(Transcribed from Dr. Kalliecharan’s lecture, 1 Feb 2000 by Brian Buschman)
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Bone is a tissue that functions
to provide mechanical support for internal organs, provide locomotion, protect
vital organs and works as a metabolic reservoir of mineral salts. It is made up cells, an organic matrix
(called osteoid) and inorganic mineral salts.
The bone matrix is made of inorganic material, largely Ca and PO4 in the form of hydroxyapatite crystals, collagen fibrils and some Mg, K and Na. It also has an organic component made primarily of collagen type I and a small amount of amorphous ground substance made of glycosaminoglycans, proteoglycans and glycoproteins. Most of the proteoglycans lack the acidic SO4 groups which are common in cartilage and this situation makes bone stain basophilic. The bone matrix alone stains acidophilic due to the SO4 units in it’s collagen.
Developing bone has four types of cells; osteogenic, osteoblasts, osteoclasts and osteocytes.
Osteogenic cells are able to differentiate into osteoblasts, chrondroblasts, fibroblasts or adipose cells as the need arises. They are flat, inconspicuous, have elongated nucleuses and sparse cytoplasm. They form the inner cellular layer of the periosteum.
Osteoblasts are cuboidal or columnar cells that lie in the outer layer of bone that is developing. They secrete the osteoid. They also have matrix vesicles containing alkaline phosphatase which when released will cause PO4 to be released from solution, bind with Ca2+ and deposited into the bone matrix.
Osteocytes are osteoblasts that secrete matrix and completely surround themselves in it. All working osteoblasts will become osteocytes as they do their job. The space they sit in is called a lacuna so on the practical when he shows the osteocyte it’s best to write osteocyte in lacuna. They appear as flat almond shaped cells.
The bone has tubes called canaliculi that are spaces for osteocytes to send cytoplasmic processes that are used to connect the osteocytes to the next osteocytes. They are essential for supplying the osteocytes since nutrients for these cells are unable to diffuse through the bone. This provides a chain of osteocytes that support each other and up to 15 cells can be supported in one chain. These cells are involved in bone maintenance so if they die the work of the osteoclasts goes unopposed so the bone is reabsorbed.
Osteocytes can be found in three states:
1) The quiescent state which is the resting state. They appear small and don’t do anything.
2) The formative state where they are able to make small repairs to the bone.
3) The resorptive state where they function in osteocytic osteolysis.
Osteoclasts are derived from monocytes. They are large and contain between five and fifty nuclei each. They have a ruffled cytoplasmic boarder to increase their resorptive surface area. They secrete enzymes with extracellular activity to reabsorb the bone in two phases:
1) They enzymatically dissolve the mineralized (inorganic) portion of the bone.
2) They then degrade the collagen matrix
As they work they create a lacuna to sit in that is specifically called Howship’s lacuna.
They are controlled by two hormones:
1) PTH stimulates osteoblasts to release osteoclast stimulating factor which increases the number of osteoclasts.
2) Calcitonin from parafollicular cells inhibits the activity of the osteoclasts.
Bone can be found in two types depending on it’s state of development.
1) Woven (primary) bone. It is characterized by a random arrangement of collagen fibers. This type of bone is very weak. One example of this in the adult is the bone that makes up the sockets for your teeth (alveolar bone).
2) Lamellar (secondary) bone is the most common form of bone in adults. It has a parallel arrangement of collagen fibers and replaces women bone as most bones mature. The collagen is arranged into sheets, called lamellae, that are very strong.
As there are two types of bone as it develops the bone develops into two types known as spongy and compact bone.
1) Spongy bone, also called trabeculated or calcaellous bone, has a network of many interconnecting cavities. These cavities house the bone marrow which is either red marrow or white marrow.
a) Red marrow is where blood cells are formed. It s located in the sternum, ribs and ends of long bones.
b) White marrow is located in the diaphyseal cavity of long bones an is primarily made of fat cells.
2) Compact bone is a solid mass with few interconnecting spaces. It forms a solid outer shell, the bone collar, which helps it resist deformity.
Bone is arranged in what is known as haversion systems also called osteons.
They are centered around a tube called the haversion canal which supplies nutrients to the cells which have cytoplasmic extensions touching the canal. The haversion canals are connected by tubes that are known as Volkman’s canals.
The actual bone material is arranged into four structures in the bone:
1) The haversion system which, as stated, is centered around a haversion canal and the bone forms in concentric rings outwards from the canal.
2) Outer circumfrencial lamellae are layers of bone that are not part of an individual haversion system but are rings around the entire bone.
3) Inner circumfrencial lamellae are layers similar to the outer circumfrencial lamellae but are located on the inner part of a bone.
4) Interstitial lamellae are sections haversion systems that still exist in the bone after their haversion system has mostly be reabsorbed.
The bone is surrounded by a periosteum that grows osteogenic cells, leads to intramembranous ossification and has fibers, known as Sharpey’s fibers that turn 90 degrees and go into the bone to anchor the periosteum down to the bone. There is no periosteum at the articulating surface of joints.
Endosteum is a thin layer of cells that lines all inner cavities and openings of bones.
Bone grows only by appositional growth but there are two methods my which this happens:
1) Intramembranous ossification which is where bone is directly laid down from the sides of the bone where the periosteum lies. The mesenchyme give rise to osteogenic cells which become osteoblasts which first lay bone spicules (small fragments to build onto). The bone then grows into a full woven bone and is then remodeled into lamellar bone.
2) Enochondral ossification is where an outline of hyaline cartilage is initially put down and it is replaced by bone. It starts with the intra membranous ossification of the perichondrium that will form a bone collar which will become the periosteum. It will trap the chondrocytes inside where they hypertrophy and die. Then the osteogenic bud enters the bone collar and adds osteogenic cells that become osteoblastic cells. They lay down the bone matrix and form a calcified cartilage. Osteoclasts join the crew to remove remains of the cartilage and the osteoblasts become osteocytes and finish the job.
The epiphyseal plate is the secondary ossification center (the bone collar is the primary) and the location where bones grow in length. They have five different regions that are very significant and you can be guaranteed that Dr. Kalliecharan will ask one of these on the first lab practical. This plate is a site of enochondral ossification.
1) The resting zone is located closest to the end of the bone and is a place where the cartilage cells just sit and rest.
2) The proliferating zone is where the mitotic activity of the chondrocytes takes place. It appears in section like stacks of coins.
3) The hypertrophic zone is where the chondrocytes get really large.
4) The Calcified cartilage zone is the location of the dead chondrocytes where the inorganic components of bone have begun to be deposited.
5) The ossification zone is the end nearest the center of the bone where the final ossification is made and primary (spongy) bone is produced.
If the proliferating zone is not producing as many new cells as the hypertrophic zone is killing the result will be loss of the epiphyseal plate and the growth of long bones will cease. Early loss of the epiphyseal plate is one cause of people being small in stature.
There are two vitamins that we have discussed that play important roles in the formation of bone:
1) Vitamin C is essential in the proper production of collagen. Without sufficient vitamin C the osteoid would not form properly.
2) Vitamin D is necessary for proper bone mineralization.
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