Bones and Skeletal Tissues: Structure, Types, and Functions

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Bones and Skeletal Tissues

Introduction

The skeletal system is composed of bones and associated connective tissues, including cartilage. These structures provide support, protection, movement, and play key roles in mineral storage and blood cell formation.

Cartilages

General Properties of Cartilage

Cartilage is a resilient, flexible connective tissue that initially forms the skeleton in embryos and is later replaced by bone in most locations.
It is avascular (contains no blood vessels) and lacks nerves.
The perichondrium is a dense irregular connective tissue layer that surrounds cartilage, providing nutrients and resisting outward expansion.
Chondrocytes are the primary cells of cartilage, residing in small spaces called lacunae within the extracellular matrix.

Types and Locations of Cartilage

Hyaline cartilage: Provides support and flexibility; most abundant type; contains only collagen fibers.
- Locations: Articular surfaces (joints), costal cartilage (ribs), respiratory structures (larynx), and nose tip.
Elastic cartilage: Provides elasticity; similar to hyaline but contains more elastic fibers.
- Locations: External ear and epiglottis.
Fibrocartilage: Provides cushioning and tensile strength; contains thick collagen fibers.
- Locations: Intervertebral discs, menisci of knee.

Growth of Cartilage

Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix on the surface of cartilage.
Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Functions of Bones

Bones perform several vital functions in the body:

Support: Provide structural framework for the body and soft organs.
Protection: Shield the brain, spinal cord, and vital organs.
Movement: Serve as levers for muscle action.
Mineral and growth factor storage: Store calcium, phosphorus, and growth factors.
Blood cell formation (Hematopoiesis): Occurs in red marrow cavities of certain bones.
Triglyceride (fat) storage: Fat is stored in yellow marrow cavities for energy.
Hormone production: Osteocalcin, produced by osteoblasts, regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

Axial and Appendicular Skeleton

Axial skeleton: Forms the long axis of the body; includes the skull, vertebral column, rib cage, sternum, and costal cartilages.
Appendicular skeleton: Comprises the upper and lower limbs and the girdles (shoulder and hip bones) that attach limbs to the axial skeleton.

Bone Shapes

Long bones: Longer than they are wide (e.g., femur, humerus).
Short bones: Cube-shaped (e.g., bones of the wrist and ankle); includes sesamoid bones (e.g., patella) that form within tendons.
Flat bones: Thin, flattened, and usually curved (e.g., sternum, ribs, most skull bones).
Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

Structure of Bone

Gross Anatomy

Compact bone: Dense outer layer; smooth and solid.
Spongy bone: Internal honeycomb of trabeculae (small needle-like or flat pieces); spaces filled with bone marrow.

Structure of Short, Irregular, and Flat Bones

Consist of thin plates of spongy bone (diploë) covered by compact bone.
Periosteum covers the outside; endosteum covers the inside.
No shaft or expanded ends; bone marrow is present but no defined marrow cavity.

Structure of Long Bones

Diaphysis: Shaft forming the long axis; composed of compact bone surrounding a medullary cavity (contains yellow marrow in adults).
Epiphyses: Bone ends; consist of compact bone externally and spongy bone internally; articular cartilage covers joint surfaces.
Epiphyseal line/plate: Remnant of childhood growth plate where bone lengthening occurs.

Membranes

Periosteum: Double-layered membrane covering external bone surfaces (except joints); outer fibrous layer (dense irregular connective tissue) and inner osteogenic layer (contains stem cells).
Endosteum: Delicate membrane covering internal bone surfaces, including trabeculae of spongy bone and canals of compact bone; contains osteogenic cells.

Bone Markings

Bones display various markings that serve as sites for muscle, ligament, and tendon attachment, joint formation, or as conduits for blood vessels and nerves.

Projections: Bulges where muscles and ligaments attach or where joints are formed.
Depressions and openings: Grooves, holes, or canals that allow passage of blood vessels and nerves.

Microscopic Anatomy of Bone

Bone Cells

Osteogenic (osteoprogenitor) cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.
Osteoblasts: Bone-forming cells; 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 sensors.
Bone-lining cells: Flat cells on bone surfaces; help maintain matrix.
Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown); derived from hematopoietic stem cells.

Compact Bone Structure

Osteon (Haversian system): Structural unit of compact bone; elongated cylinder parallel to bone's long axis; consists of concentric lamellae (rings) of bone matrix.
Lamellae: Collagen fibers in adjacent lamellae 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: Perpendicular to central canals; connect blood vessels and nerves of periosteum, medullary cavity, and central canal.
Lacunae: Small cavities containing osteocytes.
Canaliculi: Tiny canals connecting lacunae; allow communication and nutrient/waste exchange between osteocytes.
Interstitial and circumferential lamellae: Fill gaps between osteons and extend around the entire circumference of the diaphysis, respectively.

Spongy Bone Structure

Trabeculae align along lines of stress, providing strength without excessive weight.
Contains irregularly arranged lamellae and osteocytes interconnected by canaliculi.
Spaces between trabeculae are filled with bone marrow.

Chemical Composition of Bone

Organic components: Include bone cells (osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts) and osteoid (ground substance and collagen fibers), which provide tensile strength and flexibility.
Inorganic components: Mainly hydroxyapatite (mineral salts, primarily calcium phosphate), accounting for about 65% of bone mass; responsible for bone's hardness and resistance to compression.

Key Equations

Hydroxyapatite chemical formula:

Summary Table: Types of Cartilage

Type	Main Fiber	Function	Location
Hyaline	Collagen	Support, flexibility	Joints, ribs, larynx, nose
Elastic	Elastic fibers	Flexibility, elasticity	Ear, epiglottis
Fibrocartilage	Thick collagen	Shock absorption, tensile strength	Intervertebral discs, knee menisci

Additional info:

Bone remodeling is a dynamic process involving osteoblasts (formation) and osteoclasts (resorption).
Disorders of bone cells can lead to diseases such as osteoporosis (increased osteoclast activity) or osteopetrosis (decreased osteoclast activity).