Bones and Skeletal Tissues: Structure, Function, and Growth

Bones and Skeletal Tissues

Overview of Cartilages in the Body

Cartilage is a resilient and smooth elastic tissue, a major component of the skeletal system. It is found in various locations throughout the body, providing support and flexibility where needed.

• Locations: Cartilage is present in the external ear, nose, joints, rib cage, intervertebral discs, and respiratory structures.

• Types: The three main types of cartilage are hyaline, elastic, and fibrocartilage.

• Function: Cartilage cushions joints, supports soft tissues, and forms the embryonic skeleton.

Types of Cartilage Tissue

Cartilage tissue is classified based on its composition and function. Each type has unique structural features and locations in the body.

• Hyaline Cartilage: Most abundant; provides support with flexibility. Found in articular surfaces, costal cartilages, nose, and respiratory passages.

• Elastic Cartilage: Contains elastic fibers; maintains shape and flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers; resists compression and tension. Found in intervertebral discs and pubic symphysis.

Growth of Cartilage

Cartilage grows by two mechanisms, both essential during development and repair.

• Appositional Growth: Chondroblasts in the perichondrium produce new cartilage on the surface.

• Interstitial Growth: Chondrocytes within the cartilage divide and secrete new matrix internally.

• Growth Limitation: Cartilage growth ceases when the skeleton stops growing.

Tissues in Bone

Bones are complex organs composed of several tissue types, each contributing to their function and structure.

• Bone Connective Tissue: Dominates bone structure.

• Nervous Tissue: Present in bone for sensory and regulatory functions.

• Blood Connective Tissue: Supplies nutrients and removes waste.

• Cartilage: Found in articular surfaces.

• Epithelial Tissue: Lines blood vessels within bone.

Functions of Bones

Bones serve multiple essential functions in the human body.

• Support: Provide a rigid framework for the body.

• Movement: Act as levers for muscles.

• Protection: Shield vital organs (e.g., brain, heart).

• Mineral Storage: Store calcium and phosphate.

• Blood Cell Formation: Red marrow produces blood cells.

• Energy Metabolism: Osteoblasts secrete osteocalcin, influencing metabolism.

Classification of Bones

Bones are classified by their shape and structure, which relates to their function.

• Long Bones: Longer than wide; have a shaft and two ends (e.g., femur).

• Short Bones: Cube-shaped (e.g., carpals).

• Flat Bones: Thin, flattened, often curved (e.g., sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae).

Gross Anatomy of Bones

Bones have two main structural types: compact and spongy bone.

• Compact Bone: Dense outer layer providing strength.

• Spongy (Cancellous) Bone: Internal network of trabeculae, filled with marrow.

Structure of a Typical Long Bone

Long bones have distinct anatomical regions and features.

• Diaphysis: Shaft of the bone.

• Epiphyses: Ends of the bone.

• Medullary Cavity: Central cavity containing yellow marrow.

• Membranes: Periosteum (outer), endosteum (inner), and Sharpey's fibers (collagen bundles).

• Blood Vessels: Bones are highly vascularized.

Structure of a Flat Bone

Flat bones consist of two layers of compact bone enclosing a layer of spongy bone (diploë).

Bone Markings

Bone markings are features on the surface of bones that reflect stresses and serve as sites for muscle attachment, joint formation, or passage of vessels and nerves.

• Projections: For muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Surfaces: Form joints (e.g., head, facet, condyle).

• Depressions and Openings: Allow passage of vessels and nerves (e.g., foramen, groove, fissure).

Name of Bone Marking	Description	Illustration
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	Femur
Line	Narrow ridge of bone; less prominent than crest	Femur
Tubercle	Small rounded projection or process	Femur
Epicondyle	Raised area on or above a condyle	Femur
Spine	Sharp, slender, often pointed projection	Spinous process
Process	Any bony prominence	Any bone

Surfaces That Form Joints	Description	Illustration
Head	Bony expansion carried on a narrow neck	Head of femur
Facet	Smooth, nearly flat articular surface	Facet of rib
Condyle	Rounded articular projection	Condyle of mandible

Depressions and Openings	Description	Illustration
Foramen	Round or oval opening through a bone	Skull
Groove	Furrow	Skull
Fissure	Narrow, slitlike opening	Skull
Notch	Indentation at the edge of a structure	Skull
Fossa	Shallow basinlike depression	Skull
Meatus	Canal-like passageway	Skull
Sinus	Cavity within a bone, filled with air and lined with mucosa	Skull

Microscopic Structure of Compact Bone

Compact bone is organized into osteons, which are cylindrical structures that provide support and house blood vessels and nerves.

• Osteons (Haversian Systems): Resemble tree rings in cross-section.

• Lamellae: Concentric layers of bone matrix.

• Central Canal: Contains blood vessels and nerves.

• Perforating Canals: Connect central canals.

• Canaliculi: Small channels connecting osteocytes.

Microscopic Structure of Spongy Bone

Spongy bone is less dense and consists of trabeculae, which are small, beam-like structures containing lamellae and osteocytes.

• Trabeculae: Network of bone beams.

• Marrow Spaces: Filled with red or yellow marrow.

• Osteocytes: Bone cells within trabeculae.

• Endosteum: Membrane lining internal bone surfaces.

Organization of Cartilage Within the Epiphyseal Plate

The epiphyseal plate is a region of growing long bones, organized into distinct zones that facilitate bone elongation.

• Resting Zone: Quiescent cartilage cells.

• Proliferation Zone: Rapidly dividing cartilage cells.

• Hypertrophic Zone: Enlarged cartilage cells.

• Calcification Zone: Matrix becomes calcified; cells die.

• Ossification Zone: New bone forms.

Remodeling of Spongy Bone

Bone remodeling is a continuous process involving the resorption of old bone and deposition of new bone by osteoclasts and osteoblasts.

• Osteoclasts: Cells that break down bone matrix.

• Osteoblasts: Cells that build new bone matrix.

• Remodeling: Maintains bone strength and mineral homeostasis.

Key Equations and Concepts

• Bone Matrix Composition: $\text{Bone Matrix} = \text{Collagen Fibers} + \text{Hydroxyapatite (Ca}_3\text{(PO}_4\text{)}_2\text{)}$

• Osteon Structure: $\text{Osteon} = \text{Central Canal} + \text{Lamellae} + \text{Osteocytes in Lacunae} + \text{Canaliculi}$

Summary Table: Bone Types and Features

Bone Type	Shape	Example
Long Bone	Longer than wide	Femur
Short Bone	Cube-shaped	Carpals
Flat Bone	Thin, flattened, curved	Sternum
Irregular Bone	Complex shape	Vertebrae

Additional info:

• Bone remodeling is influenced by mechanical stress and hormonal regulation.

• Osteocalcin, secreted by osteoblasts, plays a role in energy metabolism and regulation of blood glucose.