Examples of flat bones are the sternum breast bone , ribs, scapulae shoulder blades , and the roof of the skull Figure 1. Irregular bones are bones with complex shapes. These bones may have short, flat, notched, or ridged surfaces. Examples of irregular bones are the vertebrae, hip bones, and several skull bones. Sesamoid bones are small, flat bones and are shaped similarly to a sesame seed. The patellae are sesamoid bones. Sesamoid bones develop inside tendons and may be found near joints at the knees, hands, and feet see Figure 3.
Sutural bones are small, flat, irregularly shaped bones. They may be found between the flat bones of the skull. They vary in number, shape, size, and position. Improve this page Learn More. Skip to main content. Module The Musculoskeletal System.
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. 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.
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.
The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.
Watch this video to see the microscopic features of a bone. Skip to main content. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms humerus, ulna, radius and legs femur, tibia, fibula , as well as in the fingers metacarpals, phalanges and toes metatarsals, phalanges.
Long bones function as levers; they move when muscles contract. A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles.
Short bones provide stability and support as well as some limited motion. Examples include the cranial skull bones, the scapulae shoulder blades , the sternum breastbone , and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs. An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification.
These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones. A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed.
These bones form in tendons the sheaths of tissue that connect bones to muscles where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces.
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