Lichen, Lichen, Everywhere

Medicine has a lot of descriptive terms. Sometimes it's related to food - ex: "currant jelly" sputum seen in Klebsiella infection, anchovy-paste pus of an amoebic liver abscess, but sometimes, it's not. To a novice in dermatology, there seems to be an awful lot of reference to lichen. Lichen planus, lichen sclerosis et atrophicus, lichen simplex chronicus, lichen nitidus, lichen striatus, to name a few, which are all dermatologic diagnoses that look very different to the naked eye.

Whoever came up with these names obviously knew their lichens. For those of us who grew up in the city, lichen is about as greek as it gets. What in the world is this lichen thing, and why does it look so different?? So for those of you in the same boat, here's what I've learned:

Lichen is a cominbation of two organisms - a fungus and an algae. The latter produces food for the fungus via photoxynthesis, and the fungus provides the algae with minerals and water absorbed from the base they grow on (tree, soil, rock, etc). Most of the lichen's bulk is made up of fungus, and the shape of the lichen is usually determined by the fungal partner. Since they have little control over the amount of water in their environment, lichens are "poikilohydric," which means they can tolerate and survive through long periods with no water. As such, they can be found in more extreme environments where other plant forms may not be able to survive.

Lichen have different jobs as part of the environment. When they grow on soil, they assist help stabilize the sand and retain water. When they grow on rocks, some lichen contribute to the weathering of rocks to soil. They also serve as a food source for animals such as reindeer and butterflies.

So how come all these diagnoses look different but contain the name "lichen?" Turns out lichen differ greatly in what they look like and are classified into categories based on appearance: crustose (flat, paint-like), filamentose (hair-like), foliose (leafy), fruticose (branched), leprose (powdery), squamulose (scale-like), gelatinous. Who woulda thunk?