At this point, the cell's synthesis activity stops, and the cytoplasm decreases to become a bone cell. Bone cells can produce a new matrix, change the crystal fluid, so that the bone tissue calcium and phosphorus deposition release in a stable state to maintain blood calcium balance. Osteocytes play a role in bone resorption and bone formation and are the principal cells for maintaining the metabolism of mature bone. The cellular components of bone tissue include osteocytes, osteogenic cells, osteoblasts, and osteoclasts.
Only bone cells exist in the bone tissue, and the other three cells are located at the edge of the bone tissue. The osteocytes are oval-shaped, multi-protuberant cells, and contain a single nucleus that is located toward the vascular side and has one or two nucleoli and a membrane. Under the electron microscope, there were a few lysosomes, mitochondria, and rough endoplasmic reticulum in the cytoplasm, and the Golgi complex was also underdeveloped.
Bone cells are sandwiched between adjacent two layers of bone plates or dispersed within the bone plate. B: bone matrix; N: nucleus.
Figure 5. Schematic summary of bone tissue showing bone cells and the relationships among them and with bone matrix B. Then, osteoclast becomes polarized through its cytoskeleton reorganization; the ruffled border RB and clear zone CZ are membrane specializations observed in the portion of the osteoclast juxtaposed to the bone resorption surface, Howship lacuna HL.
After dissolution of mineral phase, osteoclast Oc releases cathepsin Cp , matrix metalloproteinase-9 MMP-9 , and tartrate-resistant acid phosphatase TRAP that degrade the organic matrix. Sema4D produced by osteoclasts inhibits osteoblasts, while Sema3A secreted by osteoblasts inhibits osteoclasts. Osteocytes Ot are located within lacunae surrounded by mineralized bone matrix B. Its cytoplasmic processes cross canaliculi to make connection with other neighboring osteocytes processes by gap junctions, mainly composed by connexin 43 Cx3 , as well as to cytoplasmic processes of osteoblasts Ob and bone lining cells BLC on bone surface.
Conversely, osteocytes produce OPG that inhibits osteoclastogenesis; moreover, osteocytes produce sclerostin and dickkopf WNT signaling pathway inhibitor DKK-1 that decrease osteoblast activity.
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Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it. As a result, it changes in structure, becoming an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space lacuna surrounded by bone tissue.
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. Process of endochondral ossification : Endochondral ossification is the process of bone development from hyaline cartilage. The periosteum is the connective tissue on the outside of bone that acts as the interface between bone, blood vessels, tendons, and ligaments.
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, while 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. Long bones lengthen at the epiphyseal plate with the addition of bone tissue and increase in width by a process called appositional growth.
Long bones continue to lengthen potentially throughout adolescence through the addition of bone tissue at the epiphyseal plate. They also increase in width through appositional growth. The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed.
On the diaphyseal side, cartilage is ossified, allowing the diaphysis to grow in length. The metaphysis is the wide portion of a long bone between the epiphysis and the narrow diaphysis. It is considered a part of the growth plate: the part of the bone that grows during childhood, which, as it grows, ossifies near the diaphysis and the epiphyses. After the zone of calcified matrix, there is the zone of ossification, which is actually part of the metaphysis.
Arteries from the metaphysis branch through the newly-formed trabeculae in this zone. The newly-deposited bone tissue at the top of the zone of ossification is called the primary spongiosa. The older bone at the bottom of the zone of ossification is called the secondary spongiosa. Longitudinal bone growth : The epiphyseal plate is responsible for longitudinal bone growth.
This illustration shows the zones bordering the epiphyseal plate of the epiphysis. The topmost layer of the epiphysis is the reserve zone. The second zone, the proliferative zone, is where chondrocytes are continually undergoing mitosis.
The next zone is the zone of maturation and hypertrophy where lipids, glycogen, and alkaline phosphatase accumulate, causing the cartilaginous matrix to calcify. The following zone is the calcified matrix where the chondrocytes have hardened and die as the matrix around them has calcified. The bottom-most row is the zone of ossification which is part of the metaphysis. The newly-deposited bone tissue at the top of the zone of ossification is called the primary spongiosa, while the older bone is labeled the secondary spongiosa.
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