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Essentials of Human Anatomy & Physiology, Chapter 5: The Skeletal System

• The skeletal system comprises bones, joints, cartilages, and ligaments
• The skeleton is divided into two parts: axial and appendicular.
• The human adult skeleton consists of 206 bones.
•  Two primary types of bone tissue are compact and spongy bone, characterized by their density and internal structure.
• Flat bones consist of a layer of spongy bone between two thin layers of compact bone.
• Other classifications of bones include long, short, and irregular bones. 
• Long bones are typically longer than they are wide, with a shaft and enlarged ends. The majority of limb bones (excluding wrist, ankle, and kneecap bones) fall into this category. Examples include the femur and humerus. 
• Flat bones are thin, flattened, and usually curved, with two layers of compact bone sandwiching a layer of spongy bone. Examples are ribs, sternum, and most skull bones. 
• Short bones are generally cube-shaped, containing mostly spongy bone with an outer layer of compact bone. Examples include carpals (wrist bones) and tarsals (ankle bones).
• Irregular bones have irregular shapes and do not fit into the other bone classification categories. Examples include vertebrae and hip bones. 

Functions of Bones

• Support the body
• Protect soft organs (e.g., skull protects brain, rib cage protects thoracic organs)
• Allow movement via attached muscles
• Store minerals and fats (e.g., calcium, phosphorus
• Support hematopoiesis (blood cell formation) which happens within internal marrow cavities

Structure of Bone

• Diaphysis (shaft): Composed mostly of compact bone, forms the central part of a long bone.
• Periosteum: Fibrous connective tissue membrane covering the outer surface of the diaphysis, secured to underlying bone by perforating fibers.
• Endosteum: Connective tissue membrane lining the inner surface of the diaphysis.
• Medullary cavity: Cavity within the shaft that contains yellow marrow (mostly fat) in adults, and red marrow for blood cell production until about age 6 or 7.
• Epiphyses (ends): Contain spongy bone enclosed by a thin layer of compact bone, covered externally by articular cartilage to reduce friction at joint surfaces.
• Epiphyseal line: Remnant of the epiphyseal plate (flat plate of hyaline cartilage) in adult bones. The epiphyseal plate is responsible for lengthwise bone growth during development.

Microscopic Anatomy of Bone

• Osteocytes: Mature bone cells located in the bone matrix.
• Lacunae: Cavities in the bone matrix that house osteocytes.
• Lamellae: Concentric circles of lacunae arranged around the central (Haversian) canal.
• Central (Haversian) canal: Opening in the center of an osteon (Haversian system), running lengthwise through bone and carrying blood vessels and nerves.
• Osteon (Haversian system): A unit of compact bone containing central canal and matrix rings, serving as the structural and functional unit.
• Canaliculi: Tiny canals that connect the central canal to lacunae, forming a transport system connecting all bone cells to a nutrient supply.
• Perforating (Volkmann's) canal: Canal perpendicular to the central canal, carrying blood vessels and nerves.

Bone Properties

• Bone is relatively lightweight but resists tension and other forces.
• Organic parts (collagen fibers) make bone flexible and resilient.
• Calcium salts make bone hard and resistant to compressive forces.

Bone Formation, Growth, and Remodeling

• Ossification is the process of bone formation.

• It begins on hyaline cartilage models or fibrous membranes.

• Long bone growth involves two major phases:

  • Osteoblasts form bone matrix on a cartilage model or fibrous membrane.
  • Encased cartilage is replaced by bone and the cartilage is digested, creating a medullary cavity.

• Bones continue to grow in width (appositional growth) by osteoblasts adding new bone matrix on the outside and osteoclasts removing bone from the inside.

• Hormones, especially growth hormone and sex hormones, control bone growth.

• Throughout life, bones are remodeled, responding to calcium ion levels in the blood and the pull of gravity and muscles on the skeleton.

Bone Fractures

• A fracture is a break in a bone.
• Types: closed (simple) fracture vs. open (compound) fracture.
• Repair involves four major stages: hematoma formation, fibrocartilage callus formation, bony callus formation, and bone remodeling.

Axial Skeleton

• Forms the central axis of the body and consists of:
  • Skull: Cranium and facial bones. 
  • Vertebral column: 26 vertebrae separated by intervertebral discs (cervical, thoracic, lumbar, sacrum and coccyx vertebrae).
  • Bony thorax: Sternum and ribs (true ribs, false ribs, and floating ribs). 

Appendicular Skeleton

• Includes 126 bones of the limbs (appendages).
• Pectoral girdle (clavicle and scapula)
• Pelvic girdle (hips)

Joints

• Joints are articulations where bones meet. Functionally joints either hold bones together securely or allow for mobility.
• Two classifications (functional and structural). 
• Functional: Synarthroses (immovable), Amphiarthroses (slightly movable), and Diarthroses (freely movable).
• Structural: Fibrous (e.g. sutures, syndesmoses, gomphoses, generally immovable or slightly movable), cartilaginous (e.g. synchondroses, symphyses, immovable or slightly movable), synovial (freely movable). Synovial joints have articular cartilage, a joint cavity, an articular capsule, and reinforcing ligaments. 
• Common synovial joint types include plane, hinge, pivot, condylar, saddle, and ball-and-socket.
• Bursae and tendon sheathes are flattened fibrous sacs lined with synovial membrane that reduce friction in synovial joints.

Developmental Aspects of the Skeleton

•  Skeletal development occurs from birth to adulthood and involves the transformation of initial cartilage and fibrous membrane models into adult bone.
• Fetal skulls have fontanels (soft spots) which allow for brain growth during pregnancy and infancy; they eventually ossify.