# Practical: Dissection of Gas Exchange Surfaces in Fish & Insects ## Why Dissections Matter - Dissections are a fundamental technique in biological research - They enable scientists to examine the internal structures of organs to understand how they function - Observing real tissue provides insights that diagrams alone cannot convey

Ethical considerations include animal welfare during rearing and slaughter, and respect for religious beliefs

- Biological specimens must be sourced from reputable suppliers and disposed of properly - When comparing multiple specimens, select organisms from the same species and similar age group to ensure consistency ## Equipment Required | Item | Purpose | |------|---------| | Scissors | Cutting large sections of tissue where precision is not critical | | Scalpel | Making fine, precise incisions | | Tweezers/Forceps | Manipulating and moving tissue sections | | Dissection board | Providing a stable working surface | | Paper towels | Cleaning and absorbing fluids | | Pins | Securing tissue sections to expose structures | ## General Dissection Technique - Wear a lab coat, gloves and eye protection throughout - This prevents contamination with biological material that could trigger allergic reactions or infections - Position the specimen securely on the dissection board - Use appropriate tools to access the target structure: - When using a scalpel, always cut away from your body - Keep fingers well clear of the blade to prevent accidental cuts - Use scissors for bulk tissue removal where neat edges are unnecessary - A sharp scalpel ensures clean, precise cuts for detailed work - Employ pins to hold tissue sections aside, revealing the structure of interest ## Limitations of Dissection - Smaller, delicate structures within organs can be difficult to distinguish with the naked eye - Preserved specimens appear different from living tissue - Loss of colour, texture changes, and structural collapse can occur - Examining only a single specimen risks missing normal variation or overlooking anomalies specific to that individual ## Dissecting Mammalian Lungs The major visible structures in a dissected mammalian lung include: - **Trachea** - the main airway leading from the throat to the bronchi - **Bronchi** - the two branches that split from the trachea, one entering each lung - **Bronchioles** - smaller airways branching from the bronchi throughout the lung tissue

Alveoli are challenging to observe in dissected lungs because they are microscopic and collapsed without air

- Even though alveoli are the gas exchange sites, their tiny size and deflated state make them hard to identify during dissection - Microscopy is required to properly examine alveolar structure ## Dissecting Bony Fish Gills Key structures visible in dissected fish gills: - **Gill arch** - the skeletal support structure (resembles a curved bone) - **Filaments** - thin, flat projections extending from each gill arch in two rows - Filaments give gills their feathery appearance

Lamellae are the tiny plates on each filament where gas exchange occurs, but they can be difficult to distinguish without magnification

- To access the gills, carefully remove the operculum (gill cover) on the side of the fish's head - The gills are usually dark red in fresh specimens due to their extensive blood supply ## Dissecting Insect Tracheal Systems - Insect respiratory systems are extremely small, requiring specialised techniques - The tracheal network consists of tubes that deliver air directly to tissues - **Spiracles** are the external openings visible on the insect's body surface

Due to their minute size, insect tracheal systems often require specialised equipment and microscopy to examine properly

- Standard dissection tools may be too large for precise work on insects - Advanced skills in microscopy and possibly electron microscopy are necessary for detailed observation ## Observing Gas Exchange Surfaces with Microscopy Microscopic examination reveals cellular details of gas exchange surfaces Photomicrographs often look very different from textbook diagrams, so practice identifying key features ### Mammalian Lung Tissue Under the Microscope When examining stained lung tissue sections: - **Alveoli** appear as irregular spaces of varying sizes and shapes - They no longer maintain the rounded, inflated appearance seen in living lungs - Their walls have collapsed without air pressure - **Nuclei** of epithelial cells appear as dark-stained dots lining the alveolar walls - **Blood vessels** (capillaries and larger vessels) can be identified between the alveoli - Sometimes contain visible red blood cells - Occasionally white blood cells may be present in tissue samples

The thin alveolar walls and adjacent capillaries create a short diffusion distance for efficient gas exchange

### Fish Gill Tissue Under the Microscope Stained sections of fish gills (commonly from dogfish) reveal: - **Gill arch** - appears as a central supporting structure (like a backbone) - **Filaments** - extend outward from the gill arch on both sides - Multiple filaments are visible in cross-section - **Lamellae** - thin plates projecting from each filament - These create the large surface area for gas exchange - Many lamellae are visible along each filament - Their regular arrangement resembles the teeth of a comb The staining typically highlights the blood vessels within the lamellae, showing the extensive capillary network ### Insect Spiracles Under the Microscope - Spiracles appear as circular or oval openings in the insect's exoskeleton - The opening often contains structures to prevent water loss and filter particles

Electron microscopy provides the clearest images of spiracles, revealing intricate valve structures and surrounding hairs

A spiracle from a caterpillar, for example, shows: - The external opening through the cuticle - Valve mechanisms that can close to reduce water loss - Fine hairs that filter incoming air ## Practical Skills and Exam Relevance When describing dissection methods in exams: - Name specific tools (e.g., "use scissors to remove..." or "use forceps to separate...") - Specify the sequence of steps clearly - Mention safety precautions like cutting away from the body For microscopy identification: - Practice recognizing structures in photomicrographs, not just diagrams - Be able to justify your identification based on visible features - Understand why structures appear different when not in a living state