Skeletal System. Bone Classification. Bones are classified according to shape Long bones Short bones Flat bones Irregular bones Sesamoid bones (round). Long Bones. Long bones – longer than they are wide. They consist of a long shaft with two bulky ends.
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Bone Classification • Bones are classified according to shape • Long bones • Short bones • Flat bones • Irregular bones • Sesamoid bones (round)
Long Bones • Long bones – longer than they are wide. They consist of a long shaft with two bulky ends. • Examples: Thigh, leg, arm, and forearm bones. • The walls of the shaft of a long bone are made of compact bone and the ends (epiphyses) are made of spongy bone.
Long bones • Diaphysis – shaft of long bone. • Medullary cavity – hollow space – A thin layer of cells called the endosteum lines the cavity, and marrow fills it. • Epiphysis – At the end of the diaphysis. • End of the epiphysis is covered by cartilage which allows smooth movement against other bones.
Epiphyseal plate – in growing bones – it is a plate of hyaline cartilage between the epiphysis and diaphysis. This is where growth takes place. • It is gradually replaced by bone tissue. Growth stops when the bone has completely taken the place of cartilage.
Periosteum – a tough, fibrous connective tissue that covers long bones. • Periosteum has many nerves, blood vessels and osteoblasts(cells that create new bone) • Nutrient foramina are small openings through which blood vessels enter into the bone.
Short bones • Roughly cube-shaped • Primarily made of spongy bone, covered by a thin layer of compact bone. • Bones of the wrist and ankle
Flat bones • Thin, flattened, and often curved. • Usually arranged like a sandwich. They have a middle layer of spongy bone, covered on each side by a later of compact bone. • Examples: bones of the skull
Irregular bones • Don’t fit in any of the other categories. • Primarily spongy bone, covered with a thin layer of compact bone. • Examples: vertebrae and some skull bones.
Sesamoid Bones • Small and nodular bones. • Usually embedded within a tendon adjacent to a joint • Example: patella and sesamoid bones of foot
Compact bone • Bone cells are called osteocytes • The Haversian system – consists of a central canal surrounded by lamellae (rings) of matrix. Between the rings the osteocytes are located in spaces called lacunae. • Small channels called canaliculi radiate from the lacunae to the Haversian canal and connect the osteocytes. • This cylindrical shaped unit is called an osteon.
Spongy Bone • Composed of osteocytes and extracellular matrix. Bone cells here do not surround a central canal. • Cells lie within the trabeculae(bony spaces) and get nutrients through diffusion
Bone development and growth • Osteogenesis/ossification – either term is used to indicate the process of bone formation. • Parts of the skeleton form in the first few weeks after conception. Bone development continues throughout adulthood. • Bone development may continue for repair of fractures or to remodel bone to meet changing lifestyles.
Cells involved in bone development • Osteoblasts – bone forming cells • Osteocytes – mature bone cells • Osteoclasts – break down and reabsorb bone.
Intramembranous Ossification • The replacement of sheet-like connective tissue membranes with bony tissue. • Bones formed in this way are called intramembranous bones. • Examples: Flat bones of the skull, and some irregular bones.
Endochondral ossification • Endochondral ossification – replacement of hyaline cartilage with bony tissues. • Most bones of the skeleton formed this way. • Bones called endochondral bones.
Growth • Long bones continue to grow (lengthen) while the cartilaginous cells of the epiphyseal plates are active. • Once the ossification centers meet and the epiphyseal plates ossify – no further growth occurs. • An injury to an epiphyseal plate may cause uneven growth. For this reason injuries to the epiphysis are treated very carefully.
Homeostasis of bone • Throughout life, osteoclasts resorb bone matrix, and osteoblasts replace it. • Hormones that regulate bone calcium control these opposing processes. • So in an adult, total bone mass stays about the same throughout life. • Vitamin D, calcium, and physical exercise are necessary for bone development, growth, and repair.
Functions of the Skeletal System • Support – a rigid framework • Protection – of the soft body parts. • Movement – Sites for muscle attachment. Bones and muscles work as a lever system. • Storage – Intercellular matrix of bone contains calcium salts. Bone also contains sodium, magnesium, potassium, and stores fat. • Blood cell formation – takes place mostly in bone marrow. This process is called hematopoiesis. Red marrow helps form RBCs, WBCs, & platelets. • Yellow marrow stores fat.
Skeleton • 206 named bones • Other bones: • Wormian bones – small bones in the joints between certain cranial bones. • Sesamoid bones – small bones that grow in certain tendons.
Axial skeleton – 80 bones • Head, vertebral column, ribs, sternum, and hyoid Appendicular skeleton – 126 bones Upper and lower limbs, and their attachments
Axial Skeleton • Skull – 28 bones • Bones in the skull are tightly interlocked along irregular lines called sutures. • Some skull bones contains sinuses – air filled cavities lined with mucous membranes. • Numerous openings or foramina to allow blood vessels and nerves to enter. Anterior skull
Skull • Frontal bone • Anterior portion of the skull above the eyes, part of the nose, and the upper eye socket • Supraorbital foramen or notch – blood vessels and nerves pass to forehead. • Frontal sinuses – above the eyes – cavity.
Skull • Parietal bones – 2 of them. • Joined to each other in the midline by the sagittal suture and to the frontal bone by the coronal suture
Skull • Occipital bone • Most of the dorsal part of the skull. • Lambdoid suture joined to the parietal bones. • Foramen magnum is the large opening on the lower surface of the occipital bone- spinal cord passes through foramen magnum
Skull • Temporal bones – the sides and base of the cranium • External auditory meatus – opening to the middle ear. • Mastoid process – a knob that contains air cells • Styloid process • Zygomatic process
Skull • Sphenoid – crosses the cranial floor • Forms the sides of the skull and base of the cranium • The pituitary gland sits in the sellaturcica, a part of the sphenoid bone.
Skull • Ethmoid bone – forms most of the bony area between the nasal cavity and orbits.
Newborn skull • Skull of newborns not completely joined together. • There are areas called fontanels or soft spots.
