the marrow resulting in a suppressed immune system. To aid the diagnosis of diseases that may involve the bone marrow, an aspiration of the bone marrow can be performed by using a hollow needle to obtain a sample under local or general anesthesia.
Stem cells derived from the bone marrow also has a wide application in regenerative medicine. Hematopoietic stem cells can be removed from one individual and infused into another if both donor and recipient are compatible. The stem cells can be harvested from the red marrow under general anesthesia. It is considered to be a minimally invasive procedure with minimal scarring. Another harvesting option would be the administration of certain medications that stimulate stem cell release into the circulating blood. An intravenous catheter is then inserted into the arm and stem cells are filtered out of the blood. It is a similar procedure to platelet or blood donation.
