Skeletal System PDF

THE SKELETAL SYSTEM The 4 major organs of the skeletal system: 1, bones, 2, cartilage, 3. ligaments 4, tendons 20 percent of the body weight. Bone, or osseus fissue, is a hardened connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the s...

THE SKELETAL SYSTEM The 4 major organs of the skeletal system: 1, bones, 2, cartilage, 3. ligaments 4, tendons 20 percent of the body weight. Bone, or osseus fissue, is a hardened connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints), cartillage, a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. Fanctions for the human body: o supports the body o facilitates movement o protects Internal organs o produces blood cells Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the Interior of most bone is referred to as bone marrow. There are two types of bone marrow: 1. yellow marrow (deep) - contains adipose tissue; the triglycerides (lipids) stored in the adipocytes of the tissue can serve as a source of energy. 2. red marrow (superficial) - hematopolesis-the production of blood cells-takes place. Red blood cells, white blood cells, and platelets are all produced in the red marrow.  stores and releases minerals and fat the bone matrix acts as a reservoir for a number of minerals especially calcium, and potassium. These minerals, incorporated Into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Bones contain more calcium than any other organ. The intercellular matrix of bone contains large amounts of calcium salts, the most Important being calcium phosphate. When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs. When blood calcium levels are increased, the excess calcium is stored In the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously. Structure of the bone tissue 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. Small channels (canaliculi) radiate from the lacunae to the osteonic (haversian) canal to provide passageways through the hard matrix. In compact bone, the haversian systems are packed tightly together to form what appears to be a solid mass. The osteonic canals contain blood vessels that are parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating canals, with vessels on the surface of the bone. Spongy (Cancellous) Bone Spongy (cancellous) bone is lighter and less dense than compact bone. Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow. Bone Development & Growth Bone development continues throughout adulthood. Even after adult stature is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles. Osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodeling of bones. Osteoblasts are bone-forming cells, Osteocytes are mature bone cells, and Osteoclasts break down and reabsorb bone. There are two types of ossification: intramembranous and endochondral. Intramembranous Intramembranous ossification is the process of replacing connective tissue membranes with bony tissue, forming intramembranous bones, including some flat skull bones and irregular ones. Osteoblasts migrate to these membranes and deposit bony matrix, forming osteocytes when surrounded by the matrix. Endochondral Ossification Endochondral ossification is the process of replacing hyaline cartilage with bony tissue, forming most endochondral bones. It begins with hyaline cartilage models, which are infiltrated with blood vessels and osteoblasts. Osteoblasts form a collar of compact bone around the diaphysis, while disintegrating cartilage forms a primary ossification center. Bone Growth 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. These two processes together increase the diameter of the bone and, at the same time, keep the bone from becoming excessively heavy and bulky. Classification of bones The bones of the body come in a variety of sizes and shapes. The four principal types of bones are long, short, flat and irregular. Bones that are longer than they are wide are called long bones. They consist of a long shaft with two bulky ends or extremities. They are primarily compact bone but may have a large amount of spongy bone at the ends or extremities. Long bones include bones of the thigh, leg, arm, and forearm. Short Bones Short bones are roughly cube shaped with vertical and horizontal dimensions approximately equal. They consist primarily of spongy bone, which is covered by a thin layer of compact bone. Short bones include the bones of the wrist and ankle. Flat Bones Flat bones are thin, flattened, and usually curved. Most of the bones of the cranium are flat bones. Irregular Bones Bones that are not in any of the above three categories are classified as irregular bones. They are primarily spongy bone that is covered with a thin layer of compact bone. The vertebrae and some of the bones in the skull are irregular bones. All bones have surface markings and characteristics that make a specific bone unique. There are holes, depressions, smooth facets, lines, projections and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones or points of attachment for tendons and ligaments. Divisions of the skeleton The adult human skeleton usually consists of 206 named bones. These bones can be grouped in two divisions: axial skeleton and appendicular skeleton. The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs and breastbone or sternum. The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower extremities, or limbs, and their attachments which are called girdles. The named bones of the body are listed below by category. Axial Skeleton (80 bones) Skull (28) Cranial Bones  Parietal (2)  Temporal (2)  Frontal (1)  Occipital (1)  Ethmoid (1)  Sphenoid (1) Facial Bones  Maxilla (2)  Zygomatic (2)  Mandible (1)  Nasal (2)  Platine (2)   Inferior nasal concha (2)  Lacrimal (2)  Vomer (1) Auditory Ossicles  Malleus (2)  Incus (2)  Stapes (2) Hyoid (1) Vertebral Column  Cervical vertebrae (7)  Thoracic vertebrae (12)  Lumbar vertebrae (5)  Sacrum (1)  Coccyx (1) Thoracic Cage  Sternum (1)  Ribs (24)    Appendicular Skeleton (126 bones) Pectoral girdles  Clavicle (2)  Scapula (2) Upper Extremity  Humerus (2)  Radius (2)  Ulna (2)  Carpals (16)  Metacarpals (10)  Phalanges (28) Pelvic Girdle  Coxal, innominate, or hip bones (2) Lower Extremity  Femur (2)  Tibia (2)  Fibula (2)  Patella (2)  Tarsals (14)  Metatarsals (10)  Phalanges (28)    Articulations An articulation, or joint, is where two bones come together. In terms of the amount of movement they allow, there are three types of joints: immovable, slightly movable and freely movable. Synarthroses Synarthroses are immovable joints. The singular form is synarthrosis. In these joints, the bones come in very close contact and are separated only by a thin layer of fibrous connective tissue. The sutures in the skull are examples of immovable joints. Amphiarthroses Slightly movable joints are called amphiarthroses. The singular form is amphiarthrosis. In this type of joint, the bones are connected by hyaline cartilage or fibrocartilage. The ribs connected to the sternum by costal cartilages are slightly movable joints connected by hyaline cartilage. The symphysis pubis is a slightly movable joint in which there is a fibrocartilage pad between the two bones. The joints between the vertebrae and the intervertebral disks are also of this type. Diarthroses Most joints in the adult body are diarthroses, or freely movable joints. The singular form is diarthrosis. In this type of joint, the ends of the opposing bones are covered with hyaline cartilage, the articular cartilage, and they are separated by a space called the joint cavity. The components of the joints are enclosed in a dense fibrous joint capsule. The outer layer of the capsule consists of the ligaments that hold the bones together. The inner layer is the synovial membrane that secretes synovial fluid into the joint cavity for lubrication. Because all of these joints have a synovial membrane, they are sometimes called synovial joints. Process of bone healing Hematoma Formation: (Immediately after the fracture) This forms the key step in fracture healing. The blood vessels supplying the bone and periosteum are disrupted during the fracture, causing a hematoma to form at the fracture site, which is rich in hematopoietic cells. The hematoma clots and forms the temporary frame for subsequent healing. Macrophages, neutrophils, and platelets release pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), bone morphogenetic proteins (BMPs), platelet-derived growth factors (PDGF), transforming growth factor beta (TGF- Beta), vascular endothelial growth factor (VEGF) and interleukins (IL-1, IL-6, IL-10, IL-11, IL 12, IL-23). These cytokines further stimulate essential cellular biology at the fracture site. Granulation Tissue Formation (Primary or fibrocartilaginous callus): (Within two weeks) This provides provisional stability. Platelets are recruited to the fracture site. Among the products secreted by platelets are fibronectin (FN), platelet- derived growth factor (PDGF), and transforming growth factor-β (TGF-β), which collectively trigger an inflammatory response. Subsequently, other mesenchymal cells and inflammatory cells are recruited to the fracture site, such as fibroblasts and endothelial cells, with resultant fibrin-rich granulation tissue formation and angiogenesis. Mesenchymal stem cells begin to differentiate (driven by BMPs). As a result, chondrogenesis begins to occur, laying down a collagen-rich fibrocartilaginous network spanning the fracture ends, with a surrounding hyaline cartilage sleeve. Bony Callus Formation (If bone ends are not in contact, then a soft bridging callus forms): The endosteum and periosteum serve as primary sources for the Fibroblasts involved in fracture healing. The fibroblasts play a pivotal role by secreting the matrix constituents such as collagen, elastic and mesh fibers, and glycoproteins. Fibroblasts differentiate into osteoblasts guided by various bone morphogenic proteins (BMPs) and fibroblast growth factors (FGFs) released by the body at the fracture site. With resultant increased levels of alkaline phosphatase (ALP), total calcium content, and osteogenic marker genes encoding for integrin-binding sialoprotein (IBSP), runt-related transcription factor 2 (Runx2), and osteoblast-associated transcription factors. The cartilaginous (soft) callus begins to undergo endochondral ossification, and a medullary callus further supports the bridging soft callus. RANK-L is expressed, stimulating further differentiation of chondroblasts, chondroclasts, osteoblasts, and osteoclasts. As a result, the cartilaginous callus is resorbed and begins to calcify. Subperiosteally, woven bone continues to be laid down. The newly formed blood vessels continue to proliferate, allowing further migration of mesenchymal stem cells. Bone Remodelling (Continues for months to years after clinical union) This involves a complex interaction of signaling pathways, including BMP, fibroblast growth factor (FGF), parathyroid hormone-related peptide (PTHrP), and Indian hedgehog (Ihh). All of which are involved somehow in the differentiation of the appendicular skeleton.