January 28 – Go Climb A Tree!

Today’s factismal: There are 411 different types of cell in a healthy adult human.

In science, the interesting questions are always the weirdest ones. They are interesting because (1) most people understand that importance of the question once they hear it and (2) few people would have any idea of how to answer the question. “How many different types of cell does a healthy adult human have?” is one of those questions. It is important because the different types of cells do different things in the body and knowing the relative number of different types of cell with similar characteristics can tell us much about that characteristic’s importance. For example, 145 of those 411 different types of cell are types of neuron; obviously, thought is an important part of the human biological makeup!

So the question is clearly important, but how do we answer it? How can we know how many different types of cell a healthy adult human has? Did they go out and pick apart a politician, putting each cell type into a different pile? Sadly, no (the politicians objected). Instead, the researchers used a technique known as cladistic analysis. They looked over different tissue samples from dozens of different donors and were able to identify features that each cell type had in common. For example, mitochondria were present in most cells so the list was divided into those cells with mitochondria and those without; it “branched” at that point. The cells without mitochondria can further be broken down into those without nuclei and those with.

A tree of life cladogram (Image courtesy American Museum of Natural History)

A tree of life cladogram
(Image courtesy American Museum of Natural History)

Because a written list of all of these differences would be very hard to read (and even harder to keep track of), most scientists instead draw it as a tree-like structure known to geeks as a cladogram (“branch drawing”). You are probably familiar with the “tree of life” which was made in the same way as the tree of human cells. Once the scientists run out of meaningful differences between the different branches, the cladogram in complete and you can find out how many different types of thing there are by simply counting the branches.

Even better, the relationships between the branches often tell us a lot about how things are related. On the tree of life, we know that mammal (like us) are more closely related to lizards (like politicians) than they are to green plants (like broccoli). Those relationships can then help us understand things like “will this heart medicine be dangerous for my kidneys?”

Of course, making these cladograms takes time and effort, but it can be fun. If you’d like to get in on the fun, the folks at Citizen Sort have developed a series of sorting games that you can do to actually sort out data that they will use to understand more about biology. To get in on the fun and games, head over to:
http://citizensort.org/

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