Skeletal System: Bone Structure, Cells, and Joint Movements

Skeletal System Overview

The skeletal system provides structural support, protection, and facilitates movement in the human body. It is composed of bones, joints (articulations), and associated connective tissues. This guide covers the structure of bones, types of bone cells, bone development, and the major types of joint movements.

Bone Structure and Types

Long Bone Anatomy

• Diaphysis: The shaft or central part of a long bone. It is primarily composed of compact bone, providing strength.

• Epiphyses: The ends of long bones, usually wider than the diaphysis. They are involved in joint articulation and are often filled with spongy bone.

• Metaphysis: The region between the diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in growing bones.

Types of Bones

• Flat Bones: Thin, flattened, and usually curved. Examples: sternum, cranial bones, scapula. Structure: two layers of compact bone with a middle layer of spongy bone.

• Short Bones: Cube-shaped bones, such as carpals (wrist) and tarsals (ankle).

• Irregular Bones: Complex shapes, such as vertebrae.

• Sesamoid Bones: Small, round bones embedded within tendons. Example: patella.

Bone Tissue Types

• Compact Bone: Dense outer layer; structural unit is the osteon.

• Spongy Bone: Internal network of trabeculae; provides strength with minimal weight.

Bone Matrix

• Inorganic Component: Mainly calcium phosphate (about 2/3 of bone mass), providing hardness.

• Organic Component: Primarily collagen fibers (about 1/3 of bone mass), providing flexibility and tensile strength.

Bone Cells

Osteogenic Cells

• Stem cells found in the periosteum and endosteum.

• Differentiate into osteoblasts.

Osteoblasts

• Bone-forming cells responsible for synthesizing the organic matrix (osteoid) during osteogenesis.

• Initiate the process of bone mineralization.

Osteocytes

• Mature bone cells derived from osteoblasts.

• Reside in lacunae and maintain the bone matrix.

• Connected to other osteocytes via canaliculi for nutrient and waste exchange.

Osteoclasts

• Large, multinucleated cells responsible for bone resorption (osteolysis).

• Secrete acids and enzymes to dissolve bone matrix, releasing calcium into the blood (important for calcium homeostasis).

Bone Development (Ossification)

Intramembranous Ossification

• Formation of flat bones (e.g., cranial bones, clavicle) directly from mesenchymal tissue.

• Steps include mesenchymal cell differentiation, osteoid secretion, and mineralization.

Endochondral Ossification

• Formation of long bones from a hyaline cartilage model.

• Process begins in the diaphysis (primary ossification center) and later in the epiphyses (secondary ossification centers).

Bone Fractures and Healing

Types of Fractures

• Stress Fracture: Caused by unusual or repeated stress (e.g., sports injury, accident).

• Pathological Fracture: Occurs in bones weakened by disease (e.g., osteoporosis, cancer).

• Open (Compound) Fracture: Bone breaks through the skin.

• Closed (Simple) Fracture: Bone does not penetrate the skin.

Healing Process

1. Formation of a fracture hematoma (blood clot).

2. Development of a soft callus (fibrocartilaginous tissue).

3. Formation of a hard callus (spongy bone).

4. Bone remodeling to restore original shape and structure.

Articulations (Joints)

Definition and Types

• Articulation: The site where two or more bones meet; also called a joint.

• Types include fibrous, cartilaginous, and synovial joints.

Major Types of Joint Movements

Flexion and Extension

These movements occur in the sagittal plane and involve changes in the angle between articulating bones.

• Flexion: Decreases the angle between two body parts (e.g., bending the elbow or knee).

• Extension: Increases the angle between two body parts (e.g., straightening the elbow or knee).

• Example: Flexion and extension at the hip and knee during walking.

Abduction and Adduction

These movements occur in the frontal plane and involve movement away from or toward the midline of the body.

• Abduction: Movement away from the midline (e.g., raising the arm or leg sideways).

• Adduction: Movement toward the midline (e.g., lowering the arm or leg back to the side).

• Example: Moving the arms or legs laterally away from or toward the body.

Rotational Movements

Rotation involves turning a bone around its own longitudinal axis.

• Medial (Internal) Rotation: Rotating a limb toward the midline.

• Lateral (External) Rotation: Rotating a limb away from the midline.

• Right/Left Rotation: Rotation of the head or trunk to the right or left.

• Example: Turning the head side to side; rotating the arm at the shoulder.

Supination and Pronation

These terms describe movements of the forearm and hand.

• Supination: Rotating the forearm so the palm faces anteriorly (upward in anatomical position).

• Pronation: Rotating the forearm so the palm faces posteriorly (downward in anatomical position).

• Example: Turning the hand to receive change (supination) or to type on a keyboard (pronation).

Other Movements

• Dorsiflexion: Bending the foot upward at the ankle.

• Plantar Flexion: Pointing the foot downward at the ankle.

• Inversion: Turning the sole of the foot inward.

• Eversion: Turning the sole of the foot outward.

• Opposition: Movement of the thumb to touch the fingertips.

Summary Table: Major Bone Cells

Cell Type	Function	Location
Osteogenic Cells	Stem cells; differentiate into osteoblasts	Periosteum, endosteum
Osteoblasts	Form new bone matrix (osteoid)	Bone surfaces, periosteum
Osteocytes	Maintain bone matrix	Lacunae within bone
Osteoclasts	Resorb bone; release calcium	Bone surfaces, resorption bays

Key Terms and Definitions

• Osteoid: The unmineralized, organic portion of the bone matrix produced by osteoblasts.

• Osteolysis: The process of bone matrix breakdown by osteoclasts.

• Articulation: A joint; the point where two bones meet.

• Trabeculae: The lattice-like network of spongy bone tissue.

• Osteon: The structural unit of compact bone.

Additional info: The notes have been expanded to provide full academic context, definitions, and examples for each topic, as would be expected in a college-level Anatomy & Physiology course.