Skeletal System: Structure, Function, and Classification

Introduction

Skeleton

The skeleton is the supporting structure of the body, providing a framework for movement and protection. It consists of bones and associated cartilage, tendons, and ligaments.

• The skeletal system works with muscles to facilitate movement.

• Mineral salts form the inorganic matrix of bone, contributing to its strength.

Functions of the Skeleton

Main Functions

• Support: Stabilizes and supports surrounding tissues such as muscles, blood and lymphatic vessels, nerves, fat, and skin.

• Protection: Shields vital organs (e.g., brain, spinal cord, heart, lungs) and other soft tissues.

• Movement: Provides attachment points for muscles, enabling movement by acting as levers.

• Blood Formation: Manufactures blood cells (hematopoiesis), primarily in bone marrow.

• Storage of Minerals: Stores mineral salts, especially phosphorous, calcium, and fats.

• Cartilage: Provides a smooth surface for joint movement and is found at the ends of certain bones and in fetal development.

Bone-Related Structures

• Ligaments: Attach bone to bone.

• Tendons: Attach muscle to bone.

Growth and Formation of Bone

Developmental Stages

• A three-month fetal skeleton is primarily cartilage.

• Ossification (bone formation) and growth occur during fetal development and continue into adolescence (up to age 15 in girls, 16 in boys).

• Bone maturation continues into the twenties.

• Cartilage serves as the initial environment for bone development.

Remodeling of Bone

• Bone is continuously built up and broken down in response to physiological needs.

• Heavy use leads to stronger bones; lack of use leads to atrophy.

• Projections (e.g., tubercles) form in response to muscle pull.

• Remodeling also repairs fractures.

Deposition of Bone

• Bone develops from spindle-shaped cells (osteoblasts) derived from undifferentiated bone cells.

• Strain or pressure increases bone deposition.

• Osteoblasts: Build bone.

• Osteocytes: Mature bone cells.

• Osteoclasts: Break down bone for remodeling.

Types of Ossification

• Intramembranous: Bone forms directly from connective tissue membranes (e.g., cranial bones).

• Endochondral: Bone forms by replacing cartilage (most bones develop this way).

Ossification of Long Bone

• Begins as a strip of cartilage invaded by osteoblasts at three sites: diaphysis (shaft) and two epiphyses (ends).

• Growth plates (epiphyseal plates) allow for longitudinal growth.

Maintaining Bone

• Calcium balance is regulated by the endocrine system.

• Parathormone: increases calcium release into blood.

• Calcitonin: promotes calcium storage in bones.

Osteoporosis

• Bone resorption exceeds formation, leading to weaker, fracture-prone bones.

The Histology of Bone

Types of Bone Tissue

• Compact Bone: Dense, forms the outer layer of bones, contains Haversian systems (osteons).

• Cancellous (Spongy) Bone: Located at bone ends and center, has a meshwork of trabeculae, contains bone marrow.

Bone Cells

• Osteoblasts: Build bone matrix.

• Osteocytes: Mature bone cells within lacunae.

• Osteoclasts: Resorb bone matrix.

Bone Vascularization and Innervation

• Haversian (Central) Canals: Contain blood vessels and nerves, surrounded by concentric lamellae.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient/waste exchange.

• Periosteum: Outer covering rich in nerves and blood vessels.

• Bone Marrow: Red marrow (hematopoiesis), yellow marrow (fat storage).

Classification of Bones Based on Shape

Type	Description	Examples
Long Bones	Length exceeds width; shaft (diaphysis) with two ends (epiphyses); central medullary canal; metaphysis at each end	Clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, phalanges
Short Bones	Cube-shaped, nearly equal in length and width; thin compact bone over spongy bone	Carpals, tarsals
Flat Bones	Thin, provide protection and surface for muscle attachment	Sternum, ribs, scapula, some skull bones
Irregular Bones	Complex shapes, spongy bone with thin compact bone covering	Vertebrae, some facial bones
Sesamoid Bones	Small, round, embedded in tendons	Patella, some wrist/ankle bones

Bone Markings

Types and Functions

• Processes: Projections for muscle/ligament attachment or articulation (e.g., spine, condyle, tubercle, trochanter, crest, line, head, neck).

• Fossae: Depressions or cavities (e.g., foramen, meatus, canal, sinus, sulcus).

• Sutures: Junctions between skull bones.

• Foramen: Openings for blood vessels, nerves, ligaments.

Examples of Bone Markings

• Spine: Sharp, slender projection (e.g., vertebral spinous process).

• Tubercle: Small, rounded process (e.g., humerus).

• Trochanter: Large projection (e.g., femur).

• Crest: Narrow ridge (e.g., iliac crest).

• Head: Terminal enlargement (e.g., femur head).

• Neck: Connects head to shaft (e.g., femur neck).

• Foramen: Opening for vessels/nerves (e.g., skull).

• Sinus: Cavity within bone (e.g., nasal sinus).

• Sulcus: Furrow or groove.

Divisions of the Skeleton

• The adult skeleton typically has 206 named bones.

• It is divided into the axial skeleton (skull, vertebral column, rib cage) and appendicular skeleton (limbs and girdles).

The Axial Skeleton

Cranial Bones

• Frontal (1), parietal (2), occipital (1), temporal (2), sphenoid (1), ethmoid (1), auditory ossicles (6)

• Functions: protection, enclosure, muscle attachment for head movement and chewing

Facial Bones

• Nasal (2), palatine (2), maxillary (2), zygomatic (2), lacrimal (2), nasal conchae (2), vomer (1), mandible (1)

Other Axial Structures

• Hyoid Bone: Supports the tongue, not articulated with other bones.

• Spinal Column: 26 bones in adults; cervical (7), thoracic (12), lumbar (5), sacrum (5 fused), coccyx (3-5 fused).

• Curvatures: Cervical/lumbar (lordotic), thoracic/sacrum/coccyx (kyphotic).

• Disorders: Lordosis (exaggerated lumbar curve), scoliosis (sideways curve).

The Typical Vertebra

• Vertebral body: large, rounded anterior portion.

• Vertebral foramen: forms the spinal canal.

• Pedicles and laminae: form vertebral arch.

• Processes: transverse, spinous, superior/inferior articular (with facets).

Special Vertebrae

• Cervical Vertebrae: Small bodies, transverse foramina for vertebral artery.

• Atlas (C1): No body or spinous process; supports the skull, allows nodding "yes".

• Axis (C2): Has odontoid process (dens); allows shaking head "no".

• Sacrum: 5 fused bones, forms posterior pelvis.

• Coccyx: 3-5 fused bones, vestigial tail.

Intervertebral Discs

• Located between vertebrae; consist of annulus fibrosus (outer ring) and nucleus pulposus (gelatinous core).

• Functions: absorb shock, allow movement, protect vertebral bodies.