Bone Tissue and the Skeletal System: Structure, Function, and Classification

Bone Tissue & Skeletal System

Overview of the Skeletal System

The skeletal system is a fundamental organ system in the human body, providing structure, protection, and support. It consists of bones, cartilages, ligaments, and other connective tissues that stabilize and connect the body.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage (head, neck, trunk).

• Appendicular Skeleton: Comprises the pectoral girdle, pelvic girdle, and the upper and lower limbs.

• Connective Tissues: Cartilages and ligaments help stabilize joints and connect bones.

Functions of the Skeletal System

The skeletal system performs several vital functions essential for life and movement.

• Support: Maintains and supports body posture and structure.

• Protection: Shields internal vital organs such as the brain (skull) and heart (rib cage).

• Attachment: Provides sites for tendons and muscles to attach, enabling movement.

• Movement: Acts as levers for muscles to produce movement.

• Metabolic Function: Serves as a reservoir for minerals, especially calcium, contributing to calcium homeostasis and other mineral balances.

• Blood Cell Production: Contains marrow cavities that house hemopoietic tissue (bone marrow), which produces blood cells.

Bone Osseous Tissue

Definition and Properties

Bone (Osseous) Tissue is a specialized form of supporting connective tissue in which the matrix is modified to be hard and tensile. This unique composition allows bones to withstand compression and tension.

• Hardness: Due to mineral deposits, primarily calcium phosphate.

• Tensile Strength: Provided by collagen fibers in the matrix.

Classification of Bones

Criteria for Bone Classification

Bones are classified based on their shape, surface markings, and internal microscopic (histological) appearance.

• Shape: Long, short, flat, irregular, sesamoid, and sutural bones.

• Bone Markings: Features such as projections, depressions, and openings that serve as attachment points or passageways.

• Histology: Compact (cortical) and spongy (cancellous) bone types.

Bone Shapes

• Long Bones: Longer than they are wide; found in arms, legs, fingers, and toes (e.g., humerus, femur).

• Short Bones: Approximately equal in length and width; cube-shaped (e.g., carpals of the wrist, tarsals of the ankle).

• Flat Bones: Thin and broad with parallel surfaces (e.g., skull, sternum, ribs, scapula).

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

• Sesamoid Bones: Small, flat bones that develop within tendons (e.g., patella).

• Sutural Bones: Small, irregular bones found between the flat bones of the skull.

Bone Markings

Bone markings are anatomical features on the surface of bones that serve various functions.

• Depressions or Grooves: Allow passage of blood vessels and nerves along the bone surface.

• Projections: Sites where tendons and ligaments attach or where bones articulate.

• Openings or Tunnels: Permit the entry of blood vessels and nerves into the bone.

Histological Classification of Bone

Types of Bone Tissue

• Compact (Cortical) Bone: Densely organized tissue that surrounds a single central marrow cavity. Functions to support weight and withstand tension and twisting forces.

• Spongy (Cancellous/Trabecular) Bone: Loosely organized tissue with multiple marrow cavities. Provides strength with minimal weight and supports bone marrow.

Locations

• Compact Bone: Forms the outer shell of all bones and the shafts (diaphysis) of long bones.

• Spongy Bone: Found in the ends (epiphyses) of long bones and inside flat, irregular, and short bones.

Bone Membranes

Periosteum and Endosteum

Bones are covered and lined by specialized membranes that play roles in growth, repair, and nutrition.

• Periosteum: Covers the external surface of bones except at joints. Composed of an outer fibrous layer and an inner cellular layer containing bone-forming cells.

• Endosteum: Lines the internal marrow cavity and is active in bone growth and repair.

Functions of Periosteum

• Isolates bone from surrounding tissues.

• Provides a route for blood vessels and nerves.

• Participates in bone growth and healing.

• Serves as an attachment point for tendons and ligaments.

Sharpey's Fibers

Sharpey's fibers are collagen fibers that anchor the periosteum to the underlying bone matrix, providing stability.

General Features of Bones

Long Bones

• Diaphysis: The shaft, composed of compact bone surrounding a central medullary cavity lined with endosteum.

• Epiphyses: The ends, consisting of spongy bone covered by compact bone, enlarged for joint strength and ligament attachment.

• Metaphysis: The region between diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in growing bones.

• Articular Cartilage: Covers joint surfaces for smooth movement.

Flat Bones

• Composed of external and internal layers of compact bone with a middle layer of spongy bone (diploë) containing marrow cavities.

Histology of Bone (Osseous) Tissue

Components of Bone Tissue

• Specialized Cells: Osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

• Fibers: Collagen fibers arranged in sheets.

• Ground Substance: Organic (osteoid: proteoglycans, glycoproteins) and inorganic (calcium salts, mainly hydroxyapatite).

Types of Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Stem cells located in periosteum and endosteum; give rise to osteoblasts.

• Osteoblasts: Bone-forming cells that produce collagen and organic matrix; initiate mineralization by secreting alkaline phosphatase.

• Osteocytes: Mature bone cells trapped in the matrix; maintain bone tissue and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells responsible for bone resorption.

Bone Matrix

• Organic Component (Osteoid): Collagen fibers and ground substance provide tensile strength.

• Inorganic Component: Calcium phosphate (hydroxyapatite) deposits provide hardness.

Microscopic Structure of Bone

Compact Bone

• Osteon (Haversian System): The basic unit, consisting of concentric lamellae arranged around a central canal containing blood vessels and nerves.

• Canaliculi: Tiny channels connecting osteocytes for nutrient and waste exchange.

• Perforating (Volkmann's) Canals: Horizontal canals that connect osteons and allow blood vessels to enter from the periosteum.

• Circumferential Lamellae: Lamellae that wrap around the circumference of the bone near the periosteum and endosteum.

• Interstitial Lamellae: Lamellae found between osteons.

Spongy Bone

• Composed of irregularly arranged lamellae forming trabeculae.

• Trabeculae contain bone marrow and provide structural support with minimal weight.

Bone Marrow

Types and Distribution

• Red Bone Marrow: Active in hematopoiesis (blood cell formation); found in children and in adult axial skeleton.

• Yellow Bone Marrow: Inactive, mainly fat cells; predominates in adult appendicular skeleton.

Distribution of red and yellow marrow changes with age, with red marrow receding distally in adults.

Summary Table: Bone Types and Features

Bone Type	Shape	Examples	Main Function
Long	Longer than wide	Femur, Humerus	Movement, support
Short	Cube-shaped	Carpals, Tarsals	Stability, support
Flat	Thin, broad	Skull, Sternum	Protection
Irregular	Complex shapes	Vertebrae, Pelvis	Protection, support
Sesamoid	Small, round	Patella	Reduce friction, modify pressure
Sutural	Small, irregular	Skull sutures	Fill gaps between bones

Key Equations

• Calcium Homeostasis:

$\text{Ca}^{2+}_{\text{blood}} \longleftrightarrow \text{Ca}^{2+}_{\text{bone}}$

• Bone Growth (Osteogenesis):

$\text{Osteogenic~cells} \rightarrow \text{Osteoblasts} \rightarrow \text{Osteocytes}$

Additional info: Some details, such as the full list of bone cell functions and the structure of bone membranes, were expanded for academic completeness.