Final answer:
Osmium tetroxide is typically used as a fixative and as a stain for transmission electron microscopy due to its ability to provide contrast to lipid membranes and stabilize cell structures. Glutaraldehyde is also a fixative but does not serve as a stain.
Step-by-step explanation:
The chemical typically used as a fixative and as a stain for transmission electron microscopy (TEM) is osmium tetroxide (D). This agent is particularly useful in TEM as it fixes and stains lipid membranes, providing the needed contrast to observe the cell structures. Glutaraldehyde (A) can also be used as a fixative in TEM, as it helps to stabilize proteins and cell structures by cross-linking, but it does not act as a stain. Osmium tetroxide on the other hand serves dual purposes, making it highly valuable in TEM sample preparation.
Staining specimens is an essential practice in microscopy to enhance contrast within the sample, allowing for a clearer view of specific structures. Fixatives like formaldehyde and glutaraldehyde chemically preserve tissues by denaturing proteins and stopping biochemical reactions. Osmium tetroxide is not only a fixative but also contributes dark contrast to the specimens, hence its dual functionality. For light microscopy, different staining techniques and dyes like basic dyes (e.g., crystal violet) and acidic dyes are chosen based on the structural components of the cells or tissue being examined.