Unit 6: Epithelial Cells - Subjective Questions

Epithelial cells are cells that form the thin outer covering of body surfaces and line internal cavities. Human cheek epithelial cells are suitable for observation because:

• They are easily obtained by gently scraping the inside of the cheek.
• They are loosely arranged and are shed naturally.
• They are large enough to be observed using a light microscope.
• They have a distinct cell membrane, cytoplasm, and nucleus after staining.
• Their collection is simple, painless, and minimally invasive when performed correctly.

The following materials and apparatus are generally required:

• A clean glass microscope slide and coverslip.
• A sterile toothpick, cotton swab, or disposable wooden applicator.
• A dropper or Pasteur pipette.
• Normal saline or distilled water.
• A suitable stain, such as methylene blue.
• Forceps or a mounted needle for handling the coverslip.
• A compound light microscope.
• Lens tissue or blotting paper for cleaning excess liquid.
• Gloves and disinfectant for safe handling and disposal.

All instruments that come into contact with the mouth should be sterile or disposable.

A temporary cheek-cell preparation can be made as follows:

1. Clean a microscope slide thoroughly.
2. Place one drop of normal saline or distilled water at the center of the slide.
3. Gently scrape the inside of the cheek with a sterile toothpick or cotton swab.
4. Mix the collected material into the drop on the slide.
5. Add one drop of methylene blue stain.
6. Carefully lower a coverslip at an angle to reduce the formation of air bubbles.
7. Remove excess stain with blotting paper.
8. Examine the slide first under low power and then under high power.
9. Record observations using a labeled drawing or written description.

The slide should be observed promptly because it is a temporary preparation.

Methylene blue is used because cheek cells are almost transparent and have very little natural color. The stain improves contrast and makes cellular structures easier to identify.

• It binds strongly to acidic cell components, especially nucleic acids.
• The nucleus becomes dark blue and clearly visible.
• It helps distinguish the cell boundary from the surrounding liquid.
• It improves visibility of the cytoplasm and general cell outline.
• It allows the observer to focus and identify structures more accurately.

The stain should be used in an appropriate amount because excessive staining can obscure cellular details.

A human cheek epithelial cell usually appears as a large, thin, irregularly shaped or roughly polygonal cell. Its main features include:

• A distinct but delicate cell membrane.
• A lightly stained cytoplasm surrounding the nucleus.
• A prominent, darkly stained nucleus, usually near the center of the cell.
• No cell wall.
• No chloroplasts.
• A generally flat appearance because the cell is part of a squamous epithelium.

The exact shape and visibility of structures may vary according to the quality of the smear, amount of stain, and microscope adjustment.

Human cheek epithelial cells and plant cells differ in several important ways:

These differences reflect the different structures and functions of animal and plant cells.

Cell membrane: It forms the outer boundary of the cell and regulates the movement of substances into and out of the cell. It also helps maintain the cell's internal environment.

Cytoplasm: It is the jelly-like material inside the cell membrane. It contains cell organelles and is the site of many metabolic reactions.

Nucleus: It contains the cell's genetic material and controls many cell activities, including growth, repair, and protein production. In a stained cheek-cell preparation, the nucleus usually appears as the darkest structure.

The coverslip should be lowered gradually at an angle because this technique:

• Minimizes the formation of air bubbles.
• Spreads the sample into a thin, even layer.
• Prevents the liquid from flowing off the slide.
• Protects the microscope objective from direct contact with the sample.
• Makes the cells easier to focus and observe.

If a coverslip is dropped directly onto the liquid, trapped air bubbles may appear as large circular structures and can be mistaken for cells or obscure the specimen.

Low power should be used first because it provides a wider field of view and makes it easier to locate the specimen. It also allows the observer to center a suitable cell before increasing magnification.

After the cell is centered and focused, high power can be used to examine details such as:

• The cell membrane.
• The cytoplasm.
• The shape and position of the nucleus.
• Any staining irregularities.

Starting with high power may make the specimen difficult to locate and may increase the risk of damaging the slide or objective lens.

Normal saline or distilled water and methylene blue have different purposes.

• Normal saline or distilled water: It provides a liquid medium in which the collected cells can be spread. Normal saline helps maintain conditions that are less damaging to the cells, while distilled water is commonly used for simple temporary preparations.
• Methylene blue: It is a stain that increases contrast and makes the nucleus and other cellular features easier to see.

The liquid medium helps prepare and distribute the cells, whereas the stain improves their visibility under the microscope.

Aseptic technique is important because human cheek cells may carry microorganisms, and laboratory equipment can become contaminated.

Important safety practices include:

• Use a sterile or disposable applicator for each person.
• Do not share toothpicks, swabs, slides, or other collection materials.
• Avoid scraping the cheek forcefully, which could cause injury.
• Wear gloves and wash hands before and after the activity.
• Do not eat, drink, or touch the face during the practical.
• Dispose of used applicators and slides according to laboratory rules.
• Disinfect work surfaces after the experiment.
• Never return used materials to reagent containers.

These measures protect the student, instructor, and laboratory environment.

Improper focusing or illumination can make the specimen difficult to interpret.

• If the objective is not correctly focused, cell boundaries and nuclei appear blurred.
• Excessive light may reduce contrast and make stained structures appear pale.
• Insufficient light may hide fine details.
• An incorrectly adjusted diaphragm can produce a flat or overly bright image.
• Failure to center the specimen may cause the cell to move out of view when magnification is increased.
• Dirty lenses can create spots or hazy regions that may be mistaken for cellular structures.

Correct illumination, careful focusing, and clean lenses are therefore essential for reliable observations.

A scientific observation should be based only on structures that are actually visible. The procedure includes:

1. Observe the specimen under the appropriate magnification.
2. Select a clearly visible, undamaged cell.
3. Draw the cell using clear, continuous lines rather than artistic shading.
4. Show the relative size and position of the nucleus and cytoplasm.
5. Label visible structures such as the cell membrane, cytoplasm, and nucleus.
6. Include the microscope magnification used.
7. Add a suitable title and brief description.

The drawing should be large enough to show details and should not include structures that were not observed.

The epithelial tissue lining the cheek forms a protective covering for the inner surface of the mouth. Its functions include:

• Protecting underlying tissues from friction caused by chewing.
• Providing a barrier against microorganisms and harmful substances.
• Maintaining the moist lining of the oral cavity.
• Contributing to the repair of the mouth lining when cells are damaged.

The cells are thin and closely arranged, forming a continuous layer. This arrangement provides an effective protective barrier while allowing the lining to remain flexible during speaking, chewing, and swallowing.

Human cheek epithelial cells are described as squamous because they are thin and flattened in shape. They form part of the stratified squamous epithelium lining the inside of the cheek.

Their flattened structure is useful because:

• It allows several layers of cells to form a protective covering.
• It provides flexibility to the mouth lining.
• The surface cells can be shed and replaced.
• The tissue can withstand repeated friction from food and movement.

The cells observed in a smear are usually superficial cells that have naturally detached from the lining.

Several errors can affect the quality of the preparation:

• Insufficient scraping: Too few cells may be collected. Use a gentle but adequate scraping technique.
• Excessive scraping: The cheek may be injured. Use minimal pressure and a sterile applicator.
• Too much stain: The image may become too dark. Use only a small drop.
• Too little stain: The nucleus may be difficult to see. Apply enough stain to cover the sample.
• Air bubbles: They may obscure cells. Lower the coverslip gradually at an angle.
• Dirty slide or lens: Foreign particles may be mistaken for cells. Clean all surfaces before observation.
• Poor focusing: Cellular details may be unclear. Focus carefully, beginning with low power.
• Overcrowded smear: Cells may overlap. Spread the sample evenly in the liquid drop.

The total magnification of a compound microscope is calculated by multiplying the magnification of the eyepiece lens by the magnification of the objective lens:

For example, if the eyepiece is and the objective is :

The total magnification should be recorded with the observation because it indicates the scale at which the cells were viewed. Higher magnification increases apparent size but does not necessarily improve image clarity unless the specimen is well prepared and focused.

The cell membrane is a thin, flexible boundary present in cheek cells. It controls movement of substances into and out of the cell and helps maintain the cell's internal conditions.

A cell wall is a rigid outer layer found in plant cells, fungi, and many microorganisms, but it is absent in animal cells such as human cheek epithelial cells.

In a cheek-cell preparation, the cell outline may be faint because it is formed only by the delicate cell membrane. It does not usually appear as a thick, rigid boundary like the cell wall of a plant cell.

Cheek epithelial cells may appear either separately or in groups because of the way they are collected and spread.

• The cells are naturally shed from the surface of the cheek.
• Some cells remain attached to one another in small clusters.
• Gentle scraping may collect isolated cells and small groups.
• Strong mixing or spreading may separate the cells.
• Uneven distribution of the sample can cause some regions to contain many cells while others contain very few.

The presence of groups does not necessarily indicate a problem, although overcrowding can make individual cell structures difficult to observe.

A temporary cheek-cell preparation is useful for observing basic structures, but it has limitations:

• It usually shows only a few clearly visible structures, such as the membrane, cytoplasm, and nucleus.
• Most organelles are too small to be resolved with a standard light microscope.
• The cells may be damaged during collection or preparation.
• The staining process can alter the natural appearance of the cells.
• Cell dimensions may not be accurate if the preparation is compressed under the coverslip.
• The cells are usually viewed in isolation rather than as part of an intact tissue layer.
• The preparation cannot normally be stored for long periods.

Therefore, the slide is suitable for basic observation but not for detailed ultrastructural analysis.