Bones and Skeletal Tissues: Structure, Function, and Growth

Chapter 6: Bones and Skeletal Tissues

Overview

This chapter explores the structure, types, and functions of bones and skeletal cartilage, as well as the processes involved in bone growth and development. Understanding these foundational concepts is essential for students of Anatomy & Physiology.

Skeletal Cartilage

Basic Information

• Components: Cells, fibers, and ground substance.

• High water content: Makes cartilage resilient.

• Perichondrium: Layer of dense irregular connective tissue surrounding cartilage surfaces; contains blood vessels for nutrient delivery.

• Avascular: Cartilage itself contains no blood vessels or nerves.

Types of Skeletal Cartilage

• Hyaline cartilage: Most abundant; provides support, flexibility, and resilience. Contains only collagen fibers. Locations include articular cartilage (joints), costal cartilage (ribs), respiratory cartilage (larynx), and nasal cartilage.

• Elastic cartilage: Similar to hyaline but contains more elastic fibers, allowing for repeated bending. Found in the external ear and epiglottis.

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

Cartilage Growth

• Appositional growth: Chondroblasts in the perichondrium secrete new matrix on the surface, causing cartilage to expand outward.

• Interstitial growth: Chondrocytes within the cartilage divide and secrete new matrix, expanding cartilage from within.

Functions of Bone

Seven Major Functions

• Support: Framework for the body.

• Protection: Shields vital organs (e.g., skull protects brain).

• Movement: Leverage for muscles.

• Mineral storage: Reservoir for calcium and phosphate.

• Blood cell formation: Occurs in bone marrow (hematopoiesis).

• Triglyceride storage: Fat stored in bone cavities.

• Hormone production: Osteoblasts secrete osteocalcin, regulating bone formation and metabolism.

Bone Classification Systems

By Location

System	Function	Components
Axial Skeleton	Supports and protects body organs	Skull, vertebral column, rib cage
Appendicular Skeleton	Movement and manipulation of environment	Limbs, girdles (shoulder, pelvic)

By Shape

Type	Description	Examples
Long Bones	Longer than wide, shaft plus two ends	Femur, humerus, phalanges
Short Bones	Cube-shaped, mostly spongy bone	Carpals, tarsals
Flat Bones	Thin, flattened, usually curved	Sternum, scapulae, ribs, cranial bones
Irregular Bones	Complex shapes	Vertebrae, hip bones

Three Levels of Bone Structure

Gross Anatomy

• Compact (cortical) bone: Dense outer layer; appears smooth and solid.

• Spongy (trabecular) bone: Internal honeycomb of small needle-like or flat pieces called trabeculae; spaces filled with bone marrow.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone covered by compact bone.

• No shaft or expanded ends.

• Bone marrow found between trabeculae.

• Periosteum covers outside; endosteum covers inside.

Structure of Long Bones

• Diaphysis: Shaft; compact bone surrounds medullary cavity filled with marrow.

• Epiphyses: Ends of long bones; spongy bone interior, articular cartilage covers joint surfaces.

• Membranes: Periosteum (outer), endosteum (inner).

Microscopic Anatomy

• Osteons (Haversian systems) in compact bone.

• Lamellae, central canal, lacunae, canaliculi.

Chemical Structure

• Organic components: Cells, osteoid (collagen fibers, ground substance).

• Inorganic components: Hydroxyapatites (mineral salts, mainly calcium phosphate).

Bone Development and Growth

Formation of the Bony Skeleton

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

• Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones (e.g., cranial bones, clavicles).

Post-Natal Bone Growth

• Longitudinal growth: Occurs at epiphyseal plates; increases bone length.

• Appositional growth: Increases bone thickness.

• Hormonal regulation: Growth hormone, thyroid hormone, and sex hormones regulate bone growth.

Bone Remodeling

Process Overview

• Bone deposit: Addition of new bone by osteoblasts.

• Bone resorption: Removal of bone by osteoclasts.

• Hormonal control: Parathyroid hormone (PTH) and calcitonin regulate calcium levels and bone remodeling.

• Mechanical stress: Bone adapts to stress by remodeling (Wolff's Law).

Key Equations

• Calcium Homeostasis: $ext{Blood Calcium Level} = ext{Intestinal Absorption} + ext{Bone Resorption} - ext{Renal Excretion} - ext{Bone Deposition}$

Example

• During growth, the epiphyseal plate allows for lengthening of long bones until adulthood, after which the plate ossifies and growth ceases.

Additional info: Microscopic and chemical structure details were inferred from standard textbook content to ensure completeness.