Table of Contents
- Bone Tissue Definition and Function
- What Are the Two Types of Bone Tissue?
- Bone Tissue Anatomy
- Bone Repair
- Lesson Summary
Walking, running, jumping, and even sitting are only possible because of the structure and strength provided by bone tissue. Bone tissue is one of the strongest tissues in the body, and as such, it plays a big role in supporting other tissues, protecting fragile internal organs from damage, and facilitating movement. Bone tissue is not just an inert material, however. It contains living cells that are constantly working to remodel and repair damage to the tissue. It's also filled with bone marrow, which plays a big role in producing blood cells and other kinds of stem cells.
Bone tissue is primarily constructed of a protein known as collagen that is also found in other types of connective tissue like cartilage. In bone, collagen fibers are reinforced with calcium phosphate, which is a hard ceramic material that gives bone its strength and stability. When calcium phosphate is added to collagen fibers, the collagen becomes mineralized. Although all bone tissue is made of the same basic materials, the structure of the tissue can be quite different depending on its location.
Bone tissue can be classified into one of two types, known as spongy bone and compact bone, based on how the mineralized collagen fibers are arranged within the tissue.
In spongy bone, which is also known as cancellous bone, there are many open spaces that are connected by pieces of bone tissue, giving spongy bone the appearance of a sponge. Spongy bone is typically found in the ends of the long bones, as well as in the pelvic bones, ribs, vertebrae, shoulder blades, and skull bones. Although spongy bone is strong, the presence of all those holes makes it light and somewhat flexible. The holes in spongy bone are not empty, but are full of bone marrow, which plays an important role the production of both red and white blood cells. There are also many blood vessels present in spongy bone that provide nutrients to the bone marrow and bone tissue.
In contrast to spongy bone, compact bone, which is also known as cortical bone, is very dense. Compact bone is much stronger, stiffer, and heavier than spongy bone, and it is found in the shafts of long bones like those in the arms and legs. These bones experience a lot of mechanical stress, so they need to be very strong. In these long bones, there is also a central cavity that is filled with bone marrow. In addition to being found in the long bones, there is also a thin layer of compact bone that covers areas of spongy bone. This provides additional strength and protects the more fragile spongy bone from damage.
Bone tissue is arranged in functional units known as osteons. Within each osteon, there are layers of mineralized collagen that are arranged in concentric rings called lamellae, surrounding a central Haversian canal. These layers of bone tissue look similar to the rings inside a tree trunk. Within each Haversian canal, there are blood vessels that supply nutrients to the cells living within the bone. The blood and nerve supply of bone (remember, it's a living tissue; therefore, oxygen is delivered to bone via red blood cells and carbon dioxide is taken away) runs through the Haversian canal. In between the lamellae, there are small holes called lacunae, and inside each lacuna, there is a cell known as an osteocyte that helps maintain the bone. Osteocytes are connected to other osteocytes through very tiny channels known as canaliculi. This allows them to communicate with each other.
In compact bone, osteons are packed very tightly together to create a very dense, solid structure. In contrast, spongy bone does not contain osteons. Instead, layers of bone tissue are arranged into plates known as trabeculae that surround the open spaces in the bone.
Osteoporosis is a medical condition in which the bones become brittle and fragile from loss of tissue, typically as a result of hormonal changes, or deficiency of calcium or vitamin D. If a growing child does not receive enough minerals such as calcium in his diet, he may develop osteoporosis due to his lack of calcium.
Bone marrow is found in both spongy bone and compact bone. In long bones, there is an open space in the center of each bone that contains blood vessels, nerves, and bone marrow. In spongy bone, the holes within the tissue are also filled with bone marrow. There are two types of bone marrow that can be present: red bone marrow and yellow bone marrow.
Red bone marrow is responsible for producing blood cells and some stem cells to repair damage and allow growth. In children, almost all of the bone marrow in their body is red bone marrow because they are growing quickly and, therefore, need a continuous supply of new blood cells and stem cells. By adulthood, a lot of the red bone marrow has been converted to yellow bone marrow, which is mostly made up of fat tissue. In a typical adult, about half of the bone marrow is red and half is yellow, with the red bone marrow concentrated in the spongy bone of the pelvis, ribs, skull, and vertebrae.
Bone tissue is strong, but to maintain its strength and stability, three types of cells have to work together to repair damage and remodel bones to meet the demands our bodies place on them. These cells live within bone tissue and are known as osteoblasts, osteoclasts, and osteocytes.
Osteoblasts are cells that produce new bone tissue. They are derived from bone marrow stem cells, and they sit on the outside of bone tissue and synthesize collagen that becomes mineralized with calcium phosphate to produce new bone tissue. Osteoblasts create new bone to allow bones to grow and to repair microcracks and fractures. They also produce new bone in response to increased loading. For example, if a person starts training to run a marathon, the leg bones will suddenly experience a lot more stress than they are used to. This causes osteoblasts to begin producing new bone tissue to reinforce the leg bones and make them stronger.
Osteoclasts are cells that destroy bone tissue. That may seem like a bad thing, but osteoblasts also play a vital role in repairing and healing bone tissue. When a bone is damaged, osteoclasts must first remove all the damaged bone tissue so that osteoblasts can produce new bone. Osteoclasts also remove bone from areas where there is less stress. Through the actions of osteoclasts and osteoblasts working together, bone tissue is constantly being remodeled to meet the demands that are placed on it.
Osteocytes are the most abundant type of cell in bone tissue, making up about 95% of the total cells. They are formed from osteoblasts that get trapped within bone tissue as it forms. Osteocytes are connected to each other through tiny canaliculi. Osteocytes are constantly monitoring the bone tissue that surrounds them, and they have the ability to both produce new bone and destroy damaged bone if they are activated by stress or damage to the tissue around them. Along with osteoblasts and osteoclasts, they play a big role in maintaining and remodeling bone tissue.
Bone tissue is important because it provides structure to other body tissues, allows movement via attached muscles, protects internal organs, and produces blood cells. It is composed primarily of mineralized collagen and is produced, maintained, and repaired by three types of cells:
Bone tissue is classified into two types based on the internal structure of the tissue.
|Compact||strong, dense tissue
found in the shaft of long bones in the arms and legs
|Spongy||less dense, more flexible tissue
found in the ends of long bones, pelvis, skull, ribs, spine, and shoulder blades
Both types of bone contain bone marrow. Red bone marrow, which makes up almost 100% of the total bone marrow in children and 50% of the total bone marrow in adults, is very metabolically active and produces blood cells and stem cells. Yellow bone marrow, which makes up the other 50% of bone marrow in adults, contains mostly fat tissue.
The structural unit of compact bone is the osteon. In each osteon, there is a central Haversian canal that contains blood vessels, and it is surrounded by concentric rings of mineralized collagen known as lamellae. Between the lamellae, there are tiny holes known as lacunae that contain osteocytes. Osteocytes communicate with each other through very tiny channels known as canaliculi.
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Bone tissue is found throughout the body. The long bones in the arms and legs contain a very dense, strong type of bone tissue known as compact bone, while other bones, such as the skill, spine, and ribs, contain a less dense, more flexible type of bone tissue known as spongy bone.
Bone tissue protects internal organs, provides support to other tissues, allows motion by facilitating the attachment of muscles, and also plays a big role in the formation of both red and white blood cells.
There are two types of bone tissue: spongy (or cancellous) bone, and compact (or cortical) bone. Spongy bone is less dense and more flexible, and it is found in the ends of long bones, the pelvis, spine, skull, ribs, and shoulder blades. Compact bone is found in the shafts of the long bones in the arms and legs and is very dense and much heavier than spongy bone.
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