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Bone tissue, or osseous tissue, is a type of connective tissue used in forming bones. Bones protect the vital organs and help support the body. It is composed mainly of collagen, or ossein, fibers, and bone cells called osteocytes. There are two types of bone tissue, referred to as cortical bone and cancellous bone.
Cortical bone is also known as compact or dense bone, and most of a body's bones are made from this kind of bone. It is very strong and dense, and it makes up the hard exterior of a bone. Its surface is smooth and white, and covered by a thin membrane referred to as the periosteum, which holds capillaries and also gives tendons and muscles something to attach to.
There are two types of cortical bone tissue as well, known as lamellar and woven bone. Lamellar is very strong and made from organized layers of ossein fibers. Most of the bones in an adult's body are made of lamellar bone. Woven bone is made very quickly by the cells called osteoblasts, and it is much weaker than lamellar. The ossein fibers that make up this type of tissue are typically arranged in an unorganized pattern, and it is usually made to help repair a fracture until it can be replaced by stronger material.
Purposes of cortical bone include protection, support, and storage of minerals. Because this tissue is so strong, it is what protects vital organs and helps support the weight of the body. Minerals that the body needs, like calcium, are also stored in cortical bone until the body has a need for them.
Cancellous bone is also referred to as trabecular bone, and commonly referred to as spongy bone. This is the weaker of the two types of bone tissue. It resembles a sponge, and can usually be found inside bones, surrounded by cortical tissue, and at joints at the end of long bones.
Although it is much less dense and comparatively weaker than cortical bone tissue, cancellous tissue serves an important purpose. There are two kinds of bone marrow. Red bone marrow is almost completely made up of specialized cells that make the majority of the body's red and white blood cells, and platelets. When a child is born, all of the bone marrow in his body is typically red bone marrow.
As a person grows older, more and more red bone marrow is replaced by yellow bone marrow, which is made mostly of fat. During times of extreme starvation, the body will use these fat stores, and, in some cases, the yellow marrow can change back into red marrow if needed. Roughly half of all marrow in an adult's body is yellow.
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