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In: Mescher AL. Mescher A. Anthony L. Junqueira's Basic Histology, 14e. McGraw Hill; Accessed November 11, APA Citation Cartilage. Mescher AL. McGraw Hill. MLA Citation "Cartilage. Download citation file: RIS Zotero. Reference Manager. Autosuggest Results.
Subscribe: Institutional or Individual. In areas where these cells occur, they are located on the surfaces of forming bone and are not yet embedded in the extracellular matrix. These cells have cytoplasmic processes that bring them into contact with neighboring osteoblasts, as well as nearby osteocytes. Ultrastructure shows organelle systems typical of secretory cells. These cells are osteoblasts that have become embedded in calcified bone matrix. They reside in lacunae within the matrix and are in contact with neighboring osteocytes via cytoplasmic processes that extend through small tunnels called canaliculi.
Contacting cytoplasmic processes form gap junctions. This communication between osteocytes is important in the tranfer of nutrients to these cells and wastes out of them since they may be far removed from blood capillaries. The cells are flattened and their internal organelles exhibit the characteristics of cells that have reduced synthetic activity. Endochondral - cartilage template formed that is replaced by bone e. Intramembranous - direct formation of bone structure with no cartilagenous template e.
In the embryo, osteoblasts are derived from mesenchymal cells. These cells either aggregate where bones are to form intramembranous bone formation and lay down the matrix that will later become calcified, or they migrate into pre-existing cartilage "models " of the presumptive bone and replace the cartilage with a calcareous matrix endochondral bone formation.
Long bone growth is also endochondral in nature. Osteoblasts are different than the chondroblasts that begin the histogenesis of cartilage and should not be confused with them. As the primordial bone matrix is layed down, the osteoblasts become entrapped in lacunae within the matrix and are then known as mature osteocytes.
As bone is being formed, there is also localized removal of the bone matrix by another set of connective tissue cells known as osteoclasts. These cells are thought to differentiate from monocytes and are responsible, in part, for the internal architecture of bones in that they excavate localized portions of the forming bone and make passageways for such things as blood vessels nerves.
A third population of cells involved in bone formation are the cells of the marrow. These are the stem cells for blood cells and all their progeny see Blood below.
Mesenchymal cells aggregate and begin to secrete matrix that is characterized by bundles of collagenous fibers. The secreted osteoid matrix has a high affinity for calcium salts, that are brought into the area of bone formation by the circulatory system. These deposit within and on the matrix to form calcified bone.
As this calcification takes place, the mesenchymal cells undergo morphological changes. They loose the appearance of mesenchymal cells and round up becoming true osteoblasts.
The osteoblasts become oriented in epithelial-like layers along the forming bone. The osteoblasts and the collagen and other components of the intercellular matrix form the organic osteoid framework of the bone.
As a strand of matrix is invested with inorganic salts it forms a spicule of bone. The spicules will merge to form larger calcified structures called trabeculae.
These will thicken with the deposition of more osteoid matrix and inorganic salts as the osteoblasts continue their secretion in an appositional manner. This secretion by the osteoblasts is cyclic and results in layers of bone material called lamellae. The deposition of lamellae traps some of the osteoblasts within the osteoid matrix. Once trapped they are considered mature osteocytes. Osteocytes are characterized by cytoplasmic processes that contact similar processes of adjacent osteocytes.
Gap juctions form at points of contact allowing transfer of small molecules between cells. This transfer is important in co-ordinating bone growth and in the nourishment of osteocytes which may be separated from blood vessels by a considerable amount of calcified bone.
Channals through which cytoplasmic processes of osteocytes extend are called canaliculi. Growing adjacent trabeculae will contact and fuse forming the structure of the mature bone.
As intramembranous bones grow, selective reabsorption of bone material is also occurring due to the activities of osteoclast cells.
This results in the formation of much of the internal architecture of the bones, providing spaces for blood vessels and marrow.
Most of the bones in the mammalian body are initially formed by endochondral means. This deposition of cartilage occurs as previously discussed and is accomplished by the action of chondroblasts functioning in both interstitial and appositional growth capacities. The typical examples that are used to describe endochondral bone formation are the long bones of the limbs.
Perhaps it will be easier to understand their histogenesis if we first consider the general structure of bones. We will consider this structure as it exists in long bones, however, it should be kept in mind that girdle bones, such as the pelvic girdle, and the intramembranous flat bones of the skull are made up of the same basic components, though they may be arranged somewhat differently.
Bones of the body, including the long bones may be considered a rigid form of connective tissue. The cells of this tissue are embedded within a matrix that consists of organic and inorganic components. The organic matrix, or ground substance, consists of collagen fibers for the most part.
While we tend to think of the inorganic components as being the contributing factors in a bone's structural integrity, you should realize that the collagen fibers also contribute significantly to the bones strength and resilience. Two types of bone tissue can be distingished. These are cancellous, or spongy, bone that lies centrally within the shaft of long bones, and compact or dense bone that lies more peripherally. You should realize that the actual mineralized matrix of these two types of bone is the same.
It contains embedded osteocytes that are in communication via gap junctions at their contacting cytoplasmic processes. The difference between spongy and compact bone lies simply in the size of open spaces within the mineralized bone. The spongy bone consists of slender, irregular trabeculae with large spaces between them where blood vessels, nerves, and marrow cells are located.
Compact bone appears solid, no large cavities within it. Since the actual mineralized matrix of both types of bone is the same, there is no distinct boundary between spongy and compact bone. The shaft of a long bone consists of a medullary or central volume of spongy bone surrounded by a thick cortical layer of compact bone. The compact layer can be subdivided into an outer series of sub-layers called periosteal lamellae that were secreted by the periosteal cells during its development and growth, and an inner component consisting of multiple concentric sub-layers surrounding the halversian canals.
These radial cavities and halversian canals form a network within the compact bone that is continuous with the cavities of the spongy bone. Blood vessels and nerves extend through the channels of this network. The first step in endochondral bone formation is the histogenesis of a cartilage miniature of the bone. This takes place as discussed above via the action of chondroblasts that have migrated to the area.
The chondroblasts secrete a cartilagenous matrix that is laid down both interstitially and appossitionally. The end result is a cartilage template of the bone in miniature that contains chondrocytes embedded within the cartilage matrix.
Actual osteogenesis bone ossification begins with the establishment of a periosteum on the shaft or diaphysis of the cartilage template and the laying down of an intramembranous collar of bone on the circumference of the cartilage diaphysis.
This is followed by hypertrophy they get bigger and eventual death of the chondrocytes within the cartilage matrix. As the chondrocytes degenerate they reabsorb some of the surrounding cartilage matrix causing enlargement of the lacunae in which they reside. This process is known as hypertrophication. As this occurs, the chondrocytes loose their ability to maintain the remaining cartilage matrix and it becomes partially calcified.
The end result is an area of porous calcified cartilage within the central regions of the diaphysis. As this is occurring, osteoclasts that have arrived in the area via the circulatory system, begin excavating passageways or tunnels through the intramembranous collar surrounding the diaphysis.
These passageways provide a means through which blood vessels, nerves and undifferentiated mesenchymes cells can enter into the lacunae spaces in the remnants of the cartilage matrix that have been left by the degenerating chondrocytes.
The mesenchyme cells will differentiate into osteoblasts and hematopoietic stem cells that are distributed within the bone. The osteoblasts, blood vessels, and nerves form the osteogenic bud that comes to lie more or less centrally within the diaphysis of the forming bone. As the invading cells spread out within the diaphysis of the cartilage template and ossification begins, this central volume of active bone deposition is called a primary ossification center.
The osteoblasts begin to secrete osteoid matrix on the remnants of calcified cartilage. The osteoid matrix becomes mineralized forming cancellous bone in the shaft of the diaphysis. Some of the osteoblasts become trapped within the mineralized bone and become mature bone cells, osteocytes. As the cancellous bone is layed down, chondroclasts which are the cartilagenous equivalent of osteoclasts reabsorb the calcified cartilage as it is replaced by osteoid matrix i.
At this point, it is important to note that this means the actual bone tissue, matrix and mineralization, is the result of the action of a new group of cells, the osteoblasts. The primary ossification center rapidly extends longitudinally within the diaphysis as the shaft of the cartilage template is completely replaced by cancellous bone tissue. As the ossification center extends longitudinally, so does the calcified outer collar of bone layed down by the periostial osteocytes.
As ossification proceeds in the diaphysis, secondary ossification centers form in the cartilage of the bulbuous ends, or epiphyses, at either end of the long bone shaft.
Osteogenic tissues in these regions also act to form mineralized bone. This process is similar to the primary ossification we've just discussed with one difference. Since there is no periosteum on the surface of the epiphyses, there is no periostial external collar of bone. What we have just discussed is endochondral bone formation. This involved the deposition of cancellous, or spongy bone, within a cartilage matrix.
This is not the final step in bone formation. In fact, there really is no such thing as a final step in this process. 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.
Osteoblasts are cells that make new bone. They become osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones. If the articular cartilage at the end of one of your long bones were to degenerate, what symptoms do you think you would experience?
In what ways is the structural makeup of compact and spongy bone well suited to their respective functions? The surface features of bones vary considerably, depending on the function and location in the body. There are three general classes of bone markings: 1 articulations, 2 projections, and 3 holes.
These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation. A projection is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape is an indication of the forces exerted through the attachment to the bone.
A hole is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points.
Skip to content Learning Objectives By the end of this section, you will be able to: Describe the microscopic and gross anatomical structures of bones Identify the gross anatomical features of a bone Describe the histology of bone tissue, including the function of bone cells and matrix Compare and contrast compact and spongy bone Identify the structures that compose compact and spongy bone Describe how bones are nourished and innervated function?
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. External Website Watch this video to see the microscopic features of a bone. Chapter Review A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone.
Review Questions. Critical Thinking Questions 1. Solutions Answers for Critical Thinking Questions If the articular cartilage at the end of one of your long bones were to deteriorate, which is actually what happens in osteoarthritis, you would experience joint pain at the end of that bone and limitation of motion at that joint because there would be no cartilage to reduce friction between adjacent bones and there would be no cartilage to act as a shock absorber.
The densely packed concentric rings of matrix in compact bone are ideal for resisting compressive forces, which is the function of compact bone. The open spaces of the trabeculated network of spongy bone allow spongy bone to support shifts in weight distribution, which is the function of spongy bone.
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