The Skeletal System: Osseous Tissue and Bone Structure

The Skeletal System

Introduction

The skeletal system is a vital organ system that provides structure, protection, and support for the human body. It consists of several components that work together to maintain bodily integrity and facilitate movement.

• Skeletal bones: The rigid framework of the body, providing shape and support.

• Cartilage: Flexible connective tissue found in joints, ear, nose, and other areas.

• Ligaments: Bands of connective tissue that connect bones to other bones, stabilizing joints.

• Connective tissue: Supports and stabilizes the skeleton.

The skeleton is divided into two main divisions:

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Includes the limbs and girdles (shoulder and pelvic).

Functions of the Skeletal System

Overview of Functions

The skeletal system performs several essential functions necessary for survival and health.

• Support: Provides the framework for the attachment of other organs and tissues.

• Storage of calcium and phosphate: Bones act as reservoirs for minerals, especially calcium and phosphate, which are critical for various physiological processes.

• Blood cell production: The bone marrow produces erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments involved in clotting).

• Leverage: Muscles attach to bones and pull on them to produce movement, acting as levers.

• Protection:

  • Ribs protect the heart and lungs.

  • Skull protects the brain.

  • Vertebrae protect the spinal cord.

  • Pelvic bones protect the reproductive organs.

Osseous Tissue and Bone Structure

Bone Histology

Bone tissue, or osseous tissue, is a specialized connective tissue with a matrix hardened by calcium phosphate and other minerals. It contains several types of cells, each with distinct functions.

• Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Cells that produce new bone matrix (osteoid) and promote mineralization.

• Osteocytes: Mature bone cells derived from osteoblasts, responsible for maintaining bone tissue and communicating via filopodia and gap junctions.

• Osteoclasts: Large, multinucleated cells that dissolve bone matrix, releasing calcium and phosphate into the bloodstream.

Example: Osteoclast activity increases during periods of low blood calcium, helping to restore homeostasis.

Structure and Function of Bone

Bones are organized into two main types based on their structure and function: compact bone and spongy bone.

• Compact bone (dense bone):

  • Forms the outer walls of bones.

  • Basic structural unit is the osteon, which consists of a central canal, canaliculi, lacunae, concentric lamellae, perforating canals, interstitial lamellae, and circumferential lamellae.

• Spongy bone (trabecular or cancellous bone):

  • Composed of an open network of plates called trabeculae.

  • Does not contain osteons.

  • Surrounds the medullary cavity, which contains bone marrow.

  • Contributes to the lightweight nature of bones.

Example: The proximal end of the femur contains spongy bone, which helps absorb shock and reduce bone weight.

Functional Differences in Bone Regions

Long bones have distinct regions with specific functions:

• Epiphysis: The ends of long bones, often involved in joint articulation.

• Diaphysis: The shaft of long bones, providing strength and support.

• Metaphysis: The narrow growth zone between the epiphysis and diaphysis, important for bone growth.

Periosteum and Endosteum

Bones are covered and lined by specialized membranes:

• Periosteum:

  • Outer surface of bone.

  • Consists of a fibrous layer and an osteogenic layer.

  • Isolates and protects bone, provides attachment for circulatory and nervous supply, participates in bone growth and repair, and attaches bone to deep fascia via perforating fibers.

• Endosteum:

  • Lines the inner surface of bone, including the medullary cavity.

  • Contains osteoprogenitor cells and is involved in bone growth and repair.

Bone Development and Growth

Ossification Processes

Bone formation, or ossification, occurs through two main processes:

• Intramembranous ossification: Bone develops directly from fibrous tissue. This process forms flat bones such as the clavicle, mandible, skull, and face.

• Endochondral ossification: Bone develops from cartilage tissue. This process forms most bones, including limbs, vertebrae, and hips.

Bone growth occurs in two ways:

• Interstitial growth: Increases bone length via the epiphyseal plate.

• Appositional growth: Increases bone width by adding new layers to the surface.

Steps of Intramembranous Ossification

1. Mesenchymal cells differentiate into osteoblasts.

2. Osteoblasts secrete bone matrix and become trapped, differentiating into osteocytes.

3. New osteoblasts form and move outward, expanding the bone.

4. Blood vessels are trapped by the formation of bony spicules, leading to the development of spongy bone.

Steps of Endochondral Ossification

1. Chondrocytes in the cartilage model enlarge and die, leaving cavities.

2. Blood vessels grow around the cartilage, and osteoblasts form a periosteum.

3. Osteoblasts replace cartilage with spongy bone at the primary ossification center.

4. Secondary ossification centers form in the epiphyses, replacing cartilage with bone.

5. Growth continues at the epiphyseal plate until adulthood, when the plate ossifies and bone elongation ceases.

Additional info: The process of bone remodeling continues throughout life, balancing bone formation and resorption.

Anatomy of Skeletal Elements

Classification of Bone Shapes

Bones are classified into seven broad categories based on their shapes:

• Sutural bones: Small bones found within the sutures of the skull.

• Irregular bones: Complex shapes, such as vertebrae.

• Short bones: Box-like in appearance, such as wrist bones.

• Pneumatized bones: Bones containing air pockets or sinuses, such as the ethmoid bone.

• Flat bones: Thin, parallel surfaces, such as the sternum.

• Long bones: Long and slender, such as the femur.

• Sesamoid bones: Small bones that develop inside tendons, such as the patella.

Bone Markings

Bones have various markings that serve as sites for muscle attachment, articulation, and passage of nerves and blood vessels.

• Projections:

  • Process: Any projection or bump.

  • Ramus: Extension that forms an angle.

• Depressions:

  • Fossa: Shallow depression.

  • Sulcus: Narrow groove.

• Openings:

  • Sinus: Air-filled chamber within a bone.

  • Canal/Meatus: Passageway through bone.

  • Fissure: Deep cleft or slit.

  • Foramen: Rounded passageway for blood vessels or nerves.

Processes for Tendon and Ligament Attachment

• Trochanter: Large, rough projection (femur).

• Crest: Prominent ridge.

• Spine: Pointed process.

• Line: Low ridge.

• Tubercle: Small, rounded projection.

• Tuberosity: Rough projection.

Parts That Participate in a Joint

• Head: Expanded articular end of an epiphysis.

• Neck: Narrow connection between epiphysis and diaphysis.

• Facet: Small, flat articular surface.

• Condyle: Smooth, rounded articular process.

• Trochlea: Smooth, grooved articular process (humerus).