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Nuclear structure — in DIC!

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While I work on another post, here’s a nucleus of an amoeba to look at. Some amoebae tend to have large and fairly obvious nuclei, and, if the cell is thin enough, you can make out some nuclear ultrastructure! This specimen is flattened, so what we have here is not a completely natural representation of the nucleus, and we only have a view from one side — some nuclei have elaborate internal structures that aren’t symmetrical.

Around the edge is the nuclear envelope and, potentially, a periphery of fibres called lamina. The internal blob is presumably heterochromatin and potentially nucleolar in nature — but one can’t tell without using electron microscopy as we run into the light resolution barrier here. A true nucleolus (and there can be several, contrary to what some textbooks suggest!) should be full of ribosomal material under construction. Thus, the term ‘endosome’ (inner body) is often used to avoid jumping to conclusions about the nature of those bodies. (a neat page on amoebae and their nuclei (among other things, like ID) can be found here: Lastly, note the small granules just beneath the periphery of the nucleus. I have no idea what they are, and would probably have to go to electron microscopy to really have a clue, but they look sorta cool anyway. While the nucleus is sometimes treated as just a bag of DNA or “genome”, that DNA is in a highly organised arrangement with various regions of it existing in different states, bound to different proteins… even turned ‘on’ and ‘off’, depending on conditions or what the cell is doing.

Here, we get just a passing glimpse of that complexity… but still, I think it may serve as a nice intuitive reminder.

Here’s the view from which it came. Note the border between the clear ectoplasm at the edge of the cell, and the endoplasm where all the ‘stuff’ is. This cell is a bit squished, so you don’t get to see why it has an ectoplasm just yet…

The cytoplasm is full of refractile crystals, at least some of which may be related to urea in composition, and perhaps used to accumulate (and later dispose of) waste products. They may also be and do other things, of course.

And a general view of the amoeba. This looks like Polychaos sp., an amoebozoan amoeba with multiple pseudopods poking out in different directions (pseudopod motility is an important character for amoeba ID). Upon extending the pseudopod by laying down the ectoplasm we mentioned before, the ‘pod is soon filled with endoplasm, streaming rapidly through its centre. This is no slow amoeba. However, the nucleus is strange for a Polychaos sp., so it may well be something else, or a poorly documented Polychaos. One would probably have to resort to sequence data to figure it out, or a Russian amoeba expert (there’s a bit of a team there, and they’re really good).

Psi Wavefunction About the Author: Psi Wavefunction is a graduate student working with protists (the 'other' eukaryotes) at Dalhousie University in Halifax, Nova Scotia, and now blogs about protists and evolution at Skeptic Wonder. Follow on Twitter @Ocelloid.

The views expressed are those of the author and are not necessarily those of Scientific American.

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