Q: Would a pyknotic cell be a form of necrosis or apoptosis? Or am I totally off base here?

A. No you’re totally not off base – that’s a really good question!!

We typically use the word “pyknosis” to mean one of the three nuclear patterns seen in necrotic cells…but pyknosis can also occur in apoptosis! I’ll explain a bit more.

When cells undergo necrosis, they show a lot of different morphologic abnormalities. Overall, necrotic cells appear enlarged and more eosinophilic, and their nuclei look abnormal due to breakdown of DNA. There are three specific patterns of nuclear change in necrosis, which are:

  • Pyknosis (the nucleus shrinks and becomes dark blue/black)
  • Karyorrhexis (the nucleus breaks apart, or fragments, like a cookie crumbling into bits)
  • Karyolysis (the nucleus just fades away)

Here’s a nice diagram I found (I modified it a bit from the original) showing these changes:
When cells undergo apoptosis, they also show a lot of different morphologic features. Overall, apoptotic cells appear shrunken, with really dense, dark, eosinophilic cytoplasm, and the chromatin in the nucleus aggregates into a dense mass which can fragment. Here’s a photo from Robbins showing an apoptotic cell:

The word “pyknosis” isn’t typically used when describing an apoptotic cell – but Robbins does say that pyknosis can be a feature of apoptotic cells, so there you go.

So…the bottom line is that pyknosis is a nuclear change in which the nucleus shrinks and becomes dark blue/black. Typically, we associate the word “pyknosis” with necrotic cells – but apoptotic cells can show pyknosis too.