Apoptosis put very simply is cellular suicide. It is the natural process the cells in your body undergo in order to get rid of cells that are no longer needed, or cells that are detrimental, such as those with DNA damage or a virus. It can also occur during development to form extremities like fingers. This is different from other types of cell death like necrosis, which is cell death that is a result of an injury. Apoptosis is a much neater process. During apoptosis, the parts of the cell break up into little packages that attract immune cells, like macrophages that get rid of the packaged chunks.
DNA damaged cells will undergo apoptosis to get rid of themselves so they don’t pass on their damaged DNA. If they don’t go into apoptosis, and they are not caught by immune cells that try to trigger apoptosis for them, they become successful cancer cells, mutating.
So where does histology come in? Since there are various types of cell death, and they share some common characteristics, markers are needed to determine if a cell is going through apoptosis. One of the most common ways this is done is through flow cytometry, a process in which cells are stained with specific, fluorescently labeled antibodies. For example, changes in the mitochrondria are one of the first steps in apoptosis. This can be detected using a dye like JC-1 that demonstrates red fluorescence in active mitochondria, but green once apoptosis is occurring. Similar methods can be used to identify other phases of apoptosis. For example, Phosphatidylserine (PS), a component of the cell membrane, moves to the outer membrane of the cell as apoptosis progresses, which is a sign for the phagocytes to come engulf the cell. Using a protein like Annexin V that binds to PS will allow you to identify this phase of apoptosis. This is usually done in conjunction with a viability dye, which acts as a control to weed out necrotic cell death.
Another common method of apoptosis detection is looking for DNA fragmentation, which occurs later in apoptosis as DNA starts to essentially leak out of the cell. This is often done using staining for TUNEL. You can find additional markers used in apoptosis through the sites of the vendors that offer these assays.