Created by CK-12 Foundation/Adapted by Christine Miller

Do you recognize the food item in the top left of Figure 11.4.1? It’s roasted bone marrow, still inside the bones, and it is considered a delicacy in some cuisines. Marrow is a type of tissue found inside many animal bones, including our own. It’s a soft tissue that, in adults, may be mostly fat. You’ll learn more about bone marrow and other tissues that make up bones when you read this section.

Bones are organs that consist primarily of bone tissue, also called osseous tissue. Bone tissue is a type of connective tissue consisting mainly of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and hard mineral crystals makes bone tissue hard, without making it brittle.

There are several different types of tissues in bones, including two types of osseous tissues. Osseous tissues, in turn, consist of several different types of bone cells.

Types of Osseous Tissue

The two different types of osseous tissue are compact bone tissue (also called hard or cortical bone) and spongy bone tissue (also called cancellous or trabecular bone). Both are shown in the diagrams of a typical bone in Figures 11.4.2 and 11.4.3.

Flat bones are typically enveloped by compact bone, with a center of spongy bone.

Compact bone tissue forms the extremely hard outside layer of bones. Compact bone tissue gives bone its smooth, dense, solid appearance. It accounts for about 80% of the total bone mass of the adult skeleton. Spongy bone tissue fills part or all of the interior of many bones. As its name suggests, spongy bone is porous like a sponge, containing an irregular network of spaces, as shown in Figures 11.4.4 and 11.4.5. This makes spongy bone much less dense than compact bone. Spongy bone has a greater surface area than compact bone, but makes up only 20% of bone mass.

Both compact and spongy bone tissues have the same types of cells, but they differ in how the cells are arranged. The cells in compact bone are arranged in multiple microscopic columns, whereas the cells in spongy bone are arranged in a looser, more open network. These cellular differences explain why compact and spongy bone tissues have such different structures.

Other Tissues in Bones

Besides compact and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum.

• Bone marrow is a soft connective tissue found inside a cavity, called the marrow cavity. There are two types of marrow in adults — yellow bone marrow (which consists mostly of fat) and red bone marrow. All marrow is red in newborns, but by adulthood, much of the red marrow has changed to yellow marrow. In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones. Only red bone marrow contains hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of hematopoiesis.
• Periosteum is a tough, fibrous membrane that covers the outer surface of bones. It provides a protective covering for compact bone tissue. It is also the source of new bone cells.

Bone Cells

As shown in Figure 11.4.6, bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.

• Osteoblasts are bone cells with a single nucleus that make and mineralize bone matrix. They make a protein mixture that is composed primarily of collagen and creates the organic part of the matrix. They also release calcium and phosphate ions that form mineral crystals within the matrix. In addition, they produce hormones that play a role in the mineralization of the matrix.
• Osteocytes are mainly inactive bone cells that form from osteoblasts that have become entrapped within their own bone matrix. Osteocytes help regulate the formation and breakdown of bone tissue. They have multiple cell projections that are thought to be involved in communication with other bone cells.
• Osteoclasts are bone cells with multiple nuclei that resorb bone tissue and break down bone. They dissolve the minerals in bone and release them into the blood.
• Osteogenic cells are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.

Bone is very active tissue. It is constantly remodeled by the work of osteoblasts and osteoclasts. Osteoblasts continuously make new bone, and osteoclasts keep breaking down bone. This allows for minor repair of bones, as well as homeostasis of mineral ions in the blood.

There are six types of bones in the human body, categorized based on their shape or location: long, short, flat, sesamoid, sutural, and irregular bones. You can see an example of each type of bone in Figure 11.4.7.

• Long bones are characterized by a shaft that is much longer than it is wide, and by a rounded head at each end of the shaft. Long bones are made mostly of compact bone, with lesser amounts of spongy bone and marrow. Most bones of the limbs, including those of the fingers and toes, are long bones.
• Short bones are roughly cube-shaped and have only a thin layer of compact bone surrounding a spongy bone interior. The bones of the wrists and ankles are short bones.
• Flat bones are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum (breast bone).
• Sesamoid bones are embedded in tendons, the connective tissues that bind muscles to bones. Sesamoid bones hold tendons farther away from joints, so the angle of the tendons is increased, thus increasing the leverage of muscles. The patella (knee cap) is an example of a sesamoid bone.
• Sutural bones are very small bones located between the major bones of the skull, within the joints (sutures) between the larger bones. They are not always present.
• Irregular bones are those that do not fit into any of the above categories. They generally consist of thin layers of compact bone surrounding a spongy bone interior. Their shapes are irregular and complicated. Examples of irregular bones include the vertebrae and the bones of the pelvis.

Diseased or damaged bone marrow can be replaced by donated bone marrow cells, which help treat and often cure many life-threatening conditions, including leukemia, lymphoma, sickle cell anemia, and thalassemia. If a bone marrow transplant is successful, the new bone marrow will start making healthy blood cells and improve the patient’s condition.

Learn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:

1. How does one become a potential bone marrow donor?
2. Who can and who cannot donate bone marrow?
3. How is a bone marrow donation made?
4. What risks are there in donating bone marrow?