Bones and Skeletal Tissues: Structure, Function, and Development

Bones and Skeletal Tissues

Introduction

The human skeleton is a dynamic organ system essential for support, movement, protection, and metabolic functions. Understanding bone anatomy and physiology is crucial for diagnosing and treating skeletal disorders.

Skeletal Cartilages

Basic Structure, Types, and Locations

• Skeletal cartilage is made of resilient, molded cartilage tissue, primarily water, with no blood vessels or nerves.

• Perichondrium is a dense connective tissue layer surrounding cartilage, providing nutrients and resisting expansion.

• Cartilage cells, chondrocytes, are found in lacunae within a jelly-like matrix.

Types of Cartilage

• Hyaline cartilage: Most abundant; provides support, flexibility, and resilience. Locations: articular surfaces, costal cartilage, respiratory structures, nasal cartilage.

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

• Fibrocartilage: Thick collagen fibers; high tensile strength. Locations: menisci of knee, intervertebral discs.

Growth of Cartilage

• Appositional growth: New matrix is secreted on the surface by cells in the perichondrium.

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

• Calcification: Occurs during normal bone growth and aging; calcified cartilage is not bone.

Functions of Bones

Major Functions

• Support: Framework for the body and soft organs.

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

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis in red marrow cavities.

• Triglyceride storage: Fat storage in bone cavities.

• Hormone production: Osteocalcin regulates insulin, glucose, and metabolism.

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., limb bones).

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

• Flat bones: Thin, flat, slightly curved (sternum, scapulae, ribs, skull).

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

Bone Structure

Gross Anatomy

• Compact bone: Dense outer layer; smooth and solid.

• Spongy bone: Honeycomb of trabeculae filled with marrow.

Structure of Short, Irregular, and Flat Bones

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

• Periosteum covers outside; endosteum covers inside.

• No defined marrow cavity; marrow is scattered throughout spongy bone.

Structure of a Typical Long Bone

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

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

• Epiphyseal line: Remnant of growth plate between diaphysis and epiphysis.

Membranes

• Periosteum: Double-layered membrane; outer fibrous layer (dense irregular CT), inner osteogenic layer (bone-forming cells).

• Endosteum: Delicate membrane covering internal bone surfaces, trabeculae, and canals.

Hematopoietic Tissue

• Red marrow: Found in spongy bone and diploë of flat bones; active in hematopoiesis.

• Yellow marrow: Can convert to red marrow if needed (e.g., anemia).

Bone Markings

Bone markings serve as sites for muscle, ligament, and tendon attachment, joint formation, and passageways for blood vessels and nerves.

Name of Bone Marking	Description	Illustration
Tuberosity	Large rounded projection; may be roughened	Femur
Crest	Narrow ridge of bone; usually prominent	Ilium
Trochanter	Very large, blunt, irregularly shaped process	Femur
Line	Narrow ridge of bone; less prominent than a crest	Femur
Tubercle	Small rounded projection or process	Humerus
Epicondyle	Raised area on or above a condyle	Femur
Spine	Sharp, slender, often pointed projection	Vertebra

Name of Bone Marking	Description	Illustration
Head	Bony expansion carried on a narrow neck	Femur
Facet	Smooth, nearly flat articular surface	Rib
Condyle	Rounded articular projection	Mandible
Foramen	Round or oval opening through a bone	Skull
Fossa	Shallow, basinlike depression in a bone	Skull

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic 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 in lacunae; maintain matrix and act as stress sensors.

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

• Osteoclasts: Multinucleate cells; resorb bone matrix.

Compact Bone Structure

• Osteon (Haversian system): Structural unit; concentric lamellae around a central canal.

• Canals and canaliculi: Central (Haversian) canal, perforating (Volkmann's) canals, lacunae, and canaliculi for nutrient/waste exchange.

• Lamellae: Interstitial (fill gaps) and circumferential (encircle diaphysis).

Spongy Bone Structure

• Trabeculae align along lines of stress; contain irregular lamellae and osteocytes connected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

• Organic components: Cells and osteoid (collagen, ground substance); provide tensile strength and flexibility.

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

Bone Development (Ossification)

Ossification Types

• Endochondral ossification: Bone replaces hyaline cartilage; forms most of the skeleton.

• Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones of skull and clavicle.

Postnatal Bone Growth

• Interstitial growth: Lengthening at the epiphyseal plate (five zones: resting, proliferation, hypertrophic, calcification, ossification).

• Appositional growth: Increase in bone thickness.

Hormonal Regulation

• Growth hormone: Stimulates epiphyseal plate activity.

• Thyroid hormone: Modulates growth hormone effects.

• Sex hormones: Promote growth spurts and epiphyseal plate closure.

Bone Remodeling

• Continuous process of bone deposit (by osteoblasts) and resorption (by osteoclasts).

• Regulated by hormonal controls (parathyroid hormone, calcitonin) and mechanical stress (Wolff's law).

Bone Repair

Fracture Classification

• By position: nondisplaced vs. displaced

• By completeness: complete vs. incomplete

• By skin penetration: open (compound) vs. closed (simple)

Fracture Type	Description and Comments
Comminuted	Bone fragments into three or more pieces; common in aged
Compression	Bone is crushed; common in porous bones
Spiral	Ragged break from twisting forces; common sports fracture
Epiphyseal	Epiphysis separates from diaphysis along plate
Depressed	Broken bone pressed inward; typical of skull fracture
Greenstick	Bone breaks incompletely; common in children

Fracture Repair Stages

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Bone Disorders

Osteomalacia and Rickets

• Poor mineralization; soft, weak bones; pain on weight-bearing.

• Rickets in children: bowed legs, bone deformities; caused by vitamin D or calcium deficiency.

Osteoporosis

• Bone resorption exceeds deposit; bone mass declines; common in spine and femur.

• Risk factors: age, postmenopausal women, low exercise, poor diet, smoking, genetics, hormone disorders.

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

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

Paget’s Disease

• Excessive, disorganized bone remodeling; high spongy to compact bone ratio; reduced mineralization.

• Common in spine, pelvis, femur, skull; treated with calcitonin and bisphosphonates.

Developmental Aspects of Bone

• Embryonic skeleton ossifies predictably; fetal age can be determined by X-ray.

• Most long bones ossify by 8 weeks; primary centers by week 12.

• At birth, most long bones are ossified except at epiphyses; complete ossification by ~age 25.

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