Types of Bone Tissue
The bones of the arms and legs in humans are the largest in the body and are known as long bones. Long bones are made up of two types of tissue. The central part of a long bone is composed of cortical bone. Cortical bone is very dense and strong and, therefore, difficult to fracture. Its primary purpose is providing structural support to the body and its organs and tissues. In the center of long bones is a central canal where blood vessels, nerves, and bone marrow are found. At the ends of the long bones, cortical bone gives way to cancellous bone. Cancellous bone is also found in the pelvic bones, and in the ribs and vertebrae.
In total, cancellous bone makes up about 20% of the typical adult human skeleton. Cancellous bone is much more porous and, therefore, weaker than cortical bone. Although it is still strong, it is more easily fractured than cortical bone. In addition to providing structural stability, cancellous bone contains most of the body's red bone marrow, which produces blood cells. The bone marrow found in cancellous bones also contains many stem cells that are used to repair damaged or broken bone.
Structure of Cancellous Bone
Cancellous bone is also known as spongy bone because it resembles a sponge or honeycomb, with many open spaces connected by flat planes of bone known as trabeculae. Inside the trabeculae are three types of bone cells: osteoblasts, osteocytes and osteoclasts. Osteoblasts are the cells that make new bone. They produce layers of hard tissue made primarily of calcium and phosphate until they are completely surrounded, at which point they become osteocytes. Finally, osteoclasts are larger cells that break down and destroy old or damaged bone so that it can be repaired and replaced by osteoblasts. This repair and rebuilding cycle is constantly ongoing to keep bones strong and healthy.
Cancellous Bone in Children
In babies, most of the skeleton is composed of cancellous bone, and all the bone marrow is red bone marrow. As children grow up, the bones get longer and cancellous bone is slowly converted into stronger cortical bone in the long bones. The red bone marrow is also slowly converted to yellow bone marrow, which is composed primarily of fat cells. During the teenage years, the growth plates close and cancellous bone is only left at the ends of the long bones. It remains into adulthood in the vertebrae, ribs, pelvis, and skull, and the bone marrow in these areas continues to produce blood cells. Yellow bone marrow in cortical bone can be converted back into red bone marrow even in adulthood following severe illness or blood loss.
Bone starts as cartilage in the fetus and, as it develops during infancy and childhood, the cartilage is converted first into cancellous (spongy) bone, and then finally into cortical (compact) bone in the center of the long bones in the arms and legs. Children have a much higher percentage of cancellous bone in their skeleton than adults.
Cancellous Bone and Osteoporosis
Osteoporosis, 'osteo' (bone) and 'poros' (pore or passage), is a progressive degenerative bone disease characterized by a loss of bone density and strength and resulting in an increased risk of fracture. Osteoporosis is caused when excessive amounts of bone are removed by osteoclasts and the bone is not adequately replaced by osteoblasts. Because osteoclasts live on the surface of bone tissue and cancellous bone has a much larger surface area than cortical bone, it is the primary bone tissue affected by osteoporosis.
In healthy people, bone is constantly being remodeled as osteoclasts breakdown old, damaged bone and osteoblasts replace it. In some people, particularly small, elderly females, this process becomes unbalanced and the osteoclasts breakdown bone at a much faster rate than it can be replaced. This causes the pores in cancellous bone to become larger and the trabeculae to become thinner and weaker, eventually leading to fractures in many cases. Most fractures due to osteoporosis occur at the hip, wrist, and ankle - all locations where there is a high percentage of cancellous bone.
Osteoporosis causes bone to be destroyed faster than it can be replaced, increasing the size of the pores and decreasing the amount of bone tissue, leading to an increased risk of fractures. On the left, healthy cancellous bone has relatively small pores connected by thick trabeculae of bone, while on the right, osteoporosis has caused the bone tissue to shrink and left much more open space in the bone tissue, reducing its strength.
Cancellous bone is made up of spongy, porous, bone tissue that is filled with red bone marrow. It is not as strong as cortical bone, which is found in the long bones, but it is very important for producing blood cells. It is found in the ends of long bones and in the bones of the pelvis, ribs, vertebrae, and skull. In cancellous bone, osteoblasts produce new bone, and osteoclasts remove old, damaged bone. This is a process that is normally in balance so that the total volume and density of the bone remains unchanged. However, in people with osteoporosis, osteoclasts destroy the bone tissue faster than it can be replaced, leading to a decrease in bone density and an increased risk of fracture.
Lesson at a Glance
There are two types of bone tissue found in the human body: cortical and cancellous. Cancellous bone is comprised of spongy, porous, bone tissue that is filled with red bone marrow. Mostly concentrated in the vertebrae, ribs, pelvis, and skull, cancellous bone is responsible for the production of red blood cells. Cancellous bone can easily be fractured if not careful, and the probability is higher in those with osteoporosis.
Cancellous bone is described as being spongy and porous, like an actual sponge.
Your understanding of the lesson will be displayed when you can:
- Distinguish between cortical bone and cancellous bone
- Illustrate the structure of cancellous bone
- Discuss the occurrence of cancellous bone in children
- Point out the correlation between cancellous bone and osteoporosis