Bones and Skeletal Tissues

Why This Matters

Understanding bone anatomy and the process of bone remodeling is essential for diagnosing and treating bone diseases such as osteoporosis. Knowledge of skeletal tissues underpins effective clinical practice in musculoskeletal health.

Cartilages Help Form the Skeleton

Structure and Properties of Skeletal Cartilage

• Skeletal cartilage is a highly resilient, molded tissue consisting primarily of water, which provides flexibility and cushioning.

• Cartilage contains no blood vessels or nerves, making it avascular and aneural.

• The perichondrium is a dense connective tissue layer surrounding cartilage, acting as a girdle to resist outward expansion and containing blood vessels for nutrient delivery.

• Cartilage is composed of chondrocytes, cells encased in small cavities (lacunae) within a jelly-like extracellular matrix.

Types of Cartilage

• Hyaline cartilage: Provides support, flexibility, and resilience; most abundant type; contains only collagen fibers. Found in articular (joints), costal (ribs), respiratory (larynx and passageways), and nasal cartilage.

• Elastic cartilage: Similar to hyaline but contains both collagen and elastic fibers. Located in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers for great 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 of cartilage by cells in the perichondrium.

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

• Cartilage growth ends during adolescence.

Functions of Bones

Major Functions

• Support: Provides structural 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: Stores calcium and phosphorus, essential for physiological processes.

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

• Triglyceride (fat) storage: Stores energy reserves in bone cavities.

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

Classification of Bones

By Location

• Axial skeleton: Forms the long axis of the body, including the skull, vertebral column, and rib cage.

• Appendicular skeleton: Comprises bones of the upper and lower limbs and girdles attaching limbs to the axial skeleton.

By Shape

• Long bones: Longer than wide, with a medullary cavity. Example: most limb bones.

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

• Flat bones: Thin, flat, and slightly curved. Example: sternum, ribs, cranial bones.

• Irregular bones: Complicated shapes. Example: vertebrae, hip bones.

Bone Structure

Bone as an Organ

• Bones contain various tissues: bone (osseous) tissue, nervous tissue, cartilage, dense connective tissue, muscle cells, and epithelial cells in blood vessels.

• Three levels of structure: Gross, Microscopic, and Chemical.

Compact and Spongy Bone

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

• Spongy bone: Honeycomb of small, needle-like or flat pieces called trabeculae. Spaces between trabeculae are filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

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

• Compact bone is sandwiched between connective tissue membranes: periosteum (outside) and endosteum (inside).

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

• Hyaline cartilage covers areas of bone involved in movable joints.

Structure of a Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding a central medullary cavity filled with yellow bone marrow in adults.

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

• Epiphyseal line: Remnant of childhood epiphyseal plate where bone growth occurs.

Membranes of Long Bones

• Periosteum: White, double-layered membrane covering external surfaces except joints. Contains fibrous (outer) and osteogenic (inner) layers, nerve fibers, blood vessels, and serves as anchoring points for tendons and ligaments.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone and canals in compact bone. Contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red bone marrow is found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In newborns, medullary cavities and all spongy bone contain red marrow; in adults, active hematopoiesis occurs mainly in flat bone diploë and some irregular bones (e.g., hip bone).

• Yellow bone marrow can convert to red marrow if the person becomes anemic.

Bone Markings

Categories of Bone Markings

• Projections: Sites of muscle and ligament attachment.

• Surfaces: Form joints.

• Depressions and Openings: Allow passage for blood vessels and nerves.

Microscopic Anatomy of Bone

Major Cell Types

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

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), composed of collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress/strain sensors.

• Bone-lining cells: Flat cells on bone surfaces; help maintain matrix. Called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Giant, multinucleate cells derived from hematopoietic stem cells; function in bone resorption (breakdown).

Microscopic Anatomy of Compact Bone

• Osteon (Haversian system): Structural unit; elongated cylinder running parallel to bone's long axis, composed of concentric rings (lamellae) of bone matrix.

• Lamellae contain collagen fibers running in different directions, providing strength and resistance to twisting.

• Bone salts are found between collagen fibers.

• Central (Haversian) canal: Runs through the core of each osteon; contains blood vessels and nerve fibers.

• 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 to each other and to the central canal, enabling communication and nutrient/waste exchange.

• Interstitial lamellae: Fill gaps between forming osteons or are remnants of osteons cut by remodeling.

• Circumferential lamellae: Extend around the entire surface of the diaphysis, helping resist twisting.

Microscopic Anatomy of Spongy Bone

• Composed of trabeculae arranged along lines of stress, providing strength.

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

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid.

• Osteoid: Makes up one-third of organic bone matrix; secreted by osteoblasts; consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Resilience due to sacrificial bonds between collagen molecules that dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up 65% of bone by mass; mainly tiny calcium phosphate crystals in and around collagen fibers.

• Responsible for bone's hardness and resistance to compression.

• Bones are half as strong as steel in resisting compression and as strong as steel in tension.

Bone Development

Ossification (Osteogenesis)

• Process of bone tissue formation.

• Formation of bony skeleton begins in month 2 of development.

• Postnatal bone growth continues until early adulthood; bone remodeling and repair are lifelong.

Formation of the Bony Skeleton

• Before week 8, fetal skeleton consists of fibrous membranes and hyaline cartilage, which are replaced by bone tissue.

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

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

Endochondral Ossification: Steps

1. Bone collar forms around diaphysis of cartilage model.

2. Central cartilage in diaphysis calcifies, then develops cavities.

3. Periosteal bud invades cavities, forming spongy bone.

4. Diaphysis elongates, medullary cavity forms.

5. Secondary ossification centers appear in epiphyses.

6. Epiphyses ossify; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Intramembranous Ossification: Steps

1. Ossification centers form when mesenchymal cells cluster and become osteoblasts.

2. Osteoid is secreted and then calcified.

3. Woven bone forms around blood vessels, resulting in trabeculae; outer layer forms periosteum.

4. Lamellar bone replaces woven bone; red bone marrow appears.

Growth in Length and Width of Bones

Growth in Length (Interstitial Growth)

• Requires presence of epiphyseal cartilage (epiphyseal plate).

• Plate maintains constant thickness; rate of cartilage growth balanced by bone replacement.

• Five zones: Resting, Proliferation (growth), Hypertrophic, Calcification, Ossification.

• Bone lengthening ceases at epiphyseal plate closure (females ~18 years, males ~21 years).

Growth in Width (Appositional Growth)

• Bones widen as they lengthen; can occur throughout life.

• Osteoblasts beneath periosteum secrete bone matrix; osteoclasts remove bone on endosteal surface.

• Usually more building up than breaking down, resulting in thicker, stronger bones.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity in infancy and childhood.

• Thyroid hormone: Modulates growth hormone activity for proper proportions.

• Testosterone (males) and estrogens (females): Promote adolescent growth spurts and induce epiphyseal plate closure.

• Imbalances cause abnormal skeletal growth.

Bone Remodeling

Overview

• Bone remodeling replaces 5-10% of bone annually; spongy bone every 3-4 years, compact bone every 10 years.

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

• Occurs at surfaces of periosteum and endosteum; remodeling units coordinate the process.

Bone Resorption

• Osteoclasts digest matrix by secreting lysosomal enzymes and protons (H+), converting calcium salts to soluble forms.

• Phagocytize demineralized matrix and dead osteocytes; products released into blood.

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

Bone Deposition

• Osteoblasts deposit new bone matrix (osteoid seam).

• Calcification front: transition zone between new and old bone.

• Triggers include mechanical signals, increased calcium/phosphate, matrix proteins, and alkaline phosphatase.

Control of Remodeling

• Regulated by two control loops:

  • Calcium homeostasis: Hormonal negative feedback (PTH, calcitonin) maintains blood calcium at 9-11 mg/dl.

  • Mechanical/gravitational forces: Drive remodeling to keep bone strong.

Fracture Treatment and Repair

Stages of Healing

1. Hematoma forms: Blood vessels hemorrhage, forming a mass of clotted blood.

2. Fibrocartilaginous callus forms: Capillaries grow into hematoma; fibroblasts, cartilage, and osteogenic cells reconstruct bone.

3. Bony callus forms: New trabeculae appear; callus is converted to bony (hard) callus of spongy bone.

4. Bone remodeling occurs: Excess material is removed; compact bone reconstructs shaft walls.

Bone Disorders

Osteomalacia and Rickets

• Osteomalacia: Poorly mineralized bones; osteoid produced but calcium salts not deposited, resulting in soft, weak bones and pain.

• Rickets: Osteomalacia in children; results in bowed legs and bone deformities due to enlarged, abnormally long bone ends. Caused by vitamin D deficiency or insufficient calcium.

Osteoporosis

• Bone resorption exceeds deposit; bone mass declines but matrix remains normal.

• Spongy bone of spine and neck of femur most susceptible; vertebral and hip fractures common.

• Risk factors: aged, postmenopausal females, insufficient exercise, poor diet, smoking, genetics, hormone-related conditions, alcohol, and certain medications.

• Treatments: calcium, vitamin D, weight-bearing exercise, hormone replacement therapy (controversial).

Paget's Disease

• Excessive and haphazard bone deposit and resorption; bones grow fast and develop poorly (Pagetic bone).

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

• Usually affects spine, pelvis, femur, and skull; rare before age 40.

• Cause unknown; possibly viral. Treatments include bisphosphonates and high doses of calcitonin.

Developmental and Age-Related Changes

From Birth to Adulthood

• Most long bones ossified at birth except at epiphyses; epiphyseal plates persist through childhood and adolescence.

• By age 25, all bones are completely ossified; skeletal growth ceases.

Age-Related Changes in Bone

• In children/adolescents, bone formation exceeds resorption; males tend to have greater mass.

• In young adults, formation and resorption are balanced.

• In adults, resorption exceeds formation; bone mass, mineralization, and healing ability decrease with age (except skull bones).

• Bone loss is greater in whites and females; genetics play a major role in bone density and osteoporosis risk.