Bones and Skeletal Tissues: Structure, Function, and Development

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Bones and Skeletal Tissues

Introduction

The skeletal system is composed of bones and cartilages that provide structure, support, and protection to the human body. This chapter explores the types, structure, growth, and functions of bones and skeletal cartilages.

Skeletal Cartilages

General Characteristics

Cartilage is a resilient, flexible connective tissue that initially forms the human skeleton before being replaced by bone in most areas.
Composed primarily of water, which allows it to withstand compression and return to its original shape.
Contains no blood vessels or nerves, making it aneural and avascular.
Cells called chondrocytes are encased in small cavities called lacunae within an extracellular matrix.

Types of Cartilage

Hyaline cartilage (strongest)
- Provides support, flexibility, and resilience.
- Most abundant type; contains only collagen fibers.
- Locations: articular (joints), costal (ribs), respiratory (larynx), nasal (nose).
Elastic cartilage (weakest)
- Similar to hyaline cartilage but contains elastic fibers for greater flexibility.
- Locations: external ear, epiglottis.
Fibrocartilage (middle strength)
- Contains thick collagen fibers, providing great tensile strength.
- Locations: menisci of knee, intervertebral discs.

Comparison of Cartilage and Bone

Cartilage	Bone
Surrounded by perichondrium	Surrounded by periosteum
No blood vessels or nerves (except in perichondrium)	Blood vessels and nerves throughout
Chondrocytes in lacunae	Osteocytes in lacunae
Flexible extracellular matrix	Rigid extracellular matrix (due to inorganic calcium salts)
Matrix made by chondroblasts	Matrix (organic part) made by osteoblasts
Appositional and interstitial growth	Appositional growth only

Growth of Cartilage

Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix on the surface of existing cartilage.
Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.
Cartilage growth is important during normal bone growth and in old age.

Functions of Bones

Overview

Bones perform several vital functions necessary for survival and movement.

Support: Provide a framework for the body and support soft organs.
Protection: Encase and protect the brain, spinal cord, and vital organs.
Movement: Serve as levers for muscle action.
Mineral and growth factor storage: Reservoir for minerals such as calcium and phosphorus, and storage of growth factors.
Blood cell formation: Hematopoiesis occurs in red marrow cavities of certain bones.
Triglyceride (fat) storage: Fat stored in bone cavities serves as an energy source.
Hormone production: Osteocalcin, secreted by bones, helps regulate insulin secretion, glucose levels, and metabolism.

Classification of Bones

By Shape

Long bones: Longer than they are wide (e.g., limb bones).
Short bones: Cube-shaped (e.g., wrist and ankle bones); includes sesamoid bones (e.g., patella).
Flat bones: Thin, flat, and slightly curved (e.g., sternum, scapulae, ribs, most skull bones).
Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure

Gross Anatomy

Compact bone: Dense outer layer that appears smooth and solid.
Spongy bone: Internal layer made up of a honeycomb of small, needle-like or flat pieces called trabeculae; spaces are filled with red or yellow bone marrow.

Structure of a Long Bone

Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding a medullary cavity filled with yellow marrow.
Epiphyses: Ends of long bones; consist of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.
Epiphyseal line: Remnant of the childhood epiphyseal plate where bone growth occurred.

Membranes

Periosteum: Double-layered membrane covering external bone surfaces (except at joints); contains nerve fibers, blood vessels, and serves as an anchoring point for tendons and ligaments.
Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone.

Bone Marrow

Red marrow: Site of hematopoiesis; found in trabecular cavities of spongy bone and diploë of flat bones.
Yellow marrow: Stores fat; can convert to red marrow if necessary (e.g., in anemia).

Bone Markings

Projections: Outward bulges for muscle, ligament, and tendon attachment.
Depressions: Bowl- or groove-like cut-outs for passage of vessels and nerves or joint formation.
Openings: Holes or canals for passage of blood vessels and nerves.

Microscopic Anatomy of Bone

Bone Cells

Osteogenic (osteoprogenitor) cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts.
Osteoblasts: Bone-forming cells that secrete bone matrix (osteoid), which is rich in collagen and calcium-binding proteins.
Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors.
Osteoclasts: Multinucleate cells derived from hematopoietic stem cells; responsible for bone resorption (breakdown).

Compact Bone (Lamellar Bone)

Osteon (Haversian system): Structural unit of compact bone; consists of concentric rings (lamellae) of bone matrix.
Central (Haversian) canal: Runs through the core of each osteon; 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: Tiny canals connecting lacunae, allowing communication and nutrient/waste exchange between osteocytes.
Lamellae types: Concentric (osteon), interstitial (between osteons), circumferential (around the diaphysis).

Spongy Bone

Composed of trabeculae aligned along lines of stress for strength.
No osteons, but 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, osteoclasts, and osteoid (ground substance and collagen fibers).
Osteoid makes up one-third of organic bone matrix and provides flexibility and tensile strength.

Inorganic Components

Primarily hydroxyapatite (mineral salts), mainly calcium phosphate crystals.
Responsible for bone hardness and resistance to compression.

Bone Development (Ossification)

Overview

Ossification (osteogenesis): Process of bone tissue formation.
Begins in the embryo (around week 8) and continues throughout life as growth and remodeling.

Types of Ossification

Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most bones below the skull except clavicles.
Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones (e.g., cranial bones, clavicles).

Steps in Endochondral Ossification

Primary ossification center forms in the center of the shaft.
Blood vessels infiltrate perichondrium, converting it to periosteum; mesenchymal cells become osteoblasts.
Bone collar forms around diaphysis of cartilage model.
Central cartilage in diaphysis calcifies and develops cavities.
Periosteal bud invades cavities, forming spongy bone.
Diaphysis elongates and medullary cavity forms; secondary ossification centers appear in epiphyses.
Epiphyses ossify; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Steps in Intramembranous Ossification

Ossification centers form when mesenchymal cells cluster and become osteoblasts.
Osteoid is secreted and then calcified.
Woven bone and periosteum form.
Lamellar bone replaces woven bone; red marrow appears.

Postnatal Bone Growth

Growth in Length

Occurs at the epiphyseal plate via interstitial growth of cartilage.
Epiphyseal plate closure occurs when epiphysis and diaphysis fuse, ending lengthwise growth (around age 18 in females, 21 in males).

Growth in Width (Appositional Growth)

Bones thicken in response to increased stress from muscle activity or added weight.
Osteoblasts beneath periosteum secrete bone matrix on external bone surface; osteoclasts remove bone on endosteal surface.

Hormonal Regulation of Bone Growth

Growth hormone: Stimulates epiphyseal plate activity in infancy and childhood.
Thyroid hormone: Modulates growth hormone activity for proper bone proportions.
Sex hormones (testosterone, estrogens): Promote adolescent growth spurts and induce epiphyseal plate closure.
Imbalances can cause abnormal skeletal growth.

Bone Remodeling and Repair

Bone Remodeling

Continuous process involving bone deposit (by osteoblasts) and resorption (by osteoclasts).
Regulated by hormonal controls (e.g., parathyroid hormone, calcitonin) and mechanical stress.

Hormonal Control of Blood Calcium

Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts to resorb bone.
Calcitonin: Lowers blood calcium levels (minor effect in humans).

Bone Repair

Hematoma formation: Blood vessels break, forming a clot at the fracture site.
Fibrocartilaginous callus formation: Capillaries grow into hematoma; fibroblasts and chondroblasts form a soft callus.
Bony callus formation: New trabeculae form a hard callus, converting soft callus to spongy bone.
Bone remodeling: Compact bone replaces spongy bone; bone regains original shape and strength.

Bone Disorders

Osteomalacia and Rickets

Osteomalacia: Poorly mineralized bones in adults; bones are soft and weak due to inadequate calcium deposition.
Rickets: Osteomalacia in children; results in bowed legs and bone deformities, often due to vitamin D or calcium deficiency.

Osteoporosis

Bone resorption exceeds deposition, leading to decreased bone mass and increased fracture risk.
Common in postmenopausal women due to decreased estrogen.
Prevention includes adequate calcium and vitamin D intake, weight-bearing exercise, and avoiding smoking and excessive alcohol.

Paget's Disease

Excessive and haphazard bone deposit and resorption; bone is made quickly and poorly, leading to weak spots.
Usually affects spine, pelvis, femur, and skull.

Developmental and Age-Related Aspects of Bone

Skeleton ossifies predictably, allowing fetal age estimation by X-ray or sonograms.
Bone formation exceeds resorption in children and adolescents; balanced in young adults; resorption exceeds formation in older adults.
Bone mass, mineralization, and healing ability decrease with age, especially in females and individuals of European descent.

Key Terms

Chondrocyte: Cartilage cell in a lacuna.
Osteocyte: Mature bone cell in a lacuna.
Osteoblast: Bone-forming cell.
Osteoclast: Bone-resorbing cell.
Osteoid: Unmineralized bone matrix.
Epiphyseal plate: Growth plate of cartilage in long bones.
Trabeculae: Lattice-like network in spongy bone.

Additional info: For equations related to bone mineralization, the formation of hydroxyapatite can be represented as: