Skeletal System: Structure, Function, and Bone Biology

Skeletal System Overview

The skeletal system provides the structural framework for the body, supporting and protecting internal organs and soft tissues. It is composed of bones, cartilage, tendons, and ligaments, each with specialized functions.

Components of the Skeletal System

• Bones: Rigid organs forming the main structure.

• Cartilage: Semi-rigid connective tissue providing flexibility and support.

• Tendons: Strong bands of connective tissue attaching muscles to bones.

• Ligaments: Strong bands of fibrous connective tissue connecting bones to other bones.

Functions of the Skeletal System

• Body Support

• Organ Protection

• Body Movement

• Mineral Storage (e.g., calcium, phosphate)

• Blood Cell Production (hematopoiesis in red marrow)

Cartilage

Cartilage is a flexible connective tissue found in various parts of the body, providing support and cushioning.

Types of Cartilage

• Hyaline Cartilage: Most abundant; found at joints, nose, trachea. Provides smooth surfaces for movement.

• Fibrocartilage: Contains thick collagen fibers; found in intervertebral discs and pubic symphysis. Resists compression.

• Elastic Cartilage: Contains elastic fibers; found in ear and epiglottis. Provides flexibility.

Cartilage Growth

• Appositional Growth: New cartilage is added to the surface by chondroblasts in the perichondrium.

• Interstitial Growth: Chondrocytes divide and secrete new matrix from within the cartilage.

Bone Tissue and Bone Cells

Bones are dynamic organs composed of several cell types and a mineralized matrix.

Types of Bone Cells

• Osteoblasts: Bone-forming cells; secrete bone matrix and initiate calcification.

• Osteocytes: Mature bone cells; maintain bone tissue and reside in lacunae.

• Osteoclasts: Large, multinucleated cells; break down bone matrix (bone resorption).

Bone Matrix

• Organic Component: Collagen fibers provide flexibility and tensile strength.

• Inorganic Component: Hydroxyapatite crystals (calcium phosphate) provide hardness.

Bone Remodeling

• Continuous process of bone resorption (by osteoclasts) and bone formation (by osteoblasts).

• Essential for growth, repair, and calcium homeostasis.

Bone Structure

Bones have a complex structure, including compact and spongy bone, and various surface features.

Types of Bone Tissue

• Compact Bone: Dense outer layer; provides strength and protection.

• Spongy Bone: Network of trabeculae; found at ends of long bones and inside flat bones; contains red marrow.

Bone Shapes

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

• Short Bones: Nearly equal in length and width (e.g., carpals).

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

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

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

Structure of a Long Bone

• Diaphysis: Shaft; composed of compact bone.

• Epiphyses: Ends; mostly spongy bone covered by compact bone.

• Metaphysis: Region between diaphysis and epiphysis; contains growth plate in children.

• Medullary Cavity: Central cavity; contains bone marrow.

• Periosteum: Outer fibrous covering; contains nerves and blood vessels.

• Endosteum: Lines internal bone surfaces.

Bone Development and Growth

Bones develop through two main processes: intramembranous and endochondral ossification.

Ossification Patterns

• Intramembranous Ossification: Bone forms directly from mesenchymal tissue (e.g., flat bones of skull).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

Growth in Length

• Occurs at the epiphyseal plate (growth plate) via proliferation of cartilage cells.

• Epiphyseal closure occurs when cartilage is replaced by bone, ending growth in length.

Factors Affecting Bone Growth

• Genetics: Determines potential size and shape.

• Nutrition: Adequate intake of calcium, vitamin D, and protein is essential.

• Hormones: Growth hormone, thyroid hormone, and sex hormones regulate growth and remodeling.

Calcium Homeostasis

Bone acts as a major calcium reservoir, and calcium homeostasis is tightly regulated by hormones.

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Vitamin D: Increases calcium absorption from the intestine.

Calcium Homeostasis Equation

$\text{Ca}^{2+}_{\text{blood}} = \text{Ca}^{2+}_{\text{intake}} + \text{Ca}^{2+}_{\text{bone resorption}} - \text{Ca}^{2+}_{\text{bone deposition}} - \text{Ca}^{2+}_{\text{excretion}}$

Bone Fractures

Fractures are breaks in bone continuity, classified by their characteristics and severity.

Types of Fractures

• Open (Compound) Fracture: Bone pierces the skin.

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

• Incomplete Fracture: Bone is not completely broken.

• Comminuted Fracture: Bone is shattered into several pieces.

• Greenstick Fracture: Incomplete fracture, common in children.

• Impacted Fracture: One fragment is driven into another.

• Transverse Fracture: Perpendicular to bone's axis.

• Oblique Fracture: At an angle to bone's axis.

• Spiral Fracture: Twisting injury.

Fracture Healing Process

1. Hematoma Formation: Blood clot forms at the fracture site.

2. Callus Formation: Fibrocartilaginous callus bridges the gap.

3. Callus Ossification: Callus is replaced by bone.

4. Bone Remodeling: Bone is restored to original shape.

Axial Skeleton

The axial skeleton forms the central axis of the body, protecting the brain, spinal cord, and vital organs.

Components of the Axial Skeleton

• Skull

• Hyoid Bone

• Vertebral Column

• Rib Cage (Thoracic Cage)

Skull Anatomy

• Cranial Bones: Enclose the brain (e.g., frontal, parietal, occipital, temporal).

• Facial Bones: Form the face (e.g., maxilla, mandible, zygomatic).

• Major Openings: Foramina for nerves and blood vessels (e.g., foramen magnum, optic canal).

Vertebral Column

• Cervical Vertebrae: 7 (neck region)

• Thoracic Vertebrae: 12 (chest region)

• Lumbar Vertebrae: 5 (lower back)

• Sacrum: 5 fused vertebrae

• Coccyx: 3-5 fused vertebrae (tailbone)

Rib Cage

• Ribs: 12 pairs; protect thoracic organs.

• Sternum: Breastbone; attachment for ribs.

Appendicular Skeleton

The appendicular skeleton includes the bones of the limbs and girdles, facilitating movement.

Major Components

• Pectoral Girdle: Clavicle and scapula; attaches upper limbs to trunk.

• Upper Limbs: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Hip bones (ilium, ischium, pubis); attaches lower limbs to trunk.

• Lower Limbs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Table: Comparison of Bone Cells

Cell Type	Function	Location
Osteoblast	Bone formation, matrix synthesis	Bone surfaces
Osteocyte	Maintains bone tissue	Lacunae within bone matrix
Osteoclast	Bone resorption	Bone surfaces, resorption bays

Additional info:

• Bone remodeling is influenced by mechanical stress, hormones, and nutritional status.

• Fontanels in the infant skull allow for brain growth and ease of birth.

• Red marrow is the site of hematopoiesis; yellow marrow stores fat.