Bones are considered organs because they contain various types of tissue, such as blood, connective tissue, nerves, and bone tissue. Osteocytes, the living cells of bone tissue, form the mineral matrix of bones.
There are two types of bone tissue: compact and spongy. Compact bone or cortical bone , forming the hard external layer of all bones, surrounds the medullary cavity innermost part or bone marrow. It provides protection and strength to bones.
Compact bone tissue consists of units called osteons or Haversian systems. Osteons are cylindrical structures that contain a mineral matrix and living osteocytes connected by canaliculi which transport blood. They are aligned parallel to the long axis of the bone. Osteons in compact bone tissue are aligned in the same direction along lines of stress, helping the bone resist bending or fracturing. Therefore, compact bone tissue is prominent in areas of bone at which stresses are applied in only a few directions.
Compact bone tissue forms the outer layer of all bones while spongy or cancellous bone forms the inner layer of all bones. Spongy bone tissue does not contain osteons.
Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. Spongy cancellous bone is lighter and less dense than compact bone. Spongy bone consists of plates trabeculae and bars of bone adjacent to small, irregular cavities that contain red bone marrow.
The canaliculi connect to the adjacent cavities, instead of a central haversian canal, to receive their blood supply. It may appear that the trabeculae are arranged in a haphazard manner, but they are organized to provide maximum strength similar to braces that are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.
Bone marrow biopsies are done usually to determine or diagnose certain conditions, such as leukemia, anemia, abnormal number of white blood cells, and whether cancer has spread to the bones.
You've been bombarded by headlines about stem cells over the past decade, but what do they actually do? Well, stem cells can become red blood cells which provide oxygen to tissue , white blood cells which fight infections , or platelets which help in blood clotting. They have the potential to become specialized cells, which can help in treating certain diseases.
I drink seven or eight thousand glasses of milk a day. I love it, and my love for it helps protect me against osteoporosis. I'm sure you've seen dozens of ads telling you to drink milk or take vitamins to supplement bone health Sally Field does one , but they never quite explain what osteoporosis is. Osteoporosis is a disorder, more common with increased age, in which bone tissue becomes thinner, resulting in brittle bones that are more susceptible to breaking. Sex hormones are particularly important in stimulating bone cell growth; after middle age, these hormones decrease in production, which means bone cell growth decreases.
Bones become noticeably thinner, especially for women after menopause when estrogen levels are very low. Compact bone becomes more brittle, particularly in the long bones, which is why breaks and fractures in the tibia and femur are things to keep note of when diagnosed with osteoporosis.
The femur and tibia already bear the weight of most of the body; small accidents, like simple falls, can result in injury. In spongy bone, trabeculae—small struts that are separated by cavities filled with marrow—become thinner and the spaces between them become wider, causing an overall weakening of bone structure.
While you know that milk is rich in calcium and vitamin D, the two most important nutrients for bone health, it's not the only one! Leafy greens like spinach, kale, and okra are high in calcium. Fatty fish like tuna and salmon, as well as cheese and egg yolks, are vitamin D-rich foods. So the next time you're watching TV and Sally Field tells you that you need to eat things rich in calcium and vitamin D, you should listen.
You should listen to her anyway, because she's awesome. Be sure to subscribe to the Visible Body Blog for more anatomy awesomeness!
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