Bones and Skeletal Tissues: Structure, Function, and Development

Bones and Skeletal Tissues

Introduction

The human skeleton is a dynamic organ system composed of bones and cartilages. It provides structural support, protection, and facilitates movement, while also serving as a reservoir for minerals and a site for blood cell formation.

Cartilage: Structure, Types, and Locations

Cartilage Structure

• Skeletal cartilage is made of resilient, molded tissue primarily composed of water, lacking blood vessels and nerves.

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

• Chondrocytes are the cells found in small cavities (lacunae) within the extracellular matrix of cartilage.

Types of Cartilage

• Hyaline cartilage: Provides support, flexibility, and resilience; most abundant; found in articular surfaces, costal cartilages, respiratory structures, and the nose.

• Elastic cartilage: Similar to hyaline but contains elastic fibers; found in the external ear and epiglottis.

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

Growth of Cartilage

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

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

• Cartilage growth ends during adolescence.

Functions of Bones

• Support: Framework for the body and soft organs.

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

• Anchorage: Acts as levers for muscle action.

• Mineral storage: Reservoir for calcium and phosphorus.

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

• Triglyceride storage: Fat stored in bone cavities for energy.

• Hormone production: Osteocalcin regulates insulin, glucose, and energy expenditure.

Classification of Bones

By Location

• Axial skeleton: Skull, vertebral column, rib cage (long axis of body).

• Appendicular skeleton: Limbs and girdles attaching them to the axial skeleton.

By Shape

• Long bones: Longer than wide, with a shaft and two ends (e.g., femur, humerus).

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

• Flat bones: Thin, flat, and slightly curved (e.g., sternum, ribs, cranial bones).

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

Bone Structure

Gross Anatomy

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

• Spongy bone: Internal mesh of trabeculae filled with marrow.

Structure of Short, Irregular, and Flat Bones

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

• Periosteum covers the outside; endosteum lines the inside.

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

• Hyaline cartilage covers joint surfaces.

Structure of a Typical Long Bone

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

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

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

• Membranes: Periosteum (outer, fibrous and inner osteogenic layers) and endosteum (lines internal surfaces).

Hematopoietic Tissue in Bones

• Red bone marrow: Found in trabecular cavities of spongy bone and diploë of flat bones; site of blood cell formation.

• In adults, most active in flat bones and some irregular bones (e.g., hip bone).

• Yellow marrow can revert to red marrow if necessary (e.g., anemia).

Bone Markings

Bone markings are anatomical features that serve as sites for muscle, ligament, and tendon attachment, joint formation, or passageways for blood vessels and nerves. They are classified as:

• Projections (e.g., tuberosity, crest, trochanter)

• Surfaces (e.g., facet, condyle)

• Depressions and openings (e.g., foramen, fossa, groove)

Microscopic Anatomy of Bone

Bone Cells

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

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

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

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

• Osteoclasts: Multinucleate cells from hematopoietic lineage; resorb bone matrix.

Compact Bone (Lamellar Bone)

• Osteon (Haversian system): Structural unit; elongated cylinder with concentric lamellae.

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

• Central (Haversian) canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae, allowing communication and nutrient/waste exchange.

• Interstitial and circumferential lamellae: Fill gaps between osteons and encircle the diaphysis, respectively.

Spongy Bone

• Composed of trabeculae aligned along lines of stress.

• Trabeculae contain irregular lamellae and osteocytes connected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

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

• Inorganic components: Hydroxyapatites (mineral salts, mainly calcium phosphate); provide hardness and resistance to compression.

Bone Development (Ossification)

Overview

• Ossification (osteogenesis): Process of bone tissue formation.

• Begins in the embryo (~8 weeks), continues through childhood, and persists as remodeling throughout life.

Endochondral Ossification

• Bone forms by replacing hyaline cartilage models (except clavicles).

• Primary ossification center forms in the diaphysis; secondary centers in epiphyses.

• Hyaline cartilage remains at epiphyseal plates and articular surfaces.

Intramembranous Ossification

• Bone develops from fibrous membranes (e.g., cranial bones, clavicles).

• Ossification centers form, osteoid is secreted and calcifies, woven bone and periosteum form, then lamellar bone replaces woven bone.

Postnatal Bone Growth

Growth in Length (Interstitial Growth)

• Occurs at the epiphyseal plate, which consists of five zones: resting, proliferation, hypertrophic, calcification, and ossification.

• Chondrocytes divide, enlarge, and are replaced by bone tissue, lengthening the bone.

• Epiphyseal plate closure marks the end of longitudinal growth (females ~18 years, males ~21 years).

Growth in Width (Appositional Growth)

• Bones thicken in response to stress by osteoblast activity beneath the periosteum and osteoclast activity on the endosteal surface.

• Usually, more bone is built than removed, resulting in stronger bones.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity.

• Thyroid hormone: Modulates growth hormone effects.

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

Bone Remodeling

• Bone is continuously renewed by the coordinated actions of osteoblasts (deposition) and osteoclasts (resorption).

• Remodeling is regulated by hormonal (calcium homeostasis) and mechanical (stress) factors.

Hormonal Control of Remodeling

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

• Calcitonin: Lowers blood calcium (minor effect in humans).

Mechanical Control (Wolff's Law)

• Bones remodel in response to mechanical stress (e.g., weight-bearing, muscle pull).

• Bone is thickest where stress is greatest; trabeculae align along lines of stress.

Bone Repair

Fracture Classification

• Position: Nondisplaced (normal alignment) vs. displaced (misaligned).

• Completeness: Complete (all the way through) vs. incomplete.

• Penetration: Open (compound) vs. closed (simple).

Common Types of Fractures

Type	Description
Comminuted	Bone fragments into three or more pieces; common in elderly.
Compression	Bone is crushed; common in osteoporotic bones.
Spiral	Ragged break from excessive twisting; common in sports injuries.
Epiphyseal	Epiphysis separates from diaphysis along growth plate.
Depressed	Broken bone portion pressed inward; typical of skull fracture.
Greenstick	Bone breaks incompletely; common in children.

Fracture Repair

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

2. Fibrocartilaginous callus formation: Collagen fibers and cartilage bridge the break.

3. Bony callus formation: Spongy bone replaces the callus.

4. Bone remodeling: Compact bone restores the shaft's original shape.

Bone Disorders

Osteomalacia and Rickets

• Osteomalacia: Poorly mineralized bones in adults; soft and weak bones, pain on weight-bearing.

• Rickets: Osteomalacia in children; results in bowed legs and deformities; caused by vitamin D deficiency or insufficient calcium.

Osteoporosis

• Bone resorption exceeds deposition; bone mass declines, especially in spongy bone of spine and femur.

• Common in postmenopausal women due to decreased estrogen.

• Risk factors: inactivity, poor diet, smoking, genetics, hormone disorders.

• Treatments: calcium, vitamin D, weight-bearing exercise, bisphosphonates, denosumab.

• Prevention: adequate calcium, reduced alcohol/soda, regular exercise.

Paget's Disease

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

• Common in spine, pelvis, femur, and skull; cause unknown, possibly viral.

• Treatment: bisphosphonates, calcitonin.

Developmental and Age-Related Aspects of Bone

• Fetal skeleton ossifies predictably; most long bones begin ossifying by 8 weeks gestation.

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

• Bone formation exceeds resorption in youth, balances in young adults, and reverses in older adults.

• Bone mass and healing ability decrease with age, except in the skull.