Mesenchymal cells, also referred to as mesenchymal stromal cells (MSCs), are a type of multipotent cell, meaning they can develop into a variety of other cell types. These include bone cells (osteoblasts), cartilage cells (chondrocytes), and fat cells (adipocytes). Originating from the embryonic connective tissue known as the mesoderm, these cells play a role in tissue repair and regeneration, and microscopy is a primary tool for observing their characteristics.
Preparing Mesenchymal Cells for Microscopic Viewing
Before mesenchymal cells can be properly observed, they must be prepared. This process begins with cell culture, where cells are grown on plastic surfaces, allowing them to multiply to sufficient numbers for study. Once an adequate population is available, they are prepared for viewing. Because cells are mostly transparent, they must be stained to make their structures visible under a standard microscope.
The preparation involves fixation, a step that uses chemicals to preserve the cell’s structure. Following fixation, stains are applied. Common staining methods include Hematoxylin and Eosin (H&E) or the Giemsa stain. For example, hematoxylin stains the cell nucleus a purplish-blue, while eosin stains the cytoplasm and extracellular matrix in shades of pink, providing clear visual contrast.
Key Morphological Features Under the Microscope
Under a microscope, undifferentiated mesenchymal cells have a distinct appearance. They are noted for their long, thin, and spindle-like shape, which is described as fibroblast-like. This elongated cell body contains a large, round nucleus that is centrally located. Within the nucleus, one or two prominent nucleoli are visible, and it is surrounded by finely dispersed chromatin particles, giving the nucleus a clear appearance.
The cytoplasm contains a small amount of other cellular machinery, such as mitochondria and endoplasmic reticulum. In a culture dish, these cells adhere to the plastic surface and are widely dispersed when the population is sparse. As they proliferate and the culture becomes more crowded, or confluent, the cells align themselves into swirling, colony-like patterns.
Common Microscopic Techniques Used
Scientists use several types of microscopes to visualize mesenchymal cells. The most common is bright-field microscopy, which is used to view cells that have been fixed and stained. This technique passes light directly through the specimen, making the colored parts of the cell, like the nucleus and cytoplasm, clearly visible against a bright background.
For viewing live, unstained cells, phase-contrast microscopy is preferred. This method enhances contrast in transparent specimens by manipulating the light that passes through them. It allows researchers to observe the shape and behavior of living MSCs without the need for staining, which kills the cells. Another tool is fluorescence microscopy, which is used to identify specific molecules on or inside the cells. In this technique, antibodies tagged with fluorescent dyes are used to bind to target proteins, causing specific parts of the cell to glow under special lighting.
Interpreting Microscopic Images of Mesenchymal Cells
Observing mesenchymal cells under a microscope provides biological insights. The characteristic spindle shape and adherence to culture plastics help scientists confirm they are working with the correct type of cell. The visual appearance can also be used to assess the health of the cells; healthy MSCs appear elongated and well-defined, while unhealthy cells may become rounded or show granular cytoplasm.
Microscopy is also used to monitor the purity of a cell culture, ensuring that it is not contaminated with other cell types. Furthermore, researchers can observe changes in cell morphology that indicate differentiation. For instance, when MSCs begin to differentiate into bone cells, they may become more cuboidal or polygonal in shape and form nodules. Observing these transformations provides direct evidence of the cells’ multipotent capabilities.