Skeletal System: Structure, Function, and Development

Skeletal System Overview

Functions of the Skeletal System

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

• Support: Provides the structural framework for the body.

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

• Movement: Serves as levers for muscles; joints allow for various types of movement.

• Mineral Storage: Stores minerals, especially calcium and phosphate, which can be released into the blood as needed.

• Blood Cell Formation: Hematopoiesis occurs in bone marrow cavities.

• Fat Storage: Yellow bone marrow stores adipose tissue, including triglycerides.

Cartilage and Bone Tissue

Types and Properties of Cartilage

Cartilage is a flexible connective tissue found in various parts of the body. It is primarily composed of water, collagen fibers, and specialized cells called chondrocytes.

• Chondrocytes: Cartilage cells located in spaces called lacunae.

• Extracellular Matrix (ECM): Gel-like ground substance rich in collagen fibers.

• Perichondrium: Dense irregular connective tissue surrounding cartilage, providing nutrients and support.

• Cartilage is avascular (lacks blood vessels) and not innervated (lacks nerves).

Types of Skeletal Cartilage

• Hyaline Cartilage: Most abundant; provides support, flexibility, and resilience. Examples: articular cartilage (ends of bones at joints), costal cartilage (ribs to sternum), respiratory cartilage (larynx, trachea), nasal cartilage (nose).

• Elastic Cartilage: Contains more elastic fibers; found in the external ear and epiglottis.

• Fibrocartilage: Highly compressible with great tensile strength; found in intervertebral discs, menisci of knees, and pubic symphysis.

Classification of Bones

Types of Bones by Shape

Bones are classified based on their shapes, which relate to their functions.

• Long Bones: Longer than they are wide; consist of a shaft (diaphysis) and two ends (epiphyses). Example: femur, humerus.

• Short Bones: Cube-shaped; found in the wrist (carpals) and ankle (tarsals).

• Flat Bones: Thin, flattened, and usually curved. Example: sternum, ribs, skull bones, scapulae.

• Irregular Bones: Complex shapes. Example: vertebrae, hip bones.

• Sesamoid Bones: Special type of short bone formed within tendons. Example: patella.

Structure of Long Bones and Compact Bone

Long Bone Anatomy

• Diaphysis: Shaft of the bone; composed of compact bone surrounding a medullary cavity (contains red or yellow marrow).

• Epiphyses: Ends of the bone; consist of spongy bone covered by a thin layer of compact bone.

• Metaphysis: Region between diaphysis and epiphysis; contains the epiphyseal plate (growth plate) in growing bones.

• Periosteum: Double-layered membrane covering the external surface of bones; contains osteoblasts and osteoclasts.

• Endosteum: Thin membrane lining the internal bone surfaces.

Histology of Compact Bone

• Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae (layers) around a central canal.

• Lamellae: Rings of bone matrix.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Volkmann's (Perforating) Canals: Run perpendicular to central canals, connecting blood vessels and nerves.

• Lacunae: Small spaces between lamellae housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

Chemical Composition of Bone

Organic and Inorganic Components

• Organic Components (mainly produced by osteoblasts):

  • Osteoid: Unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen fibers.

  • Cells: Osteogenic cells, osteoblasts (bone-forming), osteocytes (mature bone cells), osteoclasts (bone-resorbing).

• Inorganic Components:

  • Hydroxyapatites (mineral salts): Mainly calcium phosphates, responsible for bone hardness and resistance to compression.

Healthy bone is as strong as steel in resisting compression and as strong as steel in resisting tension.

Bone Development (Ossification)

Types of Ossification

• Intramembranous Ossification: Bone develops from a fibrous membrane. Forms flat bones of the skull and clavicles.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage. Forms most bones of the skeleton, except clavicles.

Steps of Intramembranous Ossification

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

2. Osteoblasts secrete osteoid, which calcifies in a few days; trapped osteoblasts become osteocytes.

3. Woven bone and periosteum form; blood vessels invade the area.

4. Compact bone replaces woven bone, and red marrow appears.

Steps of Endochondral Ossification

1. Chondroblasts in perichondrium differentiate into osteoblasts, forming a bone collar around the diaphysis of the hyaline cartilage model.

2. Cartilage in the center of the diaphysis calcifies and develops cavities.

3. Periosteal bud invades the internal cavities, and spongy bone forms.

4. Diaphysis elongates, and a medullary cavity forms; secondary ossification centers appear in the epiphyses.

5. Epiphyses ossify; hyaline cartilage remains only in the epiphyseal plates and articular cartilages.

Bone Growth

Types of Bone Growth

• Longitudinal Growth: Increase in bone length; occurs at the epiphyseal plate through zones of proliferation, hypertrophy, calcification, and ossification.

• Appositional Growth: Increase in bone thickness; osteoblasts build bone on the external surface, while osteoclasts remove bone from the internal surface.

Zones of Epiphyseal Plate (Longitudinal Growth)

• Zone of Proliferation: Rapid cell division of chondrocytes.

• Zone of Hypertrophy and Maturation: Chondrocytes enlarge and mature.

• Zone of Calcification: Matrix becomes calcified; chondrocytes die.

• Zone of Ossification: Osteoblasts lay down bone on calcified cartilage.

Bone Remodeling and Homeostasis

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption by osteoclasts and bone deposition by osteoblasts. It is influenced by hormonal and mechanical factors.

• Remodeling replaces compact bone every 10 years.

• Hormonal control (e.g., parathyroid hormone) maintains calcium homeostasis in the blood.

• Mechanical stress and gravity influence bone structure and strength.

Calcium Homeostasis

• Falling blood Ca2+ levels stimulate parathyroid hormone (PTH) release, which increases osteoclast activity to release Ca2+ from bone.

• Rising blood Ca2+ levels inhibit PTH release, reducing osteoclast activity.

Equation for Calcium Homeostasis (Negative Feedback Loop):

$\text{Low blood Ca}^{2+} \rightarrow \text{PTH release} \rightarrow \text{Osteoclast activation} \rightarrow \text{Ca}^{2+} \text{ released from bone} \rightarrow \text{Blood Ca}^{2+} \text{ rises}$

Bone Repair

Steps in Bone Fracture Repair

1. Hematoma formation at the fracture site.

2. Fibrocartilaginous callus formation by fibroblasts and chondroblasts.

3. Bony callus formation by osteoblasts.

4. Bone remodeling restores bone to original shape.

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; most abundant	Articular surfaces, costal cartilage, nose, trachea, larynx
Elastic	More elastic fibers; maintains shape	External ear, epiglottis
Fibrocartilage	Thick collagen fibers; tensile strength	Intervertebral discs, menisci, pubic symphysis

Summary Table: Types of Bones

Type	Shape	Example
Long	Longer than wide	Femur, humerus
Short	Cube-shaped	Carpals, tarsals
Flat	Thin, flat, curved	Sternum, ribs, skull
Irregular	Complex shapes	Vertebrae
Sesamoid	Special, within tendons	Patella

Additional info:

• Bone markings, sutures, sinuses, and fontanels are important anatomical features for identification in lab settings.

• The spinal column has primary (thoracic, sacral) and secondary (cervical, lumbar) curvatures, which develop at different stages of life.