💀Anatomy and Physiology I Unit 6 – Bone Tissue and the Skeletal System

Key Concepts and Terminology

• Osteology studies the structure and function of bones in the skeletal system
• Osseous tissue refers to the hard, rigid connective tissue that makes up bones
• Osteoblasts are bone-forming cells responsible for synthesizing and secreting the organic components of bone matrix
• Osteoclasts are large, multinucleated cells that break down and resorb bone tissue during bone remodeling
• Osteocytes are mature bone cells embedded within the mineralized matrix of bone tissue
  • Osteocytes maintain bone tissue and respond to mechanical stimuli
  • Communicate with each other and other cells through long cytoplasmic processes called canaliculi
• Ossification is the process of bone formation, which can occur through intramembranous or endochondral ossification
• Haversian systems (osteons) are the basic structural units of compact bone tissue
• Wolff's law states that bones adapt to the stresses and strains placed upon them by remodeling their structure

Bone Structure and Composition

• Bones are composed of both organic and inorganic components
  • Organic matrix consists primarily of type I collagen fibers and ground substance
  • Inorganic matrix is mainly hydroxyapatite, a crystalline form of calcium phosphate
• Compact bone (cortical bone) is dense and solid, found in the shafts of long bones and outer layers of other bones
  • Provides strength and protection to the skeletal system
• Spongy bone (trabecular bone) is less dense and has a honeycomb-like structure, found in the ends of long bones and inner portions of other bones
  • Provides structural support and houses red bone marrow
• Periosteum is a fibrous connective tissue membrane that covers the outer surface of bones (except at joints)
  • Contains blood vessels, nerve fibers, and osteoblasts
  • Plays a role in bone growth, repair, and nutrition
• Endosteum is a thin membrane lining the inner surfaces of bone, including the medullary cavity and trabeculae of spongy bone
• Bone marrow fills the spaces within spongy bone and the medullary cavity of long bones
  • Red bone marrow is the site of hematopoiesis (blood cell production)
  • Yellow bone marrow primarily consists of adipose tissue and serves as an energy reserve

Types of Bones and Their Functions

• Long bones are longer than they are wide and have a shaft (diaphysis) and two expanded ends (epiphyses)
  • Examples include the femur, humerus, and phalanges
  • Function in leverage, weight-bearing, and movement
• Short bones are roughly cube-shaped and are found in the wrist (carpals) and ankle (tarsals)
  • Provide stability and allow for limited movement
• Flat bones are thin and flattened, such as the skull bones, ribs, and scapulae
  • Protect internal organs and provide attachment points for muscles
• Irregular bones have complex shapes that do not fit into other categories, such as the vertebrae and sacrum
  • Contribute to specific functions based on their location
• Sesamoid bones are small, round bones embedded within tendons, such as the patella (kneecap)
  • Protect tendons from wear and tear and improve mechanical advantage
• Sutural bones (wormian bones) are small, irregular bones found within the sutures of the skull
  • Vary in number and location between individuals

Bone Formation and Growth

• Osteogenesis (ossification) is the process of bone formation, which begins during fetal development and continues throughout life
• Intramembranous ossification forms flat bones of the skull, mandible, and clavicles directly from mesenchymal tissue
  • Mesenchymal cells differentiate into osteoblasts, which secrete osteoid that mineralizes to form bone
• Endochondral ossification forms most bones of the skeleton, including long bones, through the replacement of a cartilage model
  • Cartilage model is formed by chondrocytes, which hypertrophy and die, leaving cavities for osteoblasts to invade
  • Primary and secondary ossification centers develop at the diaphysis and epiphyses, respectively
• Appositional growth occurs as osteoblasts lay down new bone tissue on the surface of existing bone
  • Allows bones to increase in diameter and thickness
• Epiphyseal plates (growth plates) are cartilaginous regions between the diaphysis and epiphyses of long bones in growing individuals
  • Chondrocytes proliferate, hypertrophy, and die, allowing for longitudinal bone growth
  • Closure of epiphyseal plates occurs at the end of puberty, stopping further lengthening of bones

Bone Remodeling and Homeostasis

• Bone remodeling is a continuous process of bone resorption and formation that maintains bone strength and mineral homeostasis
  • Osteoclasts resorb old or damaged bone tissue
  • Osteoblasts lay down new bone tissue in resorbed areas
• Bone remodeling is regulated by hormones, mechanical stress, and local factors
  • Parathyroid hormone (PTH) stimulates osteoclast activity and bone resorption
  • Calcitonin inhibits osteoclast activity and promotes bone formation
  • Mechanical stress (weight-bearing exercise) stimulates bone formation and remodeling
• Calcium homeostasis is maintained through the actions of PTH, calcitonin, and vitamin D on bones, kidneys, and intestines
  • Bones serve as a reservoir for calcium, storing and releasing it as needed
• Phosphate homeostasis is also regulated by PTH and vitamin D, which affect bone mineralization and renal excretion of phosphate

Skeletal System Organization

• Axial skeleton consists of 80 bones that form the vertical axis of the body
  • Includes the skull, vertebral column, ribs, and sternum
  • Protects the brain, spinal cord, and vital organs of the thorax
• Appendicular skeleton consists of 126 bones that make up the appendages and their attachments to the axial skeleton
  • Includes the pectoral girdle, upper limbs, pelvic girdle, and lower limbs
  • Allows for movement and interaction with the environment
• Skull is composed of 22 bones divided into the cranium (8 bones) and the facial skeleton (14 bones)
  • Protects the brain and houses the organs of special senses
• Vertebral column (spine) consists of 33 vertebrae divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal
  • Supports the head, neck, and trunk; protects the spinal cord; and allows for flexibility and movement
• Thoracic cage is formed by the 12 pairs of ribs, sternum, and thoracic vertebrae
  • Protects the heart, lungs, and other vital organs of the chest
  • Assists in breathing by allowing for changes in thoracic volume

Joints and Articulations

• Joints (articulations) are points of contact between bones, allowing for various degrees of movement
• Fibrous joints are connected by fibrous connective tissue and allow for little to no movement
  • Examples include sutures of the skull and the gomphosis between teeth and their sockets
• Cartilaginous joints are connected by cartilage and allow for limited movement
  • Examples include the pubic symphysis and the intervertebral discs
• Synovial joints are the most common and movable type of joint, featuring a synovial cavity and articular cartilage
  • Examples include the shoulder, hip, knee, and elbow joints
  • Classified by the type of movement they allow (e.g., hinge, ball-and-socket, gliding)
• Synovial joints are surrounded by a joint capsule and contain synovial fluid, which lubricates and nourishes the joint
  • Ligaments are dense connective tissue bands that stabilize joints by connecting bones to each other
  • Tendons are dense connective tissue bands that attach muscles to bones, allowing for movement
• Joint movements are described by the plane in which they occur and the type of motion
  • Flexion and extension occur in the sagittal plane
  • Abduction and adduction occur in the frontal plane
  • Rotation occurs around a longitudinal axis

Clinical Applications and Disorders

• Osteoporosis is a condition characterized by low bone mass and deterioration of bone tissue, increasing the risk of fractures
  • Often associated with aging, menopause, and hormonal imbalances
  • Prevented and managed through weight-bearing exercise, adequate calcium and vitamin D intake, and medications (e.g., bisphosphonates)
• Fractures are breaks in bone tissue caused by trauma or excessive stress
  • Classified as closed (simple) or open (compound) based on whether the skin is intact
  • Types include transverse, oblique, spiral, comminuted, and greenstick fractures
  • Healing involves hematoma formation, callus formation, and bone remodeling
• Osteoarthritis is a degenerative joint disease characterized by the breakdown of articular cartilage and inflammation
  • Often associated with aging, obesity, and joint overuse
  • Managed through pain relief, physical therapy, and joint replacement surgery in severe cases
• Rheumatoid arthritis is an autoimmune disorder that causes chronic inflammation and destruction of synovial joints
  • Affects multiple joints symmetrically, often in the hands and feet
  • Managed through anti-inflammatory and immunosuppressive medications, physical therapy, and surgery
• Osteosarcoma is a malignant bone tumor that typically affects the long bones of children and adolescents
  • Treated with a combination of chemotherapy, surgery, and radiation therapy
• Rickets (in children) and osteomalacia (in adults) are disorders caused by vitamin D deficiency, resulting in softening and weakening of bones
  • Prevented and treated by ensuring adequate vitamin D intake through diet, supplements, and sun exposure