Some effusions

The usual challenge in examing body cavity fluids (pleural fluid, peritoneal fluid) is to distinguish benign from malignant cells. This takes some training, and is made difficult by the fact that mesothelial cells (the cells lining these cavities) can often take odd shapes and form bizarre clusters. However, the telltale, purplish, 'open' chromatin of a cancerous epithelial cell (resembling a sieve - i.e. euchromatin) will always stand out.

Colon cancer in peritoneum

 40F, unknown primary, any guess?

Ovarian cancer

Note the surface blebs.

Another spill from ovary, maybe mucinous

Signet ring cells, possibly colon cancer (mesothelial cells might look like this over time, too!)

Reactive mesothelial (benign) cells

Plasmacytoid mesothelial cells in a reactive pleural effusion. The cells come in all shapes and sizes, but note that the nucleus is still compact, dark, and completely made of heterochromatin.