2.1 Skeletal system and bone structure

The skeletal system is the body's framework, providing support and protection. It consists of 206 bones, each with specific functions. From the skull guarding the brain to the vertebrae protecting the spinal cord, bones are crucial for our structure and movement.

Bone tissue is a dynamic structure, constantly remodeling to adapt to stress and repair damage. This process involves specialized cells like osteoblasts and osteoclasts, working together to maintain bone strength. Understanding bone composition and remodeling is key to optimizing skeletal health.

Human Skeletal System

Bones and Their Functions

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• The skeletal system consists of 206 bones that provide structural support, protection of vital organs, attachment points for muscles to enable movement, storage of minerals and fats, and production of blood cells
• The skull is comprised of 22 bones that protect the brain and support the structures of the face
  • The cranium includes the frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones
  • The face includes the mandible, maxilla, zygomatic, nasal, lacrimal, and palatine bones
• The vertebral column consists of 33 vertebrae separated into cervical, thoracic, lumbar, sacral, and coccygeal regions
  • It protects the spinal cord, supports the head, and serves as an attachment point for ribs, muscles, and ligaments
• The thoracic cage is made up of 12 pairs of ribs, the sternum, and 12 thoracic vertebrae
  • It protects vital organs in the chest cavity, including the heart and lungs

Limb and Girdle Bones

• The pectoral girdle consists of the clavicle and scapula, which provide attachment points for muscles that move the upper limbs
  • The upper limbs contain the humerus, radius, ulna, carpals, metacarpals, and phalanges
• The pelvic girdle is formed by the hip bones (ilium, ischium, pubis) and sacrum
  • It supports the weight of the upper body, protects reproductive and digestive organs, and provides attachment points for lower limb muscles
  • The lower limbs contain the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges

Bone Tissue Composition and Structure

Bone Composition

• Bone is a connective tissue composed of a mineralized extracellular matrix and specialized cells: osteoblasts, osteocytes, and osteoclasts
• The extracellular matrix consists of organic and inorganic components
  • The organic component is primarily type I collagen fibers, which provide tensile strength and flexibility
  • The inorganic component is mainly hydroxyapatite crystals, which provide compressive strength and rigidity

Bone Structure

• Bone tissue is organized into two types of bone: cortical (compact) and trabecular (spongy)
  • Cortical bone is dense and forms the outer layer of bones
  • Trabecular bone is porous and found in the interior of bones, particularly at the ends of long bones (femur, humerus)
• Cortical bone is composed of osteons (Haversian systems), which are cylindrical structures with a central Haversian canal surrounded by concentric lamellae
  • Osteocytes are located in lacunae between lamellae and communicate through canaliculi
• Trabecular bone consists of a network of thin, interconnected trabeculae, which provide structural support and flexibility
  • The spaces between trabeculae are filled with bone marrow, which produces blood cells (red blood cells, white blood cells, platelets)
• The outer surface of bones is covered by a fibrous connective tissue called the periosteum, which contains blood vessels, nerves, and osteoblasts
• The inner surface of bones is lined by the endosteum, a thin layer of cells that includes osteoblasts and osteoclasts

Bone Remodeling and Stress

Bone Remodeling Process

• Bone remodeling is a continuous process of bone resorption and formation that helps maintain bone strength, repair microdamage, and adapt to mechanical stresses throughout life
• The bone remodeling process involves the coordinated actions of osteoclasts (bone-resorbing cells) and osteoblasts (bone-forming cells)
  • Osteocytes, the most abundant cells in bone, act as mechanosensors and orchestrate the remodeling process
• The remodeling cycle consists of five stages: activation, resorption, reversal, formation, and quiescence
  1. During activation, osteocytes detect mechanical or biochemical signals and recruit osteoclasts to the remodeling site
  2. In the resorption phase, osteoclasts break down and remove old or damaged bone
  3. The reversal phase marks the transition from resorption to formation, with osteoblasts being recruited to the site
  4. During formation, osteoblasts synthesize new bone matrix (osteoid) and regulate its mineralization
  5. Finally, in the quiescence stage, the newly formed bone surface remains inactive until the next remodeling cycle

Mechanical Stress and Hormonal Regulation

• Bone remodeling is influenced by mechanical stress, with bone formation being stimulated by increased loading and bone resorption being promoted by decreased loading or disuse (Wolff's law)
  • This adaptive response allows bones to optimize their structure and strength in response to the mechanical demands placed upon them (weight-bearing exercise, resistance training)
• Hormones, such as parathyroid hormone, calcitonin, and estrogen, as well as local factors like cytokines and growth factors, also regulate the bone remodeling process
• Imbalances in bone remodeling can lead to disorders such as osteoporosis (excessive bone resorption) or osteopetrosis (insufficient bone resorption)
  • Maintaining a balance between resorption and formation is crucial for overall bone health and preventing fractures

Axial vs Appendicular Skeleton

Axial Skeleton

• The axial skeleton forms the central axis of the body and consists of 80 bones
  • It includes the skull, vertebral column, ribs, and sternum
• The primary functions of the axial skeleton are to protect the brain and spinal cord, provide support for the head and trunk, and serve as attachment points for muscles
• Bones of the axial skeleton are generally more rigid and less mobile compared to the bones of the appendicular skeleton

Appendicular Skeleton

• The appendicular skeleton is composed of 126 bones and includes the bones of the upper and lower limbs, as well as the pectoral and pelvic girdles
• The main functions of the appendicular skeleton are to enable movement, provide attachment points for muscles, and support the weight of the body
• The pectoral girdle, consisting of the clavicles and scapulae, connects the upper limbs to the axial skeleton
• The pelvic girdle, formed by the hip bones and sacrum, connects the lower limbs to the axial skeleton
• Bones of the appendicular skeleton are adapted for greater range of motion and mechanical loading (lifting objects, running)

Skeletal System Integration

• While the axial skeleton provides stability and protection for vital organs, the appendicular skeleton allows for a wide range of movements and interactions with the environment
• The axial and appendicular skeletons work together to provide structural support, protection, and movement for the body
  • They also play important roles in mineral homeostasis (calcium and phosphorus storage) and hematopoiesis (blood cell production in bone marrow)