Bones and Skeletal Tissues: Structure, Function, Growth, and Repair Learning Objectives

Chapter 6 – Bones and Skeletal Tissues Learning Objectives

Definitions

This section introduces key terms related to bones and skeletal tissues, essential for understanding anatomy and physiology. Mastery of these definitions is foundational for further study.

• Appositional growth: Growth in the diameter or thickness of bones or cartilage by the addition of new layers on the surface.

• Articular cartilage: Smooth, white tissue covering the ends of bones in joints, reducing friction and absorbing shock.

• Bone remodeling: The continuous process of bone resorption and formation, allowing bones to adapt to stress and repair microdamage.

• Epiphyseal plate: Also known as the growth plate; a layer of hyaline cartilage where bone growth occurs in children and adolescents.

• Osteocyte: Mature bone cell that maintains bone tissue.

• Osteoblast: Cell responsible for bone formation.

• Osteoclast: Cell responsible for bone resorption (breakdown).

• Ossification: The process of bone tissue formation.

• Fracture: A break in bone continuity.

• Calcitonin: Hormone that lowers blood calcium levels by inhibiting osteoclast activity.

• Parathyroid hormone (PTH): Hormone that increases blood calcium levels by stimulating osteoclasts.

• Wolff’s law: Principle stating that bone grows and remodels in response to the forces placed upon it.

• Endochondral ossification: Bone formation that occurs by replacing hyaline cartilage.

• Intramembranous ossification: Bone formation directly from mesenchymal tissue without a cartilage template.

• Compact bone: Dense, hard bone tissue found on the outer surface of bones.

• Spongy bone: Porous bone tissue found inside bones, containing red bone marrow.

• Epiphysis: The end part of a long bone, initially growing separately from the shaft.

• Diaphysis: The shaft or central part of a long bone.

• Periosteum: Dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints.

• Endosteum: Thin vascular membrane lining the inner surface of the bony tissue.

• Fibrocartilage: Cartilage containing collagen fibers, found in intervertebral discs and menisci.

• Hyaline cartilage: Most common type of cartilage, found on articular surfaces, ribs, nose, and trachea.

• Greenstick fracture: Incomplete fracture in which the bone bends; common in children.

• Comminuted fracture: Bone is broken into several pieces.

• Osteogenesis: The process of bone formation.

• Osteoid: Unmineralized, organic portion of the bone matrix.

• Osteoprogenitor cell: Stem cell that differentiates into osteoblasts.

• Trabeculae: Network of supporting fibers in spongy bone.

Skeletal Cartilage

Skeletal cartilage is a resilient tissue that provides support and flexibility in the skeletal system. It is found in various locations, including joints, rib cage, and respiratory passages.

• Structure: Composed of chondrocytes (cartilage cells) embedded in a matrix of collagen and elastic fibers.

• Types:

  • Hyaline cartilage: Provides support with flexibility; found in articular surfaces, costal cartilages, nose, and trachea.

  • Elastic cartilage: Contains more elastic fibers; found in the ear and epiglottis.

  • Fibrocartilage: Highly compressible; found in intervertebral discs and menisci.

• Growth:

  • Appositional growth: New cartilage is added to the surface.

  • Interstitial growth: Chondrocytes divide and secrete new matrix from within.

• Perichondrium: Dense connective tissue surrounding cartilage, providing nutrients and support.

Function of Bones

Bones serve multiple vital functions in the human body, contributing to structure, movement, and homeostasis.

• Support: Framework for the body and attachment for soft tissues.

• Protection: Shields vital organs (e.g., skull protects brain, rib cage protects heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral storage: Reservoir for minerals, especially calcium and phosphorus.

• Blood cell formation: Hematopoiesis occurs in red bone marrow.

• Triglyceride storage: Fat stored in yellow bone marrow.

• Hormone production: Osteocalcin regulates bone formation and influences energy metabolism.

Bone Structure

Bones are complex organs composed of several types of tissues, each contributing to their function and strength.

• Cells:

  • Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells.

  • Osteocytes: Mature bone cells maintaining bone matrix.

  • Osteoclasts: Bone-resorbing cells.

• Organic components: Collagen fibers and ground substance (osteoid) provide flexibility and tensile strength.

• Inorganic components: Hydroxyapatite (calcium phosphate crystals) provide hardness and resistance to compression.

• Bone tissue types:

  • Compact bone: Dense outer layer.

  • Spongy bone: Internal network of trabeculae.

Bone Development

Bone development involves two main processes: intramembranous and endochondral ossification. These processes form bones during fetal development and growth.

• Intramembranous ossification: Direct formation of bone from mesenchymal tissue; forms flat bones like the skull and clavicle.

• Endochondral ossification: Bone replaces a cartilage template; forms most bones of the body.

• Steps of endochondral ossification:

  1. Formation of bone collar around hyaline cartilage model.

  2. Cavitation of the hyaline cartilage within the model.

  3. Invasion of internal cavities by periosteal bud and spongy bone formation.

  4. Formation of medullary cavity and appearance of secondary ossification centers.

  5. Ossification of epiphyses; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Bone Growth

Bones grow in length and thickness through processes regulated by hormones and mechanical stress.

• Longitudinal growth: Occurs at the epiphyseal plate via proliferation and hypertrophy of chondrocytes.

• Appositional growth: Increases bone diameter by adding new bone tissue to the surface.

• Hormonal regulation:

  • Growth hormone: Stimulates epiphyseal plate activity.

  • Thyroid hormone: Modulates growth hormone activity.

  • Sex hormones (estrogen and testosterone): Promote growth spurts and epiphyseal plate closure.

• Epiphyseal plate zones:

  1. Resting zone

  2. Proliferation zone

  3. Hypertrophic zone

  4. Calcification zone

  5. Ossification zone

Bone Remodeling

Bone remodeling is a lifelong process involving bone resorption and formation, allowing bones to adapt to stress and repair damage.

• Cells involved: Osteoclasts (resorption), osteoblasts (formation), and osteocytes (regulation).

• Regulation:

  • Calcitonin: Decreases blood calcium by inhibiting osteoclasts.

  • Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts.

  • Physical stress: Stimulates bone formation (Wolff’s law).

• Wolff’s law: Bone adapts to the loads under which it is placed.

Bone Fractures

Bone fractures are classified by their characteristics and require a specific sequence of repair for healing.

• Classification characteristics:

  1. Position of bone ends (displaced or nondisplaced)

  2. Completeness of break (complete or incomplete)

  3. Orientation of break (linear or transverse)

• Common types of fractures:

  • Comminuted

  • Compression

  • Spiral

  • Epiphyseal

  • Depressed

  • Greenstick

• Steps of bone repair:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

Table: Types of Cartilage in the Skeleton

Type	Main Locations	Key Properties
Hyaline cartilage	Articular surfaces, costal cartilages, nose, trachea	Support, flexibility, smooth surface for joints
Elastic cartilage	Ear, epiglottis	Flexible, maintains shape
Fibrocartilage	Intervertebral discs, menisci	High tensile strength, resists compression

Key Equations

• Wolff’s Law: $\text{Bone adapts to the mechanical stress placed upon it}$

• Calcium Homeostasis: $\text{Blood Calcium Level} = \text{Bone Resorption} - \text{Bone Formation}$

Example: A greenstick fracture is common in children due to the flexibility of their bones. During bone repair, a hematoma forms first, followed by a fibrocartilaginous callus, then a bony callus, and finally remodeling restores the bone’s original shape.

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for exam preparation.