The direction of bone formation in the epiphysis plane is determined by the direction and distribution of the pressure line. Increased thickness or width of the bone is caused by deposition of new bone in the form of circumferential lamellae under the periosteum.
If bone growth continues, the lamella will be embedded behind the new bone surface and be replaced by the haversian canal system. Bone is a tissue in which the extracellular matrix has been hardened to accommodate a supporting function. The fundamental components of bone, like all connective tissues, are cells and matrix. Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts.
They each unique functions and are derived from two different cell lines Figure 1 and Table 1 [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Osteoblast synthesizes the bone matrix and are responsible for its mineralization.
They are derived from osteoprogenitor cells, a mesenchymal stem cell line. Osteocytes are inactive osteoblasts that have become trapped within the bone they have formed. Osteoclasts break down bone matrix through phagocytosis. Development of bone precursor cells. Bone precursor cells are divided into developmental stages, which are 1. Bone cells, their function, and locations [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ].
The balance between osteoblast and osteoclast activity governs bone turnover and ensures that bone is neither overproduced nor overdegraded. These cells build up and break down bone matrix, which is composed of: Osteoid, which is the unmineralized matrix composed of type I collagen and gylcosaminoglycans GAGs. Calcium hydroxyapatite, a calcium salt crystal that give bone its strength and rigidity.
Bone is divided into two types that are different structurally and functionally. Most bones of the body consist of both types of bone tissue Figure 2 [ 1 , 2 , 8 , 9 ]: Compact bone, or cortical bone, mainly serves a mechanical function. This is the area of bone to which ligaments and tendons attach. It is thick and dense. Trabecular bone, also known as cancellous bone or spongy bone, mainly serves a metabolic function.
This type of bone is located between layers of compact bone and is thin porous. Location within the trabeculae is the bone marrow. Structure of a long bone. Long bones are composed of both cortical and cancellous bone tissue. They consist of several areas Figure 3 [ 3 , 4 ]: The epiphysis is located at the end of the long bone and is the parts of the bone that participate in joint surfaces.
The diaphysis is the shaft of the bone and has walls of cortical bone and an underlying network of trabecular bone. The epiphyseal growth plate lies at the interface between the shaft and the epiphysis and is the region in which cartilage proliferates to cause the elongation of the bone.
The metaphysis is the area in which the shaft of the bone joins the epiphyseal growth plate. Bone macrostructure. Different areas of the bone are covered by different tissue [ 4 ]: The epiphysis is lined by a layer of articular cartilage, a specialized form of hyaline cartilage, which serves as protection against friction in the joints.
The outside of the diaphysis is lined by periosteum, a fibrous external layer onto which muscles, ligaments, and tendons attach. The inside of the diaphysis, at the border between the cortical and cancellous bone and lining the trabeculae, is lined by endosteum. Compact bone is organized as parallel columns, known as Haversian systems, which run lengthwise down the axis of long bones. These columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or Haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone.
The lamellae of the Haversian systems are created by osteoblasts. As these cells secrete matrix, they become trapped in spaces called lacunae and become known as osteocytes. Osteocytes communicate with the Haversian canal through cytoplasmic extensions that run through canaliculi, small interconnecting canals Figure 4 [ 1 , 2 , 8 , 9 ]:.
Bone microstructure. Compact and spongy bone structures. The layers of a long bone, beginning at the external surface, are therefore: Periosteal surface of compact bone. Bone development begins with the replacement of collagenous mesenchymal tissue by bone. This results in the formation of woven bone, a primitive form of bone with randomly organized collagen fibers that is further remodeled into mature lamellar bone, which possesses regular parallel rings of collagen.
Lamellar bone is then constantly remodeled by osteoclasts and osteoblasts. During intramembranous bone formation, the connective tissue membrane of undifferentiated mesenchymal cells changes into bone and matrix bone cells [ 10 ].
In the craniofacial cartilage bones, intramembranous ossification originates from nerve crest cells. The earliest evidence of intramembranous bone formation of the skull occurs in the mandible during the sixth prenatal week.
In the eighth week, reinforcement center appears in the calvarial and facial areas in areas where there is a mild stress strength [ 11 ]. Intramembranous bone formation is found in the growth of the skull and is also found in the sphenoid and mandible even though it consists of endochondral elements, where the endochondral and intramembranous growth process occurs in the same bone.
The basis for either bone formation or bone resorption is the same, regardless of the type of membrane involved. Periosteal bone always originates from intramembranous, but endosteal bone can originate from intramembranous as well as endochondral ossification, depending on the location and the way it is formed [ 3 , 12 ].
The statement below is the stage of intramembrane bone formation Figure 5 [ 3 , 4 , 11 , 12 ]: An ossification center appears in the fibrous connective tissue membrane.
Mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into specialized cells. Some of these cells differentiate into capillaries, while others will become osteogenic cells and osteoblasts, then forming an ossification center.
Bone matrix osteoid is secreted within the fibrous membrane. Osteoblasts produce osteoid tissue, by means of differentiating osteoblasts from the ectomesenchyme condensation center and producing bone fibrous matrix osteoid. Then osteoid is mineralized within a few days and trapped osteoblast become osteocytes.
Woven bone and periosteum form. The encapsulation of cells and blood vessels occur. When osteoid deposition by osteoblasts continues, the encased cells develop into osteocytes. Accumulating osteoid is laid down between embryonic blood vessels, which form a random network instead of lamellae of trabecular. Vascularized mesenchyme condenses on external face of the woven bone and becomes the periosteum. Production of osteoid tissue by membrane cells: osteocytes lose their ability to contribute directly to an increase in bone size, but osteoblasts on the periosteum surface produce more osteoid tissue that thickens the tissue layer on the existing bone surface for example, appositional bone growth.
Formation of a woven bone collar that is later replaced by mature lamellar bone. Spongy bone diploe , consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.
Osteoid calcification: The occurrence of bone matrix mineralization makes bones relatively impermeable to nutrients and metabolic waste. Trapped blood vessels function to supply nutrients to osteocytes as well as bone tissue and eliminate waste products. The formation of an essential membrane of bone which includes a membrane outside the bone called the bone endosteum. Bone endosteum is very important for bone survival. Disruption of the membrane or its vascular tissue can cause bone cell death and bone loss.
Bones are very sensitive to pressure. The calcified bones are hard and relatively inflexible. The stage of intramembranous ossification. The following stages are a Mesenchymal cells group into clusters, and ossification centers form. The matrix or intercellular substance of the bone becomes calcified and becomes a bone in the end.
Bone tissue that is found in the periosteum, endosteum, suture, and periodontal membrane ligaments is an example of intramembranous bone formation [ 3 , 13 ]. Intramembranous bone formation occurs in two types of bone: bundle bone and lamellar bone.
The bone bundle develops directly in connective tissue that has not been calcified. Osteoblasts, which are differentiated from the mesenchyme, secrete an intercellular substance containing collagen fibrils. This osteoid matrix calcifies by precipitating apatite crystals.
Primary ossification centers only show minimal bone calcification density. The apatite crystal deposits are mostly irregular and structured like nets that are contained in the medullary and cortical regions. Mineralization occurs very quickly several tens of thousands of millimeters per day and can occur simultaneously in large areas.
These apatite deposits increase with time. Bone tissue is only considered mature when the crystalized area is arranged in the same direction as collagen fibrils. Bone tissue is divided into two, called the outer cortical and medullary regions, these two areas are destroyed by the resorption process; which goes along with further bone formation. The surrounding connective tissue will differentiate into the periosteum.
The lining in the periosteum is rich in cells, has osteogenic function and contributes to the formation of thick bones as in the endosteum. In adults, the bundle bone is usually only formed during rapid bone remodeling. This is reinforced by the presence of lamellar bone. Unlike bundle bone formation, lamellar bone development occurs only in mineralized matrix e.
The nets in the bone bundle are filled to strengthen the lamellar bone, until compact bone is formed. Osteoblasts appear in the mineralized matrix, which then form a circle with intercellular matter surrounding the central vessels in several layers Haversian system. Lamella bone is formed from 0. The network is formed from complex fiber arrangements, responsible for its mechanical properties.
The arrangement of apatites in the concentric layer of fibrils finally meets functional requirements. Lamellar bone depends on ongoing deposition and resorption which can be influenced by environmental factors, one of this which is orthodontic treatment.
Intramembranous bone formation from desmocranium suture and periosteum is mediated by mesenchymal skeletogenetic structures and is achieved through bone deposition and resorption [ 8 ]. This development is almost entirely controlled through local epigenetic factors and local environmental factors i. Genetic factors only have a nonspecific morphogenetic effect on intramembranous bone formation and only determine external limits and increase the number of growth periods.
When secondary ossification is complete, the hyaline cartilage is totally replaced by bone except in two areas. A region of hyaline cartilage remains over the surface of the epiphysis as the articular cartilage and another area of cartilage remains between the epiphysis and diaphysis. This is the epiphyseal plate or growth region.
Bones grow in length at the epiphyseal plate by a process that is similar to endochondral ossification. The cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes, in the region next to the diaphysis, age and degenerate. Osteoblasts move in and ossify the matrix to form bone.
This process continues throughout childhood and the adolescent years until the cartilage growth slows and finally stops. When cartilage growth ceases, usually in the early twenties, the epiphyseal plate completely ossifies so that only a thin epiphyseal line remains and the bones can no longer grow in length. Bone growth is under the influence of growth hormone from the anterior pituitary gland and sex hormones from the ovaries and testes. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight.
The increase in diameter is called appositional growth. Osteoblasts in the periosteum form compact bone around the external bone surface. At the same time, osteoclasts in the endosteum break down bone on the internal bone surface, around the medullary cavity.
The development of bone from fibrous membranes is called intramembranous ossification; development from hyaline cartilage is called endochondral ossification. Bone growth continues until approximately age Bones can grow in thickness throughout life, but after age 25, ossification functions primarily in bone remodeling and repair. Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, the mandible, and the clavicles.
Ossification begins as mesenchymal cells form a template of the future bone. They then differentiate into osteoblasts at the ossification center. Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix. The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone. Connective tissue in the matrix differentiates into red bone marrow in the fetus. The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone.
Endochondral ossification is the process of bone development from hyaline cartilage. All of the bones of the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral ossification. In long bones, chondrocytes form a template of the hyaline cartilage diaphysis. Responding to complex developmental signals, the matrix begins to calcify.
This calcification prevents diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone.
Osteoclasts then break down some of the spongy bone to create a marrow, or medullary, cavity in the center of the diaphysis. Dense, irregular connective tissue forms a sheath periosteum around the bones.
The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and elongate as the cartilage cells at the epiphyses divide. In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify.
Secondary ossification centers form in the epiphyses as blood vessels and osteoblasts enter these areas and convert hyaline cartilage into spongy bone. Until adolescence, hyaline cartilage persists at the epiphyseal plate growth plate , which is the region between the diaphysis and epiphysis that is responsible for the lengthwise growth of long bones Figure 1. Figure 1. The periosteum is the connective tissue on the outside of bone that acts as the interface between bone, blood vessels, tendons, and ligaments.
Long bones continue to lengthen, potentially until adolescence, through the addition of bone tissue at the epiphyseal plate.
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