Cell components are largely transparent and nearly colorless, making them difficult to observe under a standard light microscope. Staining is a foundational technique in biology that introduces color to the specimen, which increases contrast between the cell and its surroundings and between internal structures. This process allows students and researchers to clearly distinguish the various cellular components that would otherwise be virtually invisible. The common laboratory exercise involving human cheek cells is often one of the first introductions to this necessary technique and the basic architecture of an animal cell.
The Target: Understanding Cheek Cells
The specimen observed in this introductory lab is the buccal epithelial cell, which are the flat cells that line the inside of the human cheek. These cells are easily and non-invasively collected by gently scraping the inner mouth, making them an ideal sample for educational purposes. These cells are classified as squamous epithelium. The nucleus, cytoplasm, and cell membrane are the main components of the cheek cell. In its natural, unstained state, the cell’s internal structures are pale and translucent, blending into the background, making it nearly impossible to identify the distinct boundaries of the nucleus or cytoplasm. The difficulty in discerning these parts highlights the practical need for contrast-enhancing methods in microscopy.
Methylene Blue’s Chemical Action
Methylene Blue is selected because it is a basic dye, meaning its active coloring component is positively charged (cationic). This positive charge gives the dye a strong chemical affinity for negatively charged (anionic) components within the cell. This interaction is known as differential staining, where the stain selectively binds to certain cellular materials. The primary targets of Methylene Blue are the nucleic acids—specifically deoxyribonucleic acid (DNA) concentrated in the nucleus and ribonucleic acid (RNA) found in the cytoplasm. Both DNA and RNA contain phosphate groups that give them a distinct negative charge, making them highly receptive to the cationic dye. This mechanism is why Methylene Blue is classified as a nuclear stain, as it is drawn to the high concentration of nucleic acids.
What the Stain Reveals
The selective chemical binding of the dye results in a distinct visual contrast that makes the cell’s internal architecture recognizable. The nucleus, containing the highest concentration of nucleic acids, absorbs the most dye and stains a deep blue or purplish color. This intensely colored nucleus stands out against the rest of the cell and the surrounding slide, allowing for its easy identification. The cytoplasm, which contains a lesser concentration of nucleic acids and other acidic components, stains a much lighter blue, or may remain largely unstained. This difference in color intensity between the deep blue nucleus and the lighter cytoplasm allows the observer to clearly distinguish between the nucleus and the cytoplasm. The cell membrane, which forms the outer boundary, becomes faintly outlined, completing the visualization of the cell’s basic structure and allowing for accurate observation of its size and shape.