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. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum.
Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones. Articulations are places where two bones meet.
Projections stick out from the surface of the bone and provide attachment points for tendons and ligaments. Holes are openings or depressions in the bones. Bone matrix consists of collagen fibers and organic ground substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts.
The diagram above shows a transverse view of an osteon Haversian system - the basic unit of compact bone. Some, mostly older, compact bone is remodelled to form these Haversian systems or osteons. The osteocytes sit in their lacunae in concentric rings around a central Haversian canal which runs longitudinally.
The osteocytes are arranged in concentric rings of bone matrix called lamellae little plates , and their processes run in interconnecting canaliculi. 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 forms the hard external layer of all bones and surrounds the medullary cavity, 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. Each osteon consists of lamellae , which are layers of compact matrix that surround a central canal called the Haversian canal. Osteons in compact bone tissue are aligned in the same direction along lines of stress and help 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. Figure 1. The inner layer of bones consists of spongy bone tissue. The small dark ovals in the osteon represent the living osteocytes. Whereas compact bone tissue forms the outer layer of all bones, spongy bone or cancellous bone forms the inner layer of all bones. Spongy bone tissue does not contain osteons that constitute compact bone tissue.
Instead, it consists of trabeculae , which are lamellae that are arranged as rods or plates. Slide 69 Bone, femur. The trabecular bone present in this slide is found mostly within the epiphysis and some in the bone marrow cavity. Osteoblasts are located immediately above the osteoid newly formed bone matrix. Osteocytes are found within lacunae. Giant multinucleated osteoclasts , which break down bone, are occasionally found in lacunae termed Howship's lacunae.
These are readily found in the ossification zone of the growth plate. The compact bone in this slide surrounds the marrow cavity and spongy bone. Locate the periosteum external and endosteum internal linings of the bone. Note the separation of these linings is artifact of slide preparation.
Slide 74 Bone, ground preparation. Observe the Haversian sytems or osteons of compact bone in this slide.
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