Skeletal System: Osseous Tissue Structure and Axial Division

Skeletal System Overview

Main Divisions of the Skeletal System

The human skeletal system is divided into two major parts: the axial skeleton and the appendicular skeleton. Each division has distinct components and functions essential for support, movement, and protection.

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage (ribs and sternum). It forms the central axis of the body and protects vital organs such as the brain, spinal cord, and heart.

• Appendicular Skeleton: Comprises the pectoral girdle (shoulder), pelvic girdle (hips), and the limbs (arms and legs). It is responsible for movement and interaction with the environment.

Example: The axial skeleton supports the head and trunk, while the appendicular skeleton enables walking and manipulation of objects.

Functions of Skeletal System Components

Organ/Component	Primary Functions
Bones, Cartilages, and Joints	Support and protect soft tissues; store minerals; provide leverage for movement
Ligaments	Connect bone to bone, bone to cartilage, or cartilage to cartilage; stabilize joints
Bone Marrow	Site of blood cell production (red marrow); storage of energy reserves in fat cells (yellow marrow)

Osseous Tissue Structure

Bone as a Dynamic Tissue

Bone is a living, dynamic tissue that constantly remodels itself throughout life. It consists of cells, fibers, and ground substance, and is highly vascularized to support its metabolic needs.

• Osteoblasts: Cells that build new bone by secreting matrix and facilitating mineralization.

• Osteoclasts: Cells that break down bone tissue, aiding in bone remodeling and calcium release.

• Osteocytes: Mature bone cells that maintain bone tissue.

Example: During growth and healing, osteoblast activity predominates; during aging, osteoclast activity may increase, leading to bone loss.

Bone Development & Ossification

Types of Ossification

Bone formation, or ossification, occurs through two main processes: intramembranous ossification and endochondral ossification.

• Intramembranous Ossification: Direct formation of bone from mesenchymal tissue, producing flat bones like those of the skull.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage, forming most bones of the body, including long bones.

Example: The embryonic skull forms via intramembranous ossification, while the femur forms via endochondral ossification.

Primary and Secondary Ossification Centers

• Primary Ossification Center: Located in the diaphysis (shaft) of long bones; responsible for initial bone formation.

• Secondary Ossification Centers: Develop in the epiphyses (ends) of long bones after birth, allowing for continued growth.

Bone Growth Mechanisms

Appositional vs. Interstitial Growth

Bones grow in both length and thickness through distinct mechanisms:

• Appositional Growth: Increases bone thickness by adding new layers to the outer surface.

• Interstitial Growth: Increases bone length via activity at the epiphyseal plate (growth plate).

Example: Long bones like the humerus grow longer at the epiphyseal plate and thicker via appositional growth.

Role of the Periosteum and Endosteum

• Periosteum: Outer membrane covering bone; contains osteoblasts for appositional growth.

• Endosteum: Lines the inner surface of bone; contains osteoclasts for bone resorption and remodeling.

Microscopic Structure of Bone

Compact Bone

Compact bone is dense and forms the outer layer of bones, providing strength and protection.

• Osteons (Haversian systems): Cylindrical structures containing concentric lamellae (layers) of bone matrix around a central canal.

• Lamellae: Layers of bone matrix; can be concentric (around osteons), interstitial (between osteons), or circumferential (around the bone's outer surface).

• Canals: Haversian canals (vertical) and Volkmann's canals (horizontal) allow passage of blood vessels and nerves.

Spongy Bone

Spongy bone (cancellous bone) is found at the ends of long bones and inside flat bones. It consists of a network of trabeculae, providing strength with minimal weight.

• Trabeculae: Lattice-like structures aligned along lines of stress.

• Spaces: Filled with bone marrow, which produces blood cells.

Regulation of Bone Growth and Remodeling

Key Factors Influencing Bone Development

• Minerals: Calcium, phosphate, magnesium, carbonate, and sodium are essential for bone matrix formation.

• Vitamins: Vitamin D (cholecalciferol) is crucial for calcium absorption.

• Hormones: Parathyroid hormone, calcitonin, growth hormone, thyroxine, estrogen, and testosterone regulate bone growth and remodeling.

Example: Parathyroid hormone increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts.

Aging and the Skeletal System

Changes in Bone with Age

• Osteoblast activity declines with age, leading to decreased bone formation.

• Osteoclast activity may remain the same or increase, resulting in bone loss (osteopenia or osteoporosis).

• Hormonal changes, such as reduced estrogen, accelerate bone loss in older adults.

Classification of Skeletal Elements

Types of Bones by Shape

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

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

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

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

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

Bone Markings

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

• Projections: Sites of muscle and ligament attachment (e.g., tubercle, spine).

• Depressions: Indentations or openings for nerves and vessels (e.g., fossa, foramen).

Axial Skeleton: Skull and Vertebral Column

Major Sutures and Skull Landmarks

The skull is composed of several bones joined by sutures, which are immovable joints made of dense connective tissue.

• Lambdoid Suture: Between parietal and occipital bones.

• Coronal Suture: Between frontal and parietal bones.

• Squamous Suture: Between parietal and temporal bones.

• Fontanelles: Soft spots in the fetal skull that allow for growth and brain development.

Skull Features

• Foramen: Openings for nerves and blood vessels (e.g., foramen magnum, supraorbital foramen).

• Processes: Projections for muscle attachment (e.g., mastoid process, styloid process).

• Fossa: Depressions for articulation (e.g., mandibular fossa).

Orbit and Nasal Complex

• Orbit: Formed by frontal, sphenoid, ethmoid, lacrimal, maxillary, palatine, and zygomatic bones; houses the eye.

• Nasal Complex: Includes nasal bones, conchae, ethmoid, maxilla, and palatine bones; contains paranasal sinuses that lighten the skull and resonate sound.

Hyoid Bone

The hyoid bone is unique as it does not articulate directly with any other bone. It supports the tongue and is an attachment site for muscles involved in swallowing.

Vertebral Column

Structure and Function

The vertebral column supports the head and trunk, protects the spinal cord, and provides attachment points for ribs and muscles.

• Cervical Vertebrae: Seven vertebrae in the neck region; include specialized atlas and axis for head movement.

• Thoracic Vertebrae: Twelve vertebrae; articulate with ribs.

• Lumbar Vertebrae: Five vertebrae; largest and strongest, supporting lower back.

• Sacral and Coccygeal Vertebrae: Fused to form the sacrum and coccyx.

Example: The spinous process of vertebra prominens (C7) is easily palpable at the base of the neck.

Curvatures and Specializations

• Primary Curvatures: Thoracic and sacral; present at birth.

• Secondary Curvatures: Cervical and lumbar; develop with growth and movement.

• Specialized Features: Atlas (C1) and axis (C2) allow for nodding and rotation of the head.

Thoracic Cage

Structure and Articulations

The thoracic cage consists of the ribs, sternum, and thoracic vertebrae. It protects the heart and lungs and provides attachment for muscles involved in respiration.

• Ribs: Twelve pairs; articulate with thoracic vertebrae and sternum.

• Costal Cartilage: Connects ribs to sternum, allowing flexibility.

• Landmarks: Head, neck, and costal groove of ribs; demifacets on vertebrae for rib articulation.

Example: The angle of the rib and costal groove are important for muscle attachment and protection of intercostal nerves and vessels.