Figure 5. Bone Cells. Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells. 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 Figure 5. The osteoblast. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts.
As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte , the primary cell of mature bone and the most common type of bone cell.
Each osteocyte is located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic cell.
These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts. The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release.
The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells.
Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone.
Table 2 reviews the bone cells, their functions, and locations. The differences between compact and spongy bone are best explored via their histology.
Compact bone is dense so that it can withstand compressive forces, while spongy cancellous bone has open spaces and supports shifts in weight distribution.
Compact bone is the denser, stronger of the two types of bone tissue Figure 6. It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection. Figure 6. Diagram of Compact Bone.
The microscopic structural unit of compact bone is called an osteon , or Haversian system. Running down the center of each osteon is the central canal , or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels.
As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them. Like compact bone, spongy bone , also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone.
Each epiphysis meets the diaphysis at the metaphysis. During growth, the metaphysis contains the epiphyseal plate, the site of long bone elongation described later in the chapter.
When the bone stops growing in early adulthood approximately 18—21 years , the epiphyseal plate becomes an epiphyseal line seen in the figure. These bone cells described later cause the bone to grow, repair, and remodel throughout life.
On the outside of bones there is another layer of cells that grow, repair and remodel bone as well. The cellular layer is adjacent to the cortical bone and is covered by an outer fibrous layer of dense irregular connective tissue see Figure 6. The periosteum also contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints Figure 6.
In this region, the epiphyses are covered with articular cartilage , a thin layer of hyaline cartilage that reduces friction and acts as a shock absorber. The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer. Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts Figure 6.
The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the endosteum and the cellular layer of the periosteum.
Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and other proteins. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte , the primary cell of mature bone and the most common type of bone cell.
Each osteocyte is located in a small cavity in the bone tissue called a lacuna lacunae for plural. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes.
Like osteoblasts, osteocytes lack mitotic activity. Osteocytes are connected to one another within the canaliculi via gap junctions. If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic osteoprogenitor cell. These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide.
Immature osteogenic cells are found in the cellular layer of the periosteum and the endosteum. They differentiate and develop into osteoblasts. The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release.
The cells responsible for bone resorption, or breakdown, are the osteoclasts. These multinucleated cells originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. Table 6. Although compact and spongy bone are made of the same matrix materials and cells, they are different in how they are organized.
Compact bone is dense so that it can withstand compressive forces, while spongy bone also called cancellous bone has open spaces and is supportive, but also lightweight and can be readily remodeled to accommodate changing body needs. Compact bone is the denser, stronger of the two types of osseous tissue Figure 6. It makes up the outer cortex of all bones and is in immediate contact with the periosteum.
In long bones, as you move from the outer cortical compact bone to the inner medullary cavity, the bone transitions to spongy bone. If you look at compact bone under the microscope, you will observe a highly organized arrangement of concentric circles that look like tree trunks. The end of the long bone is the epiphysis and the shaft is the diaphysis. When a human finishes growing these parts fuse together. The outside of the flat bone consists of a layer of connective tissue called the periosteum.
Cornelis Rebola Teacher. What is spongy bone also called? Cancellous bone , also called trabecular bone or spongy bone , light, porous bone enclosing numerous large spaces that give a honeycombed or spongy appearance. The bone matrix, or framework, is organized into a three-dimensional latticework of bony processes, called trabeculae, arranged along lines of stress.
Rode Arrutarte Teacher. What is the structure of compact bone? Compact bone consists of closely packed osteons or haversian systems. The osteon consists of a central canal called the osteonic haversian canal, which is surrounded by concentric rings lamellae of matrix. Between the rings of matrix , the bone cells osteocytes are located in spaces called lacunae.
Oleksiy Didrikil Teacher. What type of bone is the mandible? Anatomical terms of bone. Celiano Vermelhudo Teacher. How do bones grow? Bone Growth. Are Ribs long bones? Long bones : Long bones have a tubular shaft and articular surface at each end. The major bones of the arms humerus, radius, and ulna and the legs the femur, tibia, and fibula are all long bones. The flat bones include the scapula wingbone , the ribs , and the sternum breastbone.
Sehrish Jayme Reviewer. What is responsible for Appositional growth? This is called interstitial growth. The process of appositional growth occurs when the cartilage model also grows in thickness due to the addition of more extracellular matrix on the peripheral cartilage surface, which is accompanied by new chondroblasts that develop from the perichondrium.
Fatma Freijedo Reviewer. What is the strongest part of a bone? Compact bone is the heaviest, hardest type of bone. It needs to be very strong as it supports your body and muscles as you walk, run, and move throughout the day.
It makes up the outer layer of the bone and also helps protect the more fragile layers inside. Mabrouk Imbs Reviewer. What is a short bone? Carpus bones of wrist is classified as short bone. Short bones are designated as those bones that are as wide as they are long. Their primary function is to provide support and stability with little to no movement.
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