Michalowski has described tissues as either hierarchical or flexible, based on their cell populations. In this model, there are three cell types:
- Stem cells which exist to replicate, producing daughter cells which maintain the stem cell population or differentiate into other types of cells. These cells make up the basal layer of the skin, or live in the crypts of the gut.
- Functional cells which are fully differentiated and incapable of further division. These include the mature epithelial cells of the gut and skin, or the neutrophils and erythrocytes of the blood.
- Maturing partially differentiated cells which exist in between these two states. For example, in the bone marrow cells may follow a number of pathways between the original stem cell and the eventual blood cell. These cells may still divide and give rise to progeny; however the capability for division is limited as they do not possess telomerase.
Hierarchical tissues (H-type) have populations of all three types, with stem cells constantly giving rise to maturing cells which eventually fully differentiate and become functional cells. Hierarchical tissues include most epithelial layers and the bone marrow.
Flexible tissues (F-type) are different in that the cells rarely divide but may be induced to by damage. Their cells are functional but retain the ability to re-enter the cell cycle if required. F-type tissues include the liver, thyroid and the dermis of the skin.
Many tissues are a hybrid of these two types, and their response to radiation varies.
In general, H-type tissues respond rapidly to radiation damage as the stem cell population is killed. This prevents the production of more maturing cells and leads to symptoms related to this – eg: thinning of epithelium or decrease in blood cell production. F-type tissues may not display damage for some time, particularly if the dose is small, because not all cells enter the cell cycle immediately.