The skeleton system is an organ system that forms the supporting structure of an organism. It has structural rigidity and support as the bone forms the body’s framework. It is also used as a calcium storage via the bones. When blood calcium is low, parathyroid hormones signal bone tissue to break down and release calcium. It also provides physical protection, seen where the rib cage protects internal organs such as the lungs or in the skull protecting the brain. Many large bones also shelter bone marrow that contains stem cells that make blood. There are different types of bones. Long bones are shaped like a rod and can be found in the arm, leg and fingers. Short bones are shaped like a cube and can be found in the wrist and ankle. Flat bones are bones that are flat, such as the sternum, shoulder blades or ribs. There are also irregular bones with complicated shapes such as the vertebrae and hip. A joint is where a bone meets another bone and joints can be mobile or non-mobile. A mobile joints have a fluid known as synovial fluid that contained in a cavity to lubricate movements of the bones. Non-mobile joints connect bone to bone with cartilage or fiber. There are several types of joints. The ball and socket joint is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. It causes motion around an indefinite number of axes, which have one common center. It enables the bone to move in many places. This joint can be found in the shoulder as well as the hip. The hinge joint is a bone joint in which the bone surfaces are molded to each other in such a manner as to permit motion only in one plane. An example can be seen in the elbow. Gliding joint is a synovial joint which, under physiological conditions, allows only gliding movement. It is seen in the wrist. Immobile joint can be observed in the plates of the skull, and it is a type of joint which permits very little or no movement under normal conditions. Most immobile joints are fibrous. Humans have an endoskeleton as the skeleton is on the inside. Insects have skeleton on the outside and it is known as an exoskeleton. Cartilage is composed of cells along with extracellular matrix. Cartilage cells are also known as chondrocytes. Extracellular matrix is secreted by the cells and contains fiber meshworks that give the cartilage its characteristic properties of flexibility and resilience (can be compressed). The flexibility is seen in structures like the ear and the nose. It resilience can be observed at the ends of bones in joints, knee and between vertebrae. Bone is connected to bone by the presence of ligaments which stabilize joints. Muscle is connected to bone by a tendon and it anchors the muscle.
Bone has a solid strucuture with canals inside where blood vessel runs and holes where cells can reside. The whole structure is surrounded by a membrane that contains stem cells known as osteoblasts, and osteoclasts. Bone composes of cell mixed with extracellular matrix , which are arranged in cylinders called osteons, with blood vessel and nerve running through the middle of the cylinder. A bone cell is called an osteocyte. The extracellular matrix of bone consists of calcium salts, collagen fibers, and ground substance. Lengthwise, bone growth occurs at the ends of long bones at the knobs. An osteoblasts’ role in lengthwise bone growth is to add bone tissue at the bone ends. By itself, osteoblasts will lengthen the knobs at the ends of the bone. Osteoclasts’ role in bone growth is to remodel bone tissue by chipping away the knobs until it’s the right size and shape. Osteoblasts’ role in diameter growth of bones is to add bone tissue to the outside of the bone while osteoclasts’ role in diameter growth of bones is to remove some bone tissue from the inside of the bone. This results in the hollowing out of bones. Without osteoclasts, diameter growth would result in bones that were too thick and too heavy. Even with osteoclasts, bones would still grow relatively thicker. Osteoblasts are the stem cells that give rise to osteocytes to build bone. Osteocytes are mature bones cells that reside in bone for protection. Osteoclasts are large cells that break down bone.
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