How Anatomists Examine Organs at a Microscopic Level

The light microscope can enlarge images hundreds of times to see the details. Light must be able to pass through the object, so it has to be cut very thin. Since tissues are soft, they must be frozen very hard, or else fixed with a chemical like formaldehyde which toughens and stiffens the component molecules. Often they are then embedded in paraffin wax to make them stiffer still. They are next cut on a machine called a microtome, which produces slices (“sections”) just a few micrometers thick (one micrometer (μm) is one millionth of a meter; a typical hair is 50 μm wide). At this point the tissues are hardly visible, being so thin, and the components do not have much color. So the sections are dipped in a variety of special stains, which bind to and color different components (this is discussed in the Cell and Molecular Biology module).

Below is a section of lung seen through a light microscope; it has been stained with two dyes called hematoxylin and eosin (H&E). This is a very low-power view; the entire lung in cross-section is seen:

Now here’s a 40-fold magnification of the same section using a high-power lens:

The blue dots, stained with hematoxylin, are the nuclei of the cells; the pink material is the rest of the cell, called the cytoplasm. Around the lung air sacs there are a lot of small cells so the area is bluer; in the walls of the air tubes (bronchioles) the cells have more cytoplasm and are pinker.