Bone Marrow in Animals: Red vs Yellow Marrow

Bone marrow is a soft, spongy tissue found inside the medullary cavity of bones. It is responsible for producing blood cells, including red blood cells, white blood cells, and platelets, and also serves as a storage site for fat.

Types

There are two types of bone marrow — red and yellow bone marrow.

Red Bone Marrow

Red marrow occupies the interstices of spongy bone throughout the skeleton and the medullary cavity of long bones at birth. After birth, red marrow is gradually replaced by yellow marrow.

It is an important blood-forming tissue containing precursors of erythrocytes, granular leukocytes, giant cells that give rise to platelets, and a few fat cells.

In adults, red marrow is present only in the vertebrae, sternum, ribs, skull bones, and epiphyses of long bones.

Yellow Bone Marrow

Yellow marrow consists of ordinary adipose tissue, especially in the medullary cavities of long and short bones.

Yellow bone marrow fills the spaces of the spongy bone in short bones (carpals and tarsals) as well as the medullary cavity of long bones.

Functions

Bone marrow plays a vital role in the body as the primary site of hematopoiesis — the process of producing blood cells.

Red bone marrow is responsible for generating red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes), which are essential for oxygen transport, immune defense, and blood clotting.

Yellow bone marrow, on the other hand, mainly serves as a storage site for fat cells but can also contribute to blood cell production in certain conditions.

Distribution

The distribution and amount of bone marrow can vary among animal species. For example, in mammals like dogs and cats, red marrow is mainly found in flat bones and the ends of long bones.

In larger animals such as horses and cattle, yellow marrow predominates in the long bones due to more fat storage. Birds have many pneumatic bones that contain little or no marrow to reduce body weight for flight.

At birth, most bone marrow is red to support high blood cell production. As animals grow, red marrow is gradually replaced by yellow marrow, especially in the long bone shafts. In adults, red marrow remains mainly in flat bones and the ends of long bones, while yellow marrow fills the shafts. During severe blood loss or anemia, yellow marrow can convert back to red marrow to increase blood cell production.