Introduction

Bone marrow refers to the semi-solid tissue located within cancellous or spongy portions of bones. In mammals, bone marrow is the main site where new blood cells are produced. It consists of marrow adipose tissue, hematopoietic cells, and supportive stromal cells. In adults, the bone marrow can be found in the ribs, sternum, vertebrae, and pelvic bones. On average, it constitutes about 4 percent of the total body mass. It produces an estimated 500 billion blood cells daily. These cells join the systemic circulation through the permeable vasculature sinusoids in the medullary cavity. All hematopoietic cells are created in the bone marrow. However, some cells will need to migrate to other parts of the body to complete the maturation process. The bone marrow composition is dynamic as it shifts with age due to various systemic factors. Bone marrow can be differentiated into red or yellow marrow. Based on the prevalence of hematopoietic vs fat cells. For example, a newborn baby exclusively has red marrow that are hematopoietic cells that gradually convert to become yellow marrow with age. In situations where chronic hypoxia occurs, yellow marrow can convert to red marrow to help increase the production of blood cells.

Red and Yellow Bone Marrow

The red bone marrow refers to the red colored tissue that contains the reticular networks that are crucial in the production and development of red blood cells, white blood cells, and platelets. The red color can be attributed to the hemoglobin. It can be found in the flat and long bones such as hip bones, vertebrae, ribs, shoulder blades, and skull. The red bone marrow has an important role in the production of red blood cells, white blood cells, and platelets. Red bone marrow is also known as medulla osium rubra.

The yellow bone marrow is yellow colored tissue that can be found in the hollow parts of compact bones. The yellowish color can be attributed to the presence of carotenoid in the fat droplets. They function in the production of blood cells in life-threatening situations and the storage of fat. The fat in the yellow bone marrow is also the body’s last source when there is extreme hunger. Yellow bone marrow is also known as medulla osium flava.

Hematopoietic Components and Stroma

The main component in the brain marrow are the progenitor cells that mature into lymphoid and blood cells. The marrow contains hematopoietic stem cells that result in three classes of blood cells:

• Red blood cells (erythrocytes)

• White blood cells (leukocytes)

• Platelets (thrombocytes)

The stroma consists of tissue that is not primarily involved in the main function of hematopoiesis. Stromal cells provide a microenvironment that influences the differentiation and function of hematopoietic cells. Cells that are found in the stroma include:

• Macrophages

• Fibroblasts

• Osteoblasts

• Osteoclasts

• Adipocytes

• Endothelial cells

Function

1. Mesenchymal Stem Cells – mesenchymal stem cells or marrow stromal cells are multipotent stem cells that can differentiate into various cells such as chondrocytes, osteoblasts, marrow adipocytes, myocytes, and beta-pancreatic islet cells.

2. Lymphatic Role – the red bone marrow is a vital part of the lymphatic system as it is the main organ that generates lymphocytes from immature progenitor cells. The thymus and bone marrow contain primary lymphoid tissue that is involved in the selection and production of lymphocytes. The bone marrow also has a valve-like function that prevents lymphatic fluid to flow back into the lymphatic system.

3. Bone Marrow Barrier – the blood vessels make up the bone marrow barrier which functions to prevent immature cells from leaving the marrow. This can be attributed to mature blood cells that contain membrane proteins that are required to pass through the blood vessels. Since hematopoietic stem cells can cross the bone marrow barrier, it can be harvested from blood.

4. Compartmentalization – compartmentalization can be seen in the bone marrow as specific cell types aggregate in certain areas. For example, macrophages, erythrocytes, and their precursors usually gather around blood vessels while granulocytes gather at the bone marrow borders.

Transplant

A bone marrow transplant can be used to replace diseased and nonfunctioning bone marrow such as in diseases like sickle cell anemia, aplastic anemia, and leukemia. It can also be used to replace bone marrow function after chemotherapy or radiation. The transplant will lead to the regeneration of a new immune system that helps fight the existing conditions. Some examples of transplant types are a syngeneic transplant, autologous transplant, allogeneic transplant, and haploidentical transplant. To determine a match for bone marrow, the person will go through an HLA-typing test. Once matched, several other tests should be performed. This includes chest x-ray, computed tomography scans, pulmonary function tests, heart function tests, skeletal survey, and bone marrow biopsy.

References:

1. Difference between red and yellow bone marrow. They Differ. Accessed 3/1/2019. https://theydiffer.com/difference-between-red-and-yellow-bone-marrow/

2. Bone marrow. Wikipedia. Accessed 3/1/2019. https://en.wikipedia.org/wiki/Bone_marrow

3. Nichols H. All you need to know about bone marrow. Medical News Today. Accessed 3/1/2019. https://www.medicalnewstoday.com/articles/285666.php