Bones and Skeletal Tissues: Structure, Function, and Development

Bones and Skeletal Tissues

Introduction

The skeletal system is fundamental to human anatomy and physiology, providing structure, protection, and facilitating movement. Understanding bone anatomy and physiology is essential for diagnosing and treating bone-related diseases such as osteoporosis.

Cartilages and Their Role in the Skeleton

Cartilage Structure and Types

Cartilage is a resilient tissue that forms the initial skeleton in embryos and remains in areas requiring flexibility. It is avascular and lacks nerves, with nutrients supplied by the perichondrium.

• Hyaline cartilage: Provides support, flexibility, and resilience; most abundant; found in joints, ribs, respiratory passages, and nose.

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

• Fibrocartilage: Contains thick collagen fibers; found in knee menisci, pubic symphysis, and intervertebral discs.

Cartilage Growth

• Appositional growth: New matrix is added to the surface by cells in the perichondrium.

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

• Cartilage growth ends during adolescence.

Functions of Bones

Seven Major Functions

• Support: Framework for the body and soft organs.

• Protection: Shields brain, spinal cord, and vital organs.

• Anchorage: Acts as levers for muscle action.

• Mineral storage: Stores calcium and phosphorus.

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

• Triglyceride storage: Fat stored in bone cavities.

• Hormone production: Osteocalcin regulates insulin and energy expenditure.

Classification of Bones

By Location

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles.

By Shape

• Long bones: Longer than wide; e.g., femur, humerus.

• Short bones: Cube-shaped; e.g., wrist, ankle; includes sesamoid bones like the patella.

• Flat bones: Thin, flat, slightly curved; e.g., sternum, ribs, cranial bones.

• Irregular bones: Complex shapes; e.g., vertebrae, hip bones.

Bone Structure

Gross Anatomy: Compact and Spongy Bone

Bones are organs containing multiple tissue types. The two main types of bone tissue are:

• Compact bone: Dense, smooth outer layer.

• Spongy bone: Honeycomb structure of trabeculae, filled with bone marrow.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone (diploë) covered by compact bone.

• Periosteum covers outside; endosteum covers inside.

• Bone marrow is scattered throughout spongy bone.

• Hyaline cartilage covers joint surfaces.

Structure of a Typical Long Bone

• Diaphysis: Shaft; compact bone surrounds medullary cavity (yellow marrow in adults).

• Epiphyses: Ends; compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of growth plate.

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

Hematopoietic Tissue in Bones

• Red bone marrow: Found in trabecular cavities of spongy bone and diploë of flat bones.

• In adults, most active in flat bones and some irregular bones.

• Yellow marrow can convert to red marrow if needed.

Bone Markings

Bone markings are sites for muscle, ligament, and tendon attachment, joint formation, or passage for blood vessels and nerves.

Name of Bone Marking	Description	Illustrations
Tuberosity	Large rounded projection	Hip bone
Trochanter	Very large, blunt, irregularly shaped process	Femur
Line	Narrow ridge of bone	Femur
Spine	Sharp, slender, pointed projection	Vertebra
Process	Any bony prominence	Spinous process

Surfaces	Description	Illustrations
Head	Bony expansion on a narrow neck	Rib
Facet	Smooth, nearly flat articular surface	Condyle
Depressions and Openings	Description	Illustrations
Groove	Narrow, slitlike opening	Skull
Foramen	Round or oval opening	Skull
Sinus	Cavity within a bone	Skull

Microscopic Anatomy of Bone

Types of Bone Cells

• Osteoprogenitor cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells; secrete osteoid (collagen and calcium-binding proteins).

• Osteocytes: Mature bone cells; maintain matrix and act as stress sensors.

• Bone-lining cells: Flat cells on bone surfaces; maintain matrix.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Microscopic Anatomy of Compact Bone

• Osteon (Haversian system): Structural unit; cylindrical, parallel to bone axis; composed of concentric lamellae.

• Canals and canaliculi: Central canal contains blood vessels and nerves; canaliculi connect lacunae and allow communication.

• Lamellae: Collagen fibers run in different directions in adjacent rings, resisting twisting forces.

• Interstitial and circumferential lamellae: Fill gaps and extend around diaphysis.

Microscopic Anatomy of Spongy Bone

• Trabeculae align along lines of stress, conferring strength.

• No osteons; trabeculae contain lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes bone cells and osteoid (collagen and ground substance).

• Provides tensile strength and flexibility.

• Sacrificial bonds in collagen dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts) make up 65% of bone mass.

• Calcium phosphate crystals provide hardness and resistance to compression.

• Bones are strong and durable due to mineral composition.

Bone Development

Ossification (Osteogenesis)

Bone tissue formation begins in the second month of development and continues throughout life.

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

• Intramembranous ossification: Bone develops from fibrous membrane; forms cranial bones and clavicles.

Endochondral Ossification

• Five main steps: bone collar formation, cartilage calcification, periosteal bud invasion, diaphysis elongation, epiphyses ossification.

• Hyaline cartilage remains in epiphyseal plates and articular cartilages.

Intramembranous Ossification

• Four steps: formation of ossification centers, secretion and calcification of osteoid, formation of woven bone and periosteum, replacement by lamellar bone and appearance of red marrow.

Postnatal Bone Growth

Growth in Length of Long Bones

• Occurs at the epiphyseal plate via interstitial growth.

• Epiphyseal plate consists of five zones: resting, proliferation, hypertrophic, calcification, ossification.

• Bone lengthening ceases when epiphysis and diaphysis fuse (epiphyseal plate closure).

Growth in Width (Thickness)

• Appositional growth increases bone thickness.

• Osteoblasts build bone externally; osteoclasts remove bone internally.

• Bones thicken in response to stress.

Hormonal Regulation of Bone Growth

• Growth hormone stimulates epiphyseal plate activity.

• Thyroid hormone modulates growth hormone activity.

• Sex hormones induce growth spurts and epiphyseal plate closure.

Bone Remodeling

Bone Remodeling Process

• Bone deposit and resorption occur throughout life.

• Remodeling units consist of osteoblasts and osteoclasts.

Bone Resorption

• Osteoclasts break down bone matrix, releasing calcium into blood.

• Regulated by parathyroid hormone (PTH) and immune proteins.

Bone Deposition

• Osteoblasts deposit new bone matrix.

• Calcification front marks transition between new and old bone.

Control of Remodeling

• Hormonal control maintains blood calcium homeostasis.

• Mechanical stress determines bone strength and structure (Wolff's law).

Bone Repair

Fracture Classification

• Nondisplaced vs. displaced: Position of bone ends.

• Complete vs. incomplete: Completeness of break.

• Open vs. closed: Whether skin is penetrated.

Common Types of Fractures

Fracture Type	Description
Comminuted	Bone fragments into three or more pieces; common in older adults.
Compression	Bone is crushed; common in porous bones.
Spiral	Ragged break from twisting forces; common in sports.
Epiphyseal	Epiphysis separates from diaphysis; occurs where cartilage cells are dying.
Depressed	Broken bone portion pressed inward; typical of skull fracture.
Greenstick	Bone breaks incompletely; common in children.

Fracture Treatment and Repair

• Reduction (realignment) and immobilization are required.

• Repair involves four stages: hematoma formation, fibrocartilaginous callus formation, bony callus formation, bone remodeling.

Bone Disorders

Osteomalacia and Rickets

• Soft, weak bones due to poor mineralization.

• Rickets in children causes bone deformities; often due to vitamin D deficiency.

Osteoporosis

• Bone resorption exceeds deposit; bone mass declines.

• Risk factors: age, gender, lack of exercise, poor diet, smoking, genetics, hormone-related conditions.

• Treatments: calcium, vitamin D, exercise, hormone therapy, bisphosphonates, denosumab.

• Prevention: adequate calcium, reduced alcohol, weight-bearing exercise.

Paget’s Disease

• Excessive, haphazard bone deposit and resorption.

• High ratio of spongy to compact bone; reduced mineralization.

• Drug therapies: bisphosphonates, calcitonin.

Developmental and Age-Related Aspects of Bone

Embryonic and Fetal Development

• Skeleton ossifies predictably; fetal age can be determined by X-rays.

• Most long bones begin ossifying by 8 weeks; primary ossification centers by week 12.

Birth to Adulthood

• Epiphyseal plates persist through childhood and adolescence.

• Complete ossification and cessation of skeletal growth by age 25.

Age-Related Changes

• Bone formation exceeds resorption in youth; balanced in young adults; resorption exceeds formation in older adults.

• Bone density, mineralization, and healing decrease with age; genetics play a major role.