Skeletal System Overview

The skeletal system provides the structural framework for the human body, supporting movement, protecting organs, and serving as a reservoir for minerals and blood cell production. This section introduces the basic organization and functions of the skeletal system.

Structure, Types, and Locations of Cartilage

Introduction

Cartilage is a specialized connective tissue that plays a crucial role in the development and function of the skeletal system. It provides flexibility and support in areas where bone would be too rigid.

• Cartilage: Made of highly resilient cartilage tissue, consisting primarily of water, which allows it to withstand compression and return to its original shape.

• Perichondrium: A dense layer of connective tissue surrounding cartilage, providing nutrients and helping cartilage resist outward expansion.

• Chondrocytes: The primary cells of cartilage, located in small spaces called lacunae within a jelly-like extracellular matrix.

Types of Cartilage

• Hyaline cartilage: Most abundant type; contains only collagen fibers. Found in articular surfaces, costal cartilages, respiratory structures, and nasal cartilage.

• Elastic cartilage: Similar to hyaline but contains more elastic fibers, providing greater flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, providing great tensile strength. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

Example: The external ear is supported by elastic cartilage, while the ends of long bones are covered by hyaline cartilage.

Growth of Cartilage

Introduction

Cartilage grows by two main mechanisms, allowing it to expand and adapt during development and repair.

• Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix on the external surface of existing cartilage, increasing its width.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within and increasing its length.

Cartilage growth is most active during childhood and adolescence but can occur in adulthood to a limited extent.

Example: The growth plates in long bones rely on interstitial growth of cartilage to lengthen bones during development.

Functions of Bones

Introduction

Bones are dynamic organs that perform several essential functions necessary for survival and homeostasis.

• Support: Provide a framework for the body and support soft tissues.

• Protection: Encase and protect vital organs such as the brain, spinal cord, and thoracic organs.

• Movement: Serve as levers for muscles to act upon, enabling movement.

• Mineral storage: Store minerals, primarily calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities of certain bones.

• Triglyceride (fat) storage: Yellow marrow in long bones stores fat as an energy reserve.

• Hormone production: Bones secrete osteocalcin, a hormone involved in regulating insulin secretion, glucose homeostasis, and energy metabolism.

Example: The sternum protects the heart and lungs, while the femur supports body weight during standing and walking.

Classification of Bones

Introduction

Bones are classified by their shape and location, which relate to their function and development.

• Axial skeleton: Includes the skull, vertebral column, and rib cage; supports and protects the body's vital organs.

• Appendicular skeleton: Includes the limbs and girdles; involved in movement and manipulation of the environment.

Bone Shapes

• Long bones: Longer than they are wide (e.g., femur, humerus).

• Short bones: Cube-shaped (e.g., wrist and ankle bones); sesamoid bones (e.g., patella) form within tendons.

• Flat bones: Thin, flat, and often curved (e.g., sternum, skull bones).

• Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

Example: The patella is a sesamoid bone embedded within the quadriceps tendon.

Bone Structure

Introduction

Bones are complex organs composed of various tissues and have distinct structural features at both the macroscopic and microscopic levels.

• Compact bone: Dense outer layer that appears smooth and solid; provides strength and support.

• Spongy bone: Internal network of trabeculae (needle-like or flat pieces) that provide structural support and house bone marrow.

Example: The shaft of a long bone is primarily compact bone, while the ends contain spongy bone.

Gross Anatomy of Bones

• Long bones have a shaft (diaphysis), ends (epiphyses), and membranes (periosteum and endosteum).

• Diaphysis: Tubular shaft composed of compact bone surrounding the medullary cavity (marrow cavity).

• Epiphyses: Ends of long bones; consist of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of the epiphyseal plate (growth plate) where bone growth occurs in youth.

• Periosteum: Double-layered membrane covering the external surface of bones (except at joints); contains fibrous and osteogenic layers.

• Endosteum: Delicate membrane lining the internal surfaces of bone, including the medullary cavity.

Example: The periosteum contains blood vessels and nerves essential for bone health and repair.

Microscopic Anatomy of Bone

• Osteogenic cells: Stem cells in periosteum and endosteum that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete osteoid (unmineralized bone matrix).

• Osteocytes: Mature bone cells in lacunae that maintain bone matrix and act as mechanosensors.

• Bone lining cells: Flat cells on bone surfaces, involved in maintenance.

• Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown).

Example: Osteoclasts are activated during bone remodeling to release calcium into the bloodstream.

Compact Bone Structure

• Osteon (Haversian system): The structural unit of compact bone; consists of concentric lamellae (rings) of bone matrix surrounding a central canal.

• Lamellae: Layers of bone matrix with collagen fibers running in alternating directions to resist twisting forces.

• Canals and canaliculi: Central (Haversian) canals contain blood vessels and nerves; perforating (Volkmann's) canals connect central canals. Canaliculi are tiny channels connecting lacunae, allowing communication and nutrient exchange between osteocytes.

• Interstitial and circumferential lamellae: Fill spaces between osteons and encircle the entire bone, respectively.

Example: The osteon structure allows compact bone to withstand compressive forces.

Spongy Bone Structure

• Trabeculae: Irregularly arranged lamellae and osteocytes interconnected by canaliculi; align along lines of stress to provide strength with minimal weight.

• No osteons are present in spongy bone.

• Capillaries in endosteum supply nutrients to spongy bone.

Example: The trabecular architecture of spongy bone is adapted to resist forces from multiple directions.

Chemical Composition of Bone

Introduction

Bones are composed of both organic and inorganic components, each contributing to bone's unique properties.

• Organic components: Include bone cells and osteoid (ground substance and collagen fibers), providing tensile strength and flexibility.

• Inorganic components: Mainly hydroxyapatites (mineral salts), primarily calcium phosphate crystals, responsible for bone hardness and resistance to compression.

Example: The combination of collagen and hydroxyapatite allows bone to be both strong and resilient.

Bone Development (Ossification)

Introduction

Ossification (osteogenesis) is the process of bone tissue formation, occurring during embryonic development and throughout life for growth and repair.

• Endochondral ossification: Bone forms by replacing hyaline cartilage models; forms most bones below the skull except the clavicles.

• Intramembranous ossification: Bone develops from fibrous membranes; forms flat bones of the skull and clavicles.

Example: The femur develops through endochondral ossification, while the parietal bone forms via intramembranous ossification.

Steps of Endochondral Ossification

1. Bone collar forms around diaphysis of cartilage model.

2. Cartilage in the center calcifies and develops cavities.

3. Periosteal bud invades cavities, forming spongy bone.

4. Diaphysis elongates and medullary cavity forms.

5. Secondary ossification centers appear in epiphyses.

6. Epiphyses ossify; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Steps of Intramembranous Ossification

1. Ossification centers form within fibrous connective tissue membranes.

2. Osteoid is secreted and calcifies.

3. Woven bone and periosteum form.

4. Lamellar bone replaces woven bone; red marrow appears.

Bone Growth

Growth in Length of Long Bones

Long bones grow in length at the epiphyseal plate through a series of zones:

• Resting (quiescent) zone: Inactive cartilage on the epiphyseal side.

• Proliferation (growth) zone: Rapidly dividing chondrocytes push the epiphysis away from the diaphysis.

• Hypertrophic zone: Older chondrocytes enlarge, creating spaces.

• Calcification zone: Matrix calcifies, chondrocytes die, and matrix deteriorates.

• Ossification (osteogenic) zone: New bone forms as osteoblasts lay down bone matrix on calcified cartilage.

Growth in length ceases when the epiphyseal plate closes and is replaced by bone (epiphyseal line).

Growth in Width (Appositional Growth)

• Bones increase in thickness as osteoblasts in the periosteum secrete new bone matrix on the external surface, while osteoclasts remove bone from the internal surface.

• This process allows bones to become thicker and stronger without becoming too heavy.

Hormonal Regulation of Bone Growth

Introduction

Bone growth is regulated by several hormones to ensure proper development and maintenance.

• Growth hormone: Stimulates epiphyseal plate activity in childhood.

• Thyroid hormone: Modulates growth hormone activity to ensure proper proportions.

• Sex hormones (estrogen and testosterone): Promote growth spurts and eventual closure of the epiphyseal plate during adolescence.

Bone Remodeling and Repair

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption and deposition, allowing bones to adapt to stress and repair microdamage.

• Osteoclasts: Resorb bone by secreting enzymes and acids.

• Osteoblasts: Deposit new bone matrix.

• Remodeling is regulated by hormonal and mechanical factors.

Hormonal Control of Remodeling

• Parathyroid hormone (PTH): Released in response to low blood calcium; stimulates osteoclasts to resorb bone and release calcium.

• Calcitonin: Produced by the thyroid gland in response to high blood calcium; effects are minor in humans but can lower blood calcium at high doses.

Calcium Homeostasis Equation:

Mechanical Control of Remodeling

• Bones remodel in response to mechanical stress (Wolff's Law).

• Areas under greatest stress become thicker and stronger.

Bone Repair and Fracture Classification

Fractures are classified by the position of bone ends, completeness, and whether the skin is penetrated.

• Nondisplaced vs. Displaced: Whether bone ends are aligned.

• Complete vs. Incomplete: Whether the bone is broken through.

• Open (compound) vs. Closed (simple): Whether the bone pierces the skin.

Stages of Bone Repair:

1. Hematoma formation: Blood clot forms at the fracture site.

2. Fibrocartilaginous callus formation: Soft callus bridges the broken ends.

3. Bony callus formation: New bone trabeculae form a hard callus.

4. Bone remodeling: Bone is reshaped to resemble original structure.

Bone Disorders

Osteomalacia and Rickets

• Osteomalacia: Poorly mineralized bones in adults; bones are soft and weak.

• Rickets: Osteomalacia in children; results in bowed legs and deformities due to inadequate mineralization.

• Common cause: Vitamin D deficiency or insufficient calcium.

Osteoporosis

• Bone resorption outpaces bone deposit, leading to decreased bone mass and increased fracture risk.

• Common in postmenopausal women due to decreased estrogen.

• Risk factors: Age, low calcium/vitamin D intake, sedentary lifestyle, smoking, alcohol use.

Paget's Disease

• Excessive and disorganized bone remodeling, resulting in weak, deformed bones.

• Unknown cause; may be viral.

• Treatment includes medications like calcitonin and bisphosphonates.

Axial Skeleton: Skull and Vertebral Column

Skull

• Encloses the brain and forms the framework of the face.

• Provides attachment sites for head and neck muscles.

• Contains cavities for special senses (sight, taste, smell).

• Facilitates air and food passage, secures teeth, and anchors facial muscles.

• Paranasal sinuses: Air-filled spaces that lighten the skull and enhance voice resonance.

Developmental Aspects of the Skull

• Infant skull has more bones than adult skull; bones are connected by fontanelles (soft spots) to allow for growth and ease of birth.

• Fontanelles ossify with age, and facial bones grow to adult proportions.

Vertebral Column

• Transmits weight of the trunk to lower limbs, surrounds and protects the spinal cord, and provides attachment points for ribs and muscles.

• Consists of 26 irregular bones in five regions: cervical, thoracic, lumbar, sacral, and coccygeal.

• Curvatures increase resilience and flexibility.

Abnormal Spinal Curvatures

• Scoliosis: Lateral curvature, often in the thoracic region.

• Kyphosis: Exaggerated thoracic curvature (hunchback).

• Lordosis: Exaggerated lumbar curvature (swayback).

Intervertebral Discs

• Act as shock absorbers between vertebrae.

• Composed of a gelatinous nucleus pulposus and a tough anulus fibrosus.

• Herniated discs occur when the nucleus pulposus protrudes through the anulus fibrosus, potentially compressing nerves.

Appendicular Skeleton: Arches of the Foot

• Foot arches are maintained by bone shape, ligaments, and tendons, allowing the foot to bear weight and act as a lever.

• Lateral longitudinal arch: Low curve on the lateral side.

• Medial longitudinal arch: Higher curve on the medial side.

• Transverse arch: Runs across the foot.

• Disorders include flat feet (pes planus) and high arches (cavus foot).

Example: Proper footwear with arch support can help prevent foot arch problems.

Summary Table: Types of Cartilage

Summary Table: Bone Cells