Human cheek cells are a classic subject of school microscopy. It is easy to collect some by gently scraping the inside of your cheek. This is a high resolution phase contrast image of one of my cheek cells, put together using focus stacking of a 4 by 4 montage of 57 focus slices using one of my ImageJ macros. The detail of the nuclear structure, the granular contents of the cytoplasm and the structured surface of the cell really jump out.
This cell is quite large for mammalian cells, about 75 μm across, and around 10 times larger than the single cell Leishmania parasite I currently do much of my research on. If you have sharp eyesight you can even see human cheek cells by eye (although only just) when they are spread on a slide.
Like most mammalian cells, cheek cells are essentially transparent. If you use a microscope in the most basic way, essentially as a giant magnifying glass, shining light straight through the sample towards your eye, you see something like this:
Bright field micrograph of a human cheek cell.
This picture has even had the contrast artificially enhanced. Practically it is tough to even find the cells on the slide and get them in focus!
For many years the best alternative was oblique or dark field microscopy. Here you deliberately avoid shining light straight through the sample, and instead make sure that only light scattered by structures in the sample can collected by the objective lens and get up to your eye.
Dark field micrograph of a human cheek cell.
Images by dark field microscopy can be hard to interpret, and are typically limited to fairly low resolution.
More complex methods based on interference of light travelling through the sample were developed in the 20th Century. These methods, phase contrast and differential interference contrast, were a revolution. They allowed completely new approaches for looking at the biology of cells, particularly live cells and dynamic processes like cell division. They were such a revolution that the inventor of phase contrast microscopy, Frits Zernik, was awarded the Nobel prize in Physics in 1953 for this work.
Phase contrast micrograph of a human cheek cell.
DIC micrograph of a human cheek cell.
It was not until the development of the famous green fluorescence protein, for fluorescence microscopy in live cells in the 1990s, that there was another discovery which improved the capacity for live cell microscopy to the same extent as phase contrast and DIC.