Unit 10: The Skeletal System – Structure, Function, and Classification

Histology: Bone & Cartilage

Types of Bone Tissue

The skeletal system is composed of two main types of bone tissue: compact bone and spongy bone. Each type has a unique structure and function within the body.

• Compact Bone:

  • Forms the external surfaces of all bones.

  • Composed of structural units called osteons (Haversian systems).

  • Each osteon contains:

    • Lamellae: Concentric rings of bone matrix.

    • Lacunae: Small spaces between lamellae housing osteocytes (mature bone cells).

    • Canaliculi: Tiny channels connecting osteocytes to each other and to blood supply.

    • Central Canal: Runs longitudinally, containing blood vessels and nerves; lined with endosteum.

    • Perforating (Volkmann's) Canals: Run perpendicular to central canals, connecting blood and nerve supply from periosteum to central canals and medullary cavity.

• Spongy Bone:

  • Lacks osteons; instead, consists of trabeculae (irregularly arranged lamellae).

  • Canaliculi connect osteocytes in lacunae.

  • Located in flat and irregular bones (e.g., skull, ribs, vertebrae) and at the epiphyses and lining the medullary cavity of long bones.

  • Spaces between trabeculae contain bone marrow:

    • Red marrow: Site of blood cell production (hematopoiesis).

Anatomical Position and Directional Terms

Standard Reference for Describing the Body

The anatomical position is the standard reference: standing upright, feet forward, face and palms facing forward. Directional terms describe locations and relationships of body parts.

• Anterior (ventral) vs. Posterior (dorsal): Front vs. back.

• Proximal vs. Distal: Closer to vs. farther from the trunk or point of origin.

• Medial vs. Lateral: Toward vs. away from the midline.

• Superior vs. Inferior: Above vs. below.

• Superficial vs. Deep: Toward the surface vs. further from the surface.

Additional info: See Table 1.1 in your textbook for a comprehensive list of directional terms.

Skeletal System: Overview

Divisions and General Organization

The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton. Bones may be paired or unpaired, and their connections are called articulations (joints).

• Axial Skeleton (80 bones): Forms the central core of the body.

  • Skull

  • Hyoid bone

  • Vertebral column

  • Thoracic cage (sternum and ribs)

• Appendicular Skeleton (126 bones): Includes limbs and girdles attaching them to the axial skeleton.

  • Pectoral girdle

  • Pelvic girdle

  • Upper limb (arm, forearm, hand)

  • Lower limb (thigh, leg, foot)

Articulations are points where bones meet (e.g., the humerus articulates with the scapula).

Axial Skeleton

Skull

The skull is composed of three main bone groupings: the cranium, facial bones, and auditory ossicles.

• Cranium (8 bones): Protects the brain.

  • 1 frontal

  • 2 parietal

  • 2 temporal

  • 1 sphenoid

  • 1 ethmoid (forms superior and middle nasal conchae)

  • 1 occipital

• Facial Bones (14 bones):

  • 2 nasal

  • 2 maxillae (form part of hard palate)

  • 2 zygomatic

  • 2 lacrimal

  • 2 palatine (form part of hard palate)

  • 2 inferior nasal conchae

  • 1 vomer

  • 1 mandible

• Auditory Ossicles (6 bones): Involved in sound transmission (3 per ear: incus, malleus, stapes).

Hyoid Bone

• Does not articulate with any other bone.

• Anchors muscles of the tongue and neck; assists in swallowing.

Vertebral Column

The vertebral column consists of 26 vertebrae, divided into five regions:

• Cervical (7): C1 (atlas) and C2 (axis) are specialized for head movement; all have transverse foramina.

• Thoracic (12): Articulate with ribs via costal facets.

• Lumbar (5): Support upper body weight; have large bodies and rectangular spinous processes.

• Sacrum (1): Five fused vertebrae; articulates with ilium and L5.

• Coccyx (1): Three to five fused vertebrae (usually four); forms the tailbone.

General vertebral structure: body, spinous process, transverse processes, lamina, pedicle, vertebral foramen, articular facets, intervertebral foramina.

Curvatures:

• Cervical and lumbar: concave posteriorly

• Thoracic and sacral: convex posteriorly

Abnormal curvatures:

• Scoliosis: Lateral curvature

• Kyphosis: Exaggerated thoracic curve

• Lordosis: Exaggerated lumbar curve

Thoracic Cage

• Sternum: Fusion of three bones: manubrium (superior), body (middle), xiphoid process (inferior).

• Ribs: 12 pairs

  • 7 pairs of true ribs: attach directly to sternum via costal cartilage.

  • 5 pairs of false ribs: ribs 8-10 attach indirectly via cartilage of rib 7; ribs 11-12 are floating ribs (no anterior attachment).

Appendicular Skeleton

Pectoral Girdle

• Clavicle (collarbone): Articulates with sternum (manubrium) and scapula (acromion process); connects axial and appendicular skeletons.

• Scapula (shoulder blade): Features include spine, acromion process (articulates with clavicle), glenoid cavity (articulates with humerus), and coracoid process (muscle attachment).

Pelvic Girdle

• Composed of two os coxae (hip bones), each formed by fusion of:

  • Ilium (superior; articulates with sacrum)

  • Ischium (posterior)

  • Pubis (anterior; joined at pubic symphysis)

• Joints: pubic symphysis, sacroiliac joint, acetabulum (articulates with femur head)

Upper Limb (Anatomical Position)

• Humerus

• Radius (lateral) and Ulna (medial)

• Carpals (8): wrist bones

• Metacarpals (5): palm

• Phalanges (14): fingers and thumb

Lower Limb (Anatomical Position)

• Femur

• Patella (kneecap)

• Tibia (medial) and Fibula (lateral)

• Tarsals (7): includes talus (articulates with tibia) and calcaneus (heel bone)

• Metatarsals (5): sole

• Phalanges (14): toes

Long Bone Components

Structure of a Typical Long Bone

• Diaphysis: Shaft of the bone.

• Epiphyses: Proximal and distal ends.

• Epiphyseal Plate: Hyaline cartilage for bone growth in length (in children).

• Epiphyseal Line: Remnant of plate after growth (in adults).

• Medullary Cavity: Contains red marrow in children, yellow marrow in adults.

• Periosteum: Outer covering; two layers (outer dense irregular CT, inner osteoblasts/osteoclasts); allows growth in diameter.

• Endosteum: Lines medullary cavity and canals; contains osteoblasts and osteoclasts.

• Articular Cartilage: Hyaline cartilage at joint surfaces; reduces friction (no periosteum here).

Articulations / Joints

Classification of Joints

Joints are points of contact between bones. They are classified structurally and functionally.

Structural Classification (Anatomical)

• Fibrous Joints: No joint cavity; bones joined by fibrous connective tissue (e.g., skull sutures, fontanels in newborns).

• Cartilaginous Joints: No joint cavity; bones joined by cartilage (e.g., pubic symphysis, costal cartilages of ribs).

• Synovial Joints: Most common; bones connected by joint capsule and ligaments; features include:

  • Articular cartilage (hyaline) covers bone ends.

  • Joint cavity contains synovial fluid.

  • Articular (joint) capsule encloses cavity:

    • Outer fibrous capsule (attaches to periosteum)

    • Inner synovial membrane (secretes synovial fluid)

  • Types (based on shape of articulating bones):

    • Plane/Gliding: Flat surfaces (e.g., sacroiliac joint)

    • Hinge: Concave/convex surfaces (e.g., elbow, knee)

    • Pivot: Projection in ring (e.g., dens of axis in atlas)

    • Ball & Socket: Spherical head fits into cup-like socket (e.g., shoulder, hip)

Functional Classification (Physiological)

• Synarthrotic: Immovable (e.g., skull sutures)

• Amphiarthrotic: Slightly movable (e.g., pubic symphysis)

• Diarthrotic: Freely movable (e.g., hip, shoulder)

Summary Table: Types of Joints

Structural Type	Example	Movement (Functional Type)
Fibrous	Skull sutures	Synarthrotic (immovable)
Cartilaginous	Pubic symphysis, costal cartilage	Amphiarthrotic (slightly movable)
Synovial	Shoulder, hip, elbow, knee	Diarthrotic (freely movable)

Example: Ball & Socket Joint

• Shoulder joint: Humerus head fits into glenoid fossa of scapula.

• Hip joint: Femur head fits into acetabulum of pelvis.