For example, while it is not possible to see the membranes and ribosomes that compose the surface of the rough endoplasmic reticulum, these structures are represented in light microscope slides by clumps of basophilic material in certain regions of the cell. Using an appropriate staining technique, however, makes aggregates of these smaller structures or the regions they occupy visible by light microscopy. With light microscopy, one cannot visualize directly structures such as cell membranes, ribosomes, filaments, and small granules and vesicles. The electron microscope is necessary to see smaller organelles like ribosomes, macromolecular assemblies, and macromolecules. Thus, light microscopes allow one to visualize cells and their larger components such as nuclei, nucleoli, secretory granules, lysosomes, and large mitochondria. The limit of resolution of the light microscope is 0.2 µm, while the practical limit of resolution of the electron microscope is about 1 nanometer (nm). Microscope images in this course come from the light microscope (magnification up to 400x) and the electron microscope (magnification up to 500000x). Therefore, we must use a microscope to visualize cells in a tissue. With the unaided eye, one can only see exceptionally large cells, such as the human ovum, which has a diameter of 100 µm. Lastly, inclusions are insoluble substances in the cytosol such as glycogen and lipid droplets.
In addition, cytosol contains the cytoskeleton (microtubules, actin filaments and intermediate filaments) that organize the organelles and provide mechanical support. Cytosol is a gel-like substance that contains dissolved macromolecules, organic compounds and ions. Membrane-bound organelles concentrate enzymes and reactants, increasing biochemical efficiency and isolating harmful proteins and molecules from the rest of the cell. Many organelles are surrounded by a membrane that separates their internal environment from the cytoplasm. Organelles are assemblies of specific macromolecules organized to carry out complex functions. Cytoplasm is further divided into organelles, cytosol and inclusions. The cell’s content is divided into two main compartments: the nucleus and the cytoplasm that surrounds the nucleus. The cell is limited by the cell membrane, also known as the plasma membrane. The cells in an organ communicate and work together to perform the functions of that organ. Several different types of tissues then organize to form organs. Cells and the extracellular material they make comprise the tissues of our bodies. Cells also assemble into groups to form complex structures. Cells grow, adapt to their environment and reproduce, processes which characterize life. The cell is the fundamental unit of living organisms. It is also used as a fixative for electron microscopy. Osmium staining blackens lipids and stains the Golgi apparatus under the light microscope. In this method, periodic acid oxidizes 1,2-glycols and 2,2-amino alcohols to aldehydes, which are then stained reddish purple by the Schiff reagent. The periodic acid-Schiff method (PAS) is useful for staining structures rich in polysaccharides (glycogen), mucopolysaccharides (ground substance, basement membrane, mucous), glycoproteins (thyroglobulin), and glycolipids. Positively charged structures are therefore said to be "eosinophilic." While H&E staining is widely used, it is limited in its ability to differentiate between cytoplasmic organelles and many other tissue components.
Eosin is pink in color and is an acidic dye (negatively charged) that binds to positively charged particles like the mitochondria and many components of the cytoplasm.
Hematoxylin is a basic dye (positively charged) that binds to negatively charged DNA and RNA and is blue in color. Hematoxylin and eosin (H&E) staining is the standard method of staining in histology. These dyes react with different chemical features of proteins, nucleic acids and carbohydrates and can be used to highlight certain cellular structures. To help us visualize the structure and features of cells, dyes are used to impart a particular color to cells. Cells are difficult to see by light microscopy.