Skeletal System Overview

Number and Function of Bones

The adult human skeleton is composed of a specific number of bones, each serving vital functions in the body.

• Number of Bones: The adult skeleton typically has 206 bones.

• Functions of Bone Tissue:

  • Support: Provides structural framework for the body.

  • Protection: Shields vital organs (e.g., skull protects the brain).

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

  • Mineral Storage: Stores calcium and phosphorus.

  • Blood Cell Production: Houses red bone marrow for hematopoiesis.

  • Triglyceride Storage: Stores fat in yellow bone marrow.

Types of Osseous (Bone) Tissue

Bone tissue exists in two main forms, each with distinct structure and function.

• Compact Bone: Dense, forms the outer layer of bones, provides strength and support.

• Spongy (Cancellous) Bone: Porous, found at the ends of long bones and inside flat bones, contains red marrow.

• Comparison: Compact bone is organized into osteons (Haversian systems), while spongy bone consists of trabeculae.

Bone Cells

Several specialized cells are found within bone tissue, each with unique roles.

• Osteoblasts: Bone-forming cells; synthesize and secrete bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue and reside in lacunae.

• Osteoclasts: Large, multinucleated cells; break down bone matrix (bone resorption).

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Classification of Bones by Shape

Bones are classified into five major shapes, each with representative examples and functions.

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., sternum, skull bones).

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

• Sesamoid Bones: Embedded in tendons (e.g., patella).

Microscopic Structure of Bone

Functional Subunits of Bone Tissue

The basic structural and functional unit of compact bone is the osteon (Haversian system).

• Osteon: Consists of concentric lamellae arranged around a central (Haversian) canal.

Canals and Lamellae

• Central (Haversian) Canal: Runs longitudinally through the osteon; contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Run perpendicular to central canals; connect blood and nerve supply of periosteum to those in central canals.

• Lamellae: Layers of bone matrix; three types:

  • Circumferential Lamellae: Encircle the entire bone just deep to the periosteum.

  • Concentric Lamellae: Surround the central canal within each osteon.

  • Interstitial Lamellae: Fill spaces between osteons.

Periosteum

• Definition: A dense, fibrous membrane covering the outer surface of bones except at joints.

• Functions: Provides nourishment, serves as an attachment for tendons and ligaments, and contains osteoprogenitor cells for bone growth and repair.

• Responsible for: Bone growth in thickness (appositional growth) and repair.

Microscopic Features (Image Reference)

• Concentric Lamellae: Identified as rings around the central canal.

• Lacunae: Small spaces containing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

• Central Canal: The central dark circle in the osteon.

Additional info: In the provided image, arrows likely indicate these structures for identification.

Spongy vs. Compact Bone

• Spongy Bone: Contains trabeculae and red bone marrow; lacks osteons.

• Compact Bone: Contains osteons; provides strength and support.

Connections Between Lacunae

• Canaliculi: Microscopic canals that connect lacunae, facilitating communication and nutrient exchange between osteocytes.

Axial Skeleton

Definition and Components

The axial skeleton forms the central axis of the body and supports and protects the organs of the head, neck, and trunk.

• Main Components:

  • Skull

  • Vertebral column

  • Thoracic cage (ribs and sternum)

Accessory Structures of the Axial Skeleton

• Examples:

  • Hyoid bone

  • Auditory ossicles (malleus, incus, stapes)

  • Intervertebral discs

Additional info: The axial skeleton is distinct from the appendicular skeleton, which includes the limbs and girdles.