Skeletal system

• The functions of bone tissue are to support your body, aid in protection of soft organs (brain is covered by skull), assistance in movement, mineral homeostasis (calcium and phosphate), blood cell production (hematopoiesis), Triglyceride storage, and hormone production. 
• Osteocalcin regulates secretion of insulin. 
• Bones (osseous)  are the primary organ of the skeleton system.
• The adult skeleton has 206 bones.
• There are 22 bones in the skull, 64 bones in the shoulder girdle and upper limbs, 33 bones in the vertebral column, 25 bones in the thoracic cage, and 62 bones in the pelvic girdle and lower limbs.
• The two types of bone are compact bone and spongy bone.
• compact bone 


  homogeneous, smooth and solid. Has a dense outer layer that looks smooth and solid to the naked eye. Makes up 80 percent of total bone mass
• Spongy bone 


  porous, has a mesh of bony spines called trabeculae, filled with yellow or red bone marrow.  Makes up 20 percent of total bone mass 
• Cartilage is a flexible semi-rigid connective tissue that covers joint surfaces and growth plates to absorb shock and reduce friction. Also present at the end of growing bones and becomes bone once lengthening stops.
• Ligaments are dense connective tissue that connects bone to bone
• Tendons are dense connective tissue that connects muscle to bone. If it ruptures, cannot connect to bone and control it.
• Joints are the places of contact between bones and bones or bones and cartilage. There is an inverse relationship between mobility and stability
• Long Bones are typically longer than white and have a shift with heads at both ends. These bones mostly contain compact bone. Examples: Femur and humerus. 
• Short bones are typically cube-shaped and contain mostly spongy bone. Examples: carpals, tarsals 
• Flat bones are thin and flattened and usually curved. Thin layers of compact bone surround a layer of spongy bone. Examples: Skull, ribs, sternum
• Irregular bones are irregularly shaped and do not fit into other bone classification categories. Examples: Vertebrae and hip. 
• Diaphysis is the shaft (middle part) of the long bone and is composed of compact bone. 
• Epiphysis is the ends of the bone and is composed mostly of spongy bone. 
• Anatomy of long bone

  The periosteum is the outside covering of the diaphysis and is made up of fibrous connective tissue. Beneath that is the sharpey’s fibers which secure the periosteum to underlying bone. The arteries goes through the periosteum and innervates the bone supplying bone cells with nutrients. 
• The articular cartilage covers the external surface of the epiphyses and is made of hylaline cartilage. The cartilage prevents bone erosion by decreasing friction at joint surfaces. 
• The medullary cavity of the shaft contains yellow marrow (mostly fat) in adults and mostly red marrow (for blood cell formation) in infants. 
• Bone markings are surface features of bones and act as sites of attachments for muscles, tendons, and ligaments. They also indicate where nerves and blood vessels pass through. Projections and processes grow out from the bone surface while depressions or cavities are indentations. 
• Hematopoiesis is the process of blood cell production in the red bone marrow connective tissue which contains stem cells
• Red marrow is found within trabecular cavities of spongy bone and dipole of flat bones, such as the sternum.
• Newborns do not have much fat in their body so their medullary cavities and all spongy bone contain red marrow. In adults, red marrow is located in heads of femur and humerus, but the most active areas of hematopoiesis are flat bone dipole and some irregular bones (hip bone). 
• Yellow marrow can convert to red marrow if the person becomes anemic. 
• Osteoprogenitor (Osteogenic) cells are bone stem cells that can differentiate into bone-lining cells and osteoblasts. Some remain as osteogenic stem cells. These cells are found in the periosteum and the endosteum.
• Osteoblasts are cells formed from osteoprosenitor stem cells. They synthesize and secrete bone matrix, osteoid, which forms the bone. Osteoid is made up of collagen and calcium-binding proteins. Collagen makes up 90 percent of bone protein. The osteoblast be come entrapped within the matrix and then differentiate into osteocytes. Osteoblasts are actively mitotic. 
• Osteocytes are mature bone cells, found in lucanae, derived from osteoblasts and maintain bone matrix. They also act as stress or strain sensors that respond to mechanical stimuli such as increased forced. This information is communicated to osteoblasts and osteoclasts so bone remodeling can occur. 
• Bone-lining cells are flat cells on bone surfaces believed to also help maintain matrix. On external bone surface, lining cells are called periostea cells. On internal surfaces, they are called endosteal cells. 
• Osteoclasts are large, multicellular, phagocytic cells derived from fused bone marrow cells. They have ruffled borders that increase surface area exposed to bone. Breaks down the outer layer of bore and replaces it with a new layer (bone resorption). 
• The structural unit of compact bone is an osteon and located in every bone towards the middle.
• Each osteon is an elongated cylinder parallel to the long axis of bone and consist of hollow tube (lamellae), with rings similar to trees. 
• Bones are made up of inorganic and organic components.
• Organic components include the cells of the bone and osteoid which makes up one third of organic bone matrix and is secreted by osteoblasts.
• The resilience of bone is due to sacrificial bonds in or between collagen molecules that stretch and break to dissipate energy and prevent fractures.
• The inroganic components of bone include hydroxyapatites, which make up 65 percent of bone by mass, and is responsible for the bones hardness. 
• There are two types of bone fractures; closed (simple) fracture where the break does not penetrate the skin and open (compound) fracture where the broken bone penetrates through the skin.
• Bone fractures are treated by reduction and immobilization. 
• A comminuted fracture is where the bone breaks into many fragments