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Structure and Function of Bone Marrow in Dogs

By: Dr. Bari Spielman

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What Is Bone Marrow?

Bone marrow is the soft, spongy material found in the central cavity of the long bones of the body. It contains cells that give rise to many of the circulating blood cells. Bone marrow also contains blood, fat, connective tissue, blood vessels and small segments of bone. Bone marrow produces the three major types of blood cells: red blood cells, white blood cells and platelets.

Where Is Bone Marrow Located?

Bone marrow is situated in the central cavity of the long bones. The long bones include such bones as the femur, humerus, radius and ulna. The outer aspect of these bones is very firm and strong and is called the cortex. The inner cavity is soft and spongy and is where bone marrow is found. Bone marrow is also found in some of the smaller bones, such as the ribs and pelvis. Despite its scattered distribution throughout bones in many parts of the body, bone marrow functions as a cohesive unit.

What Is the General Structure of Bone Marrow?

Bone marrow consists of connective tissue that forms a delicate meshwork within the marrow cavity of bones, and it is permeated by numerous thin-walled blood vessels. Within the spaces of this tissue, the immature and adult stages of blood cells exist. The active bone marrow, or red marrow, is responsible for the production of red blood cells, white blood cells and platelets. It is generally found in the ends of long bones (such as the humerus and femur) and in the short, flat and irregularly shaped bones (such as the pelvis and ribs). Yellow marrow is made up mostly of fatty tissue and is located in the shafts of long bones.
What Are the Functions of Bone Marrow?

The chief function of bone marrow is the production of blood cells, all of which have different functions. Red blood cells carry oxygen and carbon dioxide; white blood cells fight infection; and platelets assist in clot formation. Each cell in the blood has a limited life span. For example, red blood cells live for only about 120 days. Some white blood cells only live a few hours. When the cell is too old to function, it is removed from circulation by the immune system. The bone marrow is then responsible for replacing that lost cell. Every day, thousands of red blood cells, white blood cells and platelets are removed from circulation and replaced by the bone marrow. The process of blood cell formation and development is called hematopoiesis.

Bone marrow also responds to various needs of the body by producing new blood cells. If more white blood cells are needed to fight infection, or to assist in some healing process, then the normal bone marrow increases the production of the specific type of cell most needed.

Bone marrow produces all blood cells from one common cell called a stem cell. Different stem cells are encouraged to form red blood cells, white blood cells, or platelets. Some of these immature blood cells remain within the bone marrow until they are mature. Other cells, particularly some white blood cells, travel to other parts of the body where they then mature.

Under normal circumstances, red blood cells are fully formed and become mature in the marrow before they enter the bloodstream. Mature red blood cells are unique in that they do not contain a nucleus. (The nucleus is the small, oval area in the cell that contains DNA genetic material.)

What Are the Common Diseases of Bone Marrow?

Diseases that affect or involve the bone marrow generally cause a change in the number or type of circulating blood cells. Such diseases can involve only the red blood cell line, one or more types of white blood cells or platelets. In some diseases, all three blood cell types are affected.

  • Certain conditions cause death of stem cells, with a subsequent decrease in one or more of the blood cells. Such conditions include toxins, drugs (griseofulvin, chloramphenicol, chemotherapeutic agents), neoplasia or cancer, radiation therapy, immune disorders, severe bacterial infections and certain viral infections (panleukopenia, feline immunodeficiency virus and feline leukemia virus).

  • Other conditions cause an increase in activity of the bone marrow, with production of new stem cells and increased production of one or more blood cell lines. Processes or diseases that increase bone marrow activity include any loss of one or more blood cell components (anemia, thrombocytopenia); tissue injury (trauma, burns) or inflammation (surgery, other forms of healing); bacterial, fungal, protozoal and parasitic infections; cancer elsewhere in the body; and certain drugs (corticosteroids, stem cell stimulants).

    What Types of Diagnostic Tests Are Used to Evaluate Bone Marrow?

  • Tests of circulating blood cells – A complete blood count (CBC) indirectly provides some information on the bone marrow by evaluating the number, size and type of red and white blood cells in the circulation. A platelet count is also an indirect assessment of the bone marrow's platelet-producing capabilities. A reticulocyte count is a blood test that helps assess the ability of the bone marrow to make young or new red blood cells.

  • Bone marrow assays – The most sensitive test used to assess the bone marrow is a bone marrow aspirate or biopsy. It is important to assess the bone marrow when one or more cell lines are abnormal in the circulating blood, especially when there is no evidence of new, young cells entering the blood stream.

    A bone marrow aspirate is the retrieval of a portion of bone marrow through a needle placed into the marrow cavity. A biopsy of the marrow involves the use of a biopsy instrument to obtain a core of marrow and some of its surrounding tissues. Bone marrow is retrieved under local anesthesia, with or without additional tranquilization. The ileum (part of the pelvis), femur (thigh bone), humerus (upper arm bone) or ribs are the bones most commonly aspirated or biopsied. Material retrieved is submitted to a veterinary pathologist who counts and evaluates all the various components of the bone marrow and offers an opinion as to the status and health of the marrow.

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