There are three types of cells that contribute to bone homeostasis. Osteoblasts are bone-forming cell , osteoclasts resorb or break down bone, and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.
Compact bone consists of closely packed osteons or haversian systems. The osteon consists of a central canal called the osteonic haversian canal, which is surrounded by concentric rings lamellae of matrix.
The femur is also known for its marrow-filled medullary cavity, which is present in all of the long bones of the limbs. The sternum and hip bone are the sites from which marrow is usually extracted, however the femur is also used. 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! Are you a professor or know someone who is?
We have awesome visuals and resources for your anatomy and physiology course! The answer lies in the properties of a third category of bone cells—the osteogenic cell. These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts. The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release.
The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells.
Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. Figure reviews the bone cells, their functions, and locations. The differences between compact and spongy bone are best explored via their histology.
Compact bone is dense so that it can withstand compressive forces, while spongy cancellous bone has open spaces and supports shifts in weight distribution. Compact bone is the denser, stronger of the two types of bone tissue Figure. It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection. The microscopic structural unit of compact bone is called an osteon , or Haversian system.
Running down the center of each osteon is the central canal , or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal.
This system allows nutrients to be transported to the osteocytes and wastes to be removed from them. Like compact bone, spongy bone , also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone.
The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs. It is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more bone is resorbed than is laid down.
The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture. Bones of the pelvis, skull, spine, and legs are the most commonly affected.
What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. X-rays may show bone deformities or areas of bone resorption.
Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis.
Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications. The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities.
As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the foramina. In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and it appears the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.
Watch this video to see the microscopic features of a bone. A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow.
The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in length.
The medullary cavity has a delicate membranous lining called the endosteum.
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