Histology is anatomy on a much smaller scale. It’s the anatomy of the cells in tissues and how they grow, connect, and interact with one another, in the same way that (gross) anatomy is about how different parts of the body grow, connect, and interact with one another.
It’s important because many diseases have distinct histological signs. For example, [here’s a piece of brain tissue from a cow with Mad Cow Disease](https://en.wikipedia.org/wiki/Bovine_spongiform_encephalopathy#/media/File:Histology_bse.jpg). The gaps in the tissue are characteristic of mad cow (compare the healthy grey matter on [the right side of this image](https://en.wikipedia.org/wiki/Grey_matter#/media/File:Grey_matter_and_white_matter_-_very_high_mag.jpg)), which lacks those gaps. Similarly, [pleomorphism](https://en.wikipedia.org/wiki/Pleomorphism_(cytology)) (a lot of variance in the size of different cells of the same type) suggests possible cancer (since it’s influenced by the same kinds of mutation that lead to cancer).
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