Unit 6: Epithelial Cells

I. Orientation: Epithelial Tissue and the Laboratory Principle

Epithelial tissue consists of closely packed cells that cover body surfaces, line internal cavities, and form the functional boundaries of organs. Human cheek epithelial cells are squamous epithelial cells from the non-keratinized stratified squamous epithelium lining the inside of the buccal cavity. In this laboratory, a small sample is transferred to a microscope slide, stained to increase contrast, and examined using progressively higher magnification.

• Primary function: Epithelial layers provide protection, secretion, absorption, filtration, and sensation. The cheek lining mainly protects underlying tissues from abrasion during chewing.
• Cell arrangement: Epithelial cells are tightly packed with minimal extracellular material between neighboring cells.
• Cell shape: Cheek surface cells are broadly flattened or squamous. Their thin form is suited to lining rather than strong mechanical support.
• Cell organization: A typical cheek cell contains a plasma membrane, cytoplasm, and nucleus. Most visible organelles are below the resolution of a school or teaching microscope.
• Tissue layering: The cheek lining is stratified squamous epithelium, meaning it has multiple cell layers with flattened cells at the exposed surface.
• Microscopy principle: Magnification enlarges an image, while resolution determines whether two nearby structures can be distinguished as separate.
• Staining convention: Methylene blue commonly stains nuclear material dark blue, making the nucleus easier to identify against lightly stained cytoplasm.
• Biological sampling: Cheek cells are shed naturally from the epithelial surface, so collecting them is generally simple and minimally invasive when sterile technique is used.
• Unit awareness: Magnification is dimensionless; microscope field diameter and cell dimensions are usually recorded in micrometres (µm), where (1\ \text{mm}=1000\ \text{µm}).

II. Observation of Human Cheek Epithelial Cells — Collection, Preparation, and Microscopy

Human cheek epithelial cells provide a convenient temporary specimen for connecting epithelial structure with the practical skills of slide preparation, staining, focusing, and biological interpretation.

A. Purpose and Experimental Principle

The purpose of this method is to prepare a stained wet mount of buccal epithelial cells and identify their major visible structures under a light microscope. The procedure depends on obtaining a thin, representative sample and using stain without obscuring the specimen.

• Specimen source: The sample is collected from the inner cheek, where superficial epithelial cells are continuously shed.
• Sampling action: A sterile cotton swab or sterile toothpick is gently rubbed against the inner cheek. Forceful scraping is unnecessary and can injure the mucosa.
• Slide preparation: The collected material is transferred to a clean glass slide in a small drop of water or physiological saline.
  • Water: Provides a transparent medium but may cause some cells to swell.
  • Physiological saline: More closely matches cellular osmotic conditions and helps reduce extreme water movement.
• Staining purpose: A dilute methylene blue solution increases contrast, especially around the nucleus, which is naturally difficult to distinguish in an unstained cheek-cell preparation.
• Temporary mount: The preparation is designed for immediate observation rather than long-term preservation; it may dry or deteriorate over time.

B. Observation of human cheek epithelial cells

Observation of human cheek epithelial cells involves locating the cells under low power, bringing them into sharp focus, and recording their shape, arrangement, and visible internal features.

• Materials: A clean slide, coverslip, sterile swab or toothpick, dropper, water or saline, methylene blue, microscope, lens tissue, and disinfectant are typically required.
• Initial preparation: Place one small drop of water or saline near the center of the slide, then transfer the cheek sample into the drop.
• Staining step: Add a small drop of methylene blue and allow brief contact, commonly about 1–2 minutes depending on laboratory instructions.
  • Excess stain: May produce a very dark background and hide cell boundaries.
  • Insufficient stain: May leave the nucleus too faint for confident identification.
• Coverslip placement: Lower the coverslip at an angle using a mounted needle or forceps. This technique reduces trapped air bubbles.
  • Air bubbles: Appear as large, sharply outlined circles and should not be mistaken for nuclei.
• Low-power search: Begin with the scanning objective, often (4\times), or the low-power objective, often (10\times).
  • Reason: The lower-power field is wider, making scattered cells easier to locate.
  • Focusing: Use the coarse adjustment only with low power, then use fine adjustment for a sharp image.
• High-power observation: After centering a cell, change to (40\times) high power and use fine adjustment only.
  • Total magnification: Calculate by multiplying eyepiece magnification by objective magnification.

Total magnification = eyepiece magnification × objective magnification

• Symbol definitions: “Total magnification” is the final image enlargement; “eyepiece magnification” is the ocular-lens power; “objective magnification” is the selected objective-lens power.
• Worked example: A (10\times) eyepiece with a (40\times) objective gives:

Total magnification = 10 × 40 = 400×

• Expected cell appearance: A cheek epithelial cell generally appears as a large, thin, irregularly polygonal or roughly rounded structure with a distinct darker nucleus.
• Plasma membrane: The cell boundary may appear as a thin outline. It is often less sharply defined than the nucleus because the membrane is delicate and transparent.
• Cytoplasm: The cytoplasm forms the lightly stained region between the nucleus and the cell boundary. It may appear pale blue after methylene-blue staining.
• Nucleus: The nucleus is usually the darkest prominent structure, often oval or rounded and centrally or slightly eccentrically located.
  • Interpretation: The dark appearance results from stronger dye binding to nucleic-acid-rich nuclear material, not from the nucleus being a separate external object.
• Absence of a cell wall: Human cheek cells are animal cells, so they lack the rigid cellulose cell wall found in plant cells.
• Absence of chloroplasts: Chloroplasts are not expected because cheek cells are non-photosynthetic animal cells.
• Vacuoles: A large, central plant-like vacuole should not be expected. Small vesicles may exist but are generally not resolved in this preparation.
• Cell arrangement: Cells may overlap, fold, or appear singly. Overlapping cells can make a single broad outline look like multiple structures.
• Field movement: Because the microscope forms an inverted image, moving the slide to the right makes the image appear to move left, and moving it toward the observer makes the image appear to move away.
• Observation record: A useful record includes objective power, total magnification, cell shape, relative size, staining intensity, nucleus position, and any preparation artifacts.

C. Measurement, Drawing, and Interpretation

Accurate interpretation requires separating genuine biological structures from artifacts produced during collection, staining, mounting, or focusing.

• Biological drawing: Draw only structures actually observed, using clear single lines rather than artistic shading.
  • Labels: Identify the plasma membrane, cytoplasm, and nucleus when visible.
  • Title: Include the specimen and magnification, for example, “Human cheek epithelial cell, total magnification (400\times).”
  • Proportion: The nucleus should be drawn in a realistic position and relative size rather than enlarged independently for convenience.
• Scale estimation: If the field diameter is known, cell size can be estimated from the fraction of the field occupied by the cell.

Estimated cell length = fraction of field occupied × field diameter

• Symbol definitions: “Estimated cell length” is the approximate longest cell dimension; “fraction of field occupied” is the cell’s length expressed as a proportion of the field diameter; “field diameter” is the visible circular width at a specified objective.
• Worked example: If a cell spans approximately (0.20) of a (450\ \text{µm}) field:

Estimated cell length = 0.20 × 450 µm = 90 µm

• Resolution versus magnification: Increasing from (100\times) to (400\times) enlarges the image, but it does not necessarily reveal more detail if the microscope’s resolving power is limiting.
• Depth of field: At high magnification, only a narrow vertical plane is sharply focused. Fine adjustment may bring the nucleus into focus while the cell edge becomes less distinct.
• Contrast: Staining improves visual contrast but can alter the apparent color and intensity of structures. A darker structure is not automatically a larger or more important structure.
• Comparison with plant cells: Cheek cells usually lack the regular rectangular outline, cell wall, chloroplasts, and large central vacuole commonly seen in leaf epidermal cells.
• Cell specialization: The broad, thin form of a surface cheek cell reflects its protective and lining role. It should not be interpreted as a complete description of every epithelial cell type.
• Nuclear interpretation: A visible nucleus indicates that the sampled cell is nucleated. It does not demonstrate that all cells in the body have the same nuclear appearance; mature mammalian red blood cells, for example, lack nuclei.
• Replicate observations: Viewing several cells improves reliability because one cell may be folded, damaged, poorly stained, or partly hidden by another cell.

D. Safety, Sources of Error, and Limitations

Safe handling and critical evaluation are essential because the specimen is human-derived and the temporary preparation has several technical limitations.

• Personal protection: Wear a laboratory coat and eye protection; wash hands before and after the activity.
• Aseptic collection: Use a sterile, single-use swab or toothpick for each person. Do not share sampling tools.
• Sharps and glass: Handle slides and coverslips carefully; dispose of broken glass in an approved sharps container.
• Biological material: Treat cheek samples as potentially infectious. Do not taste, touch the sample, or return used tools to reagent containers.
• Stain safety: Methylene blue can stain skin, clothing, and benches; avoid contact and follow local disposal instructions.
• Focus damage: Never use coarse focus with a high-power objective because the lens may strike the slide.
• Uneven sampling: A swab that collects too little material may produce an empty field, while excessive material may create thick overlapping layers.
• Air bubbles: Bubbles can resemble circular cells or nuclei; their bright outline and lack of internal cytoplasm help distinguish them.
• Stain precipitate: Dark isolated particles may be dried stain rather than cellular structures. They often appear as irregular dots outside cell boundaries.
• Drying: Evaporation changes the concentration of stain and can distort cells, so observations should be made soon after mounting.
• Optical artifacts: Dust on the eyepiece or slide may remain stationary relative to the field or appear sharply defined in a different focal plane.
• Limited resolution: A standard light microscope cannot reliably show ribosomes, individual chromosomes, or most organelles in cheek cells.
• Non-quantitative appearance: Visual estimates of size and color are approximate unless calibrated with a stage micrometer or known field diameter.
• Interpretive limitation: A cheek-cell preparation demonstrates general animal-cell organization and epithelial morphology; it does not independently diagnose disease or establish the full structure of buccal tissue.
• Reliable conclusion: The strongest conclusion is based on repeated observation of multiple stained cells showing a thin boundary, pale cytoplasm, and a darker nucleus at a stated magnification.