Functions of the Skeletal System

Overview

The skeletal system provides the framework for the human body and serves several essential functions necessary for survival and movement.

• Support: The skeleton supports the body and maintains its shape.

• Facilitation of Movement: Bones act as levers and points of attachment for muscles, enabling movement.

• Protection: Bones protect vital internal organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Blood Cell Production: Hematopoiesis occurs in red bone marrow, producing red and white blood cells and platelets.

• Mineral and Fat Storage: Bones store minerals (mainly calcium and phosphorus) and fats (in yellow bone marrow).

Bone Classification

Types of Bones

Bones are classified based on their shapes and functions. Each type has unique structural characteristics and examples.

• Long Bones: Cylindrical in shape, longer than they are wide. Examples: humerus, femur, metacarpals, metatarsals.

• Short Bones: Cube-like in shape, equal in length, width, and thickness. Examples: carpals, tarsals.

• Flat Bones: Thin and often curved. Examples: scapulae, cranial bones, sternum, ribs.

• Irregular Bones: Complex shapes that do not fit other categories. Examples: vertebrae, many facial bones.

• Sesamoid Bones: Shaped like a sesame seed; form within tendons. Examples: patella, variable number in tendons.

Terms Related to Bone Anatomy

Key Anatomical Features

• Diaphysis: Shaft of a long bone.

• Epiphysis: Proximal and distal ends of the bone.

• Metaphysis: Area of bone growth in length (location of the epiphyseal plate).

• Epiphyseal Plate: Growth plate; visible line that remains after bone growth stops.

• Medullary Cavity: Hollow region in the diaphysis filled with yellow marrow.

• Periosteum: Fibrous membrane covering the outer surface of the bone; site of ligament and tendon attachment.

• Endosteum: Thin membrane lining the medullary cavity.

• Articular Cartilage: Thin layer of hyaline cartilage at bone ends; reduces friction and absorbs shock.

• Diploë: Layer of spongy bone between compact bone in flat bones (e.g., cranium).

Terms Related to Bone Markings

Bone Surface Features

Bones have various markings that serve as attachment points for muscles, passageways for nerves and blood vessels, and articulation surfaces for joints.

• Head: Prominent rounded surface (e.g., head of femur).

• Facet: Flat surface (e.g., vertebrae).

• Condyle: Rounded surface (e.g., occipital condyles).

• Projection: Raised markings (e.g., spinous process of vertebrae).

• Protuberance: Protruding chin.

• Process: Prominent feature (e.g., transverse process, styloid process).

• Spine: Sharp process (e.g., ischial spine, scapular spine).

• Tubercle: Small, rounded process (e.g., greater and lesser tubercles of humerus).

• Tuberosity: Rough surface (e.g., deltoid tuberosity).

• Line: Slight, elongated ridge (e.g., iliac crest).

• Crest: Ridge (e.g., iliac crest).

• Fossa: Elongated basin (e.g., glenoid fossa).

• Fovea: Small pit (e.g., fovea capitis).

• Canal: Passage in bone (e.g., auditory canal).

• Fissure: Slit through bone.

• Foramen: Hole through bone (e.g., foramen magnum).

• Meatus: Opening into canal (e.g., external auditory meatus).

• Sinus: Air-filled space (e.g., maxillary sinus).

• Sulcus: Groove.

Bone Cells

Types and Functions

• Osteocyte: Most common type of bone cell; mature bone cell located in lacunae, maintains bone tissue.

• Osteoblast: Bone-forming cell; secretes bone matrix (but does not divide).

• Osteogenic Cell: Only bone cells capable of mitosis; divide and form osteoblasts (stem cells).

• Osteoclast: Responsible for bone resorption (breakdown of bone tissue).

Terms Related to Bone Structure

Microscopic and Macroscopic Structure

• Osteon (Haversian System): Structural unit of compact bone.

• Central Canal: Cylindrical cavity in the center of each osteon containing blood vessels, nerves, and lymphatic vessels.

• Perforating Canal (Volkmann's Canal): Canals that carry blood vessels, nerves, and lymphatic vessels from the central canal to the periosteum and endosteum.

• Canaliculi: Tiny canals connecting osteocytes in lacunae to each other and to the central canal.

• Lacuna: Small space containing an osteocyte; found between lamellae (rings).

• Lamellae: Concentric rings of calcified matrix.

• Trabeculae: Lattice-like network of matrix spikes in spongy bone, providing strength.

• Cancellous Bone (Spongy Bone): Type of bone with trabeculae and open spaces filled with red marrow.

• Compact Bone: Dense type of bone made of osteons.

• Hematopoiesis: Blood cell production in red bone marrow.

• Nutrient Foramen: Small openings in the diaphysis for passage of blood vessels.

Bone Formation (Ossification)

Overview

Ossification is the process by which bone tissue is formed. Most of the fetal skeleton is initially hyaline cartilage, which is gradually replaced by bone. Ossification continues throughout life, especially during growth and bone repair.

• Intramembranous Ossification: Bone develops directly from sheets of mesenchymal connective tissue. Examples: flat bones of the face and most cranial bones.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage. Examples: long bones.

Intramembranous Ossification Steps

1. Mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center.

2. Osteoblasts secrete osteoid (uncalcified bone matrix), which calcifies as mineral salts are deposited.

3. Osteoblasts become trapped and differentiate into osteocytes; trabecular matrix forms.

4. Compact bone develops superficially; red marrow appears in the spongy bone.

Endochondral Ossification Steps

1. Chondrocytes form a cartilage model of the bone.

2. Chondrocytes in the center die as the matrix calcifies, creating a cavity for blood vessels to enter.

3. Osteoblasts form a bone collar around the diaphysis; primary ossification center forms in the diaphysis.

4. Secondary ossification centers form in the epiphyses; cartilage remains at the epiphyseal plate and articular surfaces.

5. Bone replaces cartilage except at the epiphyseal plates (growth plates) and articular cartilage.

Bone Growth Terms

Growth and Remodeling

• Longitudinal Growth: Occurs at the epiphyseal plate, increasing bone length.

• Appositional Growth: Occurs in the periosteum and endosteum, increasing bone diameter.

• Modeling: Laying down new bone during growth.

• Remodeling: Resorption and deposition of bone in response to stress or injury.

Fractures

Types of Bone Fractures

Bone fractures are classified by their pattern and severity. The following table summarizes common types:

Fracture Repair

1. Hematoma forms (blood clot at the fracture site).

2. Cartilage callus forms (fibrocartilaginous tissue stabilizes the break).

3. Bony callus forms (spongy bone replaces cartilage callus).

4. Bone remodeling occurs (compact bone replaces spongy bone at the fracture site).

Bone Disorders

Common Conditions

• Osteoporosis: Decrease in bone mass, increasing fracture risk, especially in vertebrae, femur, and wrist.

• Osteogenesis Imperfecta: Genetic disorder causing brittle bones prone to fractures.

• Osteomalacia: Softening of bones due to vitamin D deficiency (called rickets in children).

• Hypercalcemia: Abnormally high blood calcium levels.

• Hypocalcemia: Abnormally low blood calcium levels.

Hormones & Bone Cells

Hormonal Regulation of Bone

• Parathyroid Hormone (PTH): Raises blood calcium levels by stimulating osteoclast activity.

• Calcitonin: Lowers blood calcium levels by stimulating osteoblast activity.

Additional info:

• Bone remodeling is a lifelong process involving the coordinated actions of osteoblasts and osteoclasts, regulated by mechanical stress and hormones.

• Calcium homeostasis is critical for muscle contraction, nerve function, and blood clotting, making bone an important reservoir for calcium.