Bones and Skeletal Tissue: Structure, Function, and Classification

Bones and Skeletal Tissue

Introduction

The skeletal system forms the supporting framework of the human body. It is composed of bones and skeletal cartilages, each with specialized structures and functions. Understanding bone anatomy and physiology is essential for diagnosing and treating bone-related diseases such as osteoporosis.

Skeletal Cartilages

Basic Structure, Types, and Locations

• Skeletal cartilage is a resilient, molded tissue consisting mainly of water, providing flexibility and cushioning in the skeleton. It contains no blood vessels or nerves.

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

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

Types of Cartilage

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

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

• Fibrocartilage: Contains thick collagen fibers; provides tensile strength. Found in menisci of the knee and 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, but calcified cartilage is not bone.

Functions of Bones

Seven Major Functions

1. Support: Provides structural framework for the body and soft organs.

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

3. Movement: Acts as levers for muscle action.

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

5. Blood cell formation: Hematopoiesis occurs in red marrow cavities.

6. Triglyceride storage: Stores fat in bone cavities for energy.

7. Hormone production: Osteocalcin regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

By Location

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

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

By Shape

• Long bones: Longer than wide (e.g., limb bones).

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

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

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

Bone Structure

Gross Anatomy

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

• Spongy bone: Honeycomb structure of trabeculae filled with red or yellow marrow.

Structure of Short, Irregular, and Flat Bones

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

• Periosteum covers the outside; endosteum covers the inside.

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

• Hyaline cartilage covers areas involved in movable joints.

Structure of Long Bones

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

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

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

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces (except joints). Contains fibrous and osteogenic layers, blood vessels, and nerves. Anchoring point for tendons and ligaments.

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

Hematopoietic Tissue in Bones

• Red marrow: Found in trabecular cavities of spongy bone and diploë of flat bones. In adults, mainly in flat bones and some irregular bones (e.g., hip bone).

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

Bone Markings

• Serve as sites for muscle, ligament, and tendon attachment, joint formation, and passageways for blood vessels and nerves.

• Three main types: projections, depressions, and openings.

Name of Bone Marking	Description	Illustrations
Tuberosity	Large rounded projection; may be roughened	Hip bone
Crest	Narrow ridge of bone; usually prominent	Hip bone
Trochanter	Very large, blunt, irregularly shaped process (only on femur)	Femur
Line	Narrow ridge of bone; less prominent than a crest	Femur
Tubercle	Small rounded projection or process	Femur
Epicondyle	Raised area on or above a condyle	Femur
Spine	Sharp, slender, often pointed projection	Vertebra
Process	Any bony prominence	Vertebra

Name of Bone Marking	Description	Illustrations
Head	Bony expansion carried on a narrow neck	Femur
Facet	Smooth, nearly flat articular surface	Rib
Condyle	Rounded articular projection	Mandible
Ramus	Armlike bar of bone	Mandible
Fossa	Shallow, basinlike depression in a bone	Skull
Meatus	Canal-like passageway	Skull
Sinus	Cavity within a bone, filled with air and lined with mucous membrane	Skull
Groove	Furrow	Skull
Fissure	Narrow, slitlike opening	Skull
Foramen	Round or oval opening through a bone	Skull
Notch	Indentation at the edge of a structure	Skull

Microscopic Anatomy of Bone

Cells of Bone Tissue

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

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

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

• Bone-lining cells: Flat cells on bone surfaces; help maintain matrix (periosteal cells externally, endosteal cells internally).

• Osteoclasts: Multinucleate cells derived from hematopoietic stem cells; responsible for bone resorption.

Compact Bone (Lamellar Bone)

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

• Lamellae: Collagen fibers in adjacent rings run in different directions, providing strength and resistance to twisting.

• 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: Hairlike canals connecting lacunae; allow communication and nutrient/waste exchange between osteocytes.

• Interstitial lamellae: Fill gaps between osteons or are remnants of old osteons.

• Circumferential lamellae: Extend around the entire diaphysis; help resist twisting.

Spongy Bone

• Composed of trabeculae aligned along lines of stress, providing strength without excess weight.

• 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: One-third of bone matrix; consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Resilience is due to sacrificial bonds in or between collagen molecules, which stretch and break to 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 hardness and resistance to compression.

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

• Mineral composition allows bones to last long after death and provide archaeological information.