Here's an interesting micrograph of a nucleus lit up by fluorescent dextran. Besides the slightly darker areas (these are nucleoli - dense structures where ribosomes are manufactured), you'll note the small round blebs on the top of this dumbbell shaped nucleus. I run into cells like this once in a while, and I'm sure that others have seen similar nuclear morphologies, yet we still have no clue how such structures could form.
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So far as I know, in eukaryote cells such structures have to be cause either by the actin cytoskeleton or by the mitotic spindles.
Which cells did these come from? Or if you don't know the cells, where was your sample from?
John:
There are many proteins besides actin and microtubules that act to shape cells and organelles, I suspect that many of these remain undiscovered. In the nucleus for example the lamins, distant orthologs of the intermediate filament proteins, are thought to be the main cytoskeletal elements that act to give nuclei their round shape.
Two years ago, someone in this lab discovered a family of proteins that are responsible for shaping the reticular network of the endoplasmic reticulum. These proteins, reticulins, are probably forming mini scaffolds in the plane of the membrane.
For more on all these topics here are some previous posts:
On the different types of cytoskeletal elements.
On how the reticulum of the ER gets its shape.
The Future of Cell Biology - Organellar Shape.
Lab Rat:
These were Hela cells, but I've seen similar nuclear morphologies in many other cell lines such as mouse NIH 3T3 fibroblasts, human U2OS cells, and monkey cervical epithelial cells (COS7 and TC7 cells). They appear more frequently when certain proteins have been knocked down by RNAi ... but I think that the link between any particular protein I've played around with and this phenotype is indirect.
Was this confocal, and if so, do you think these could be an artifact of the laser, possibly heating and distorting the membrane. I once did a FRAP experiment, and the repeated imaging damaged the membrane, resulting in Blebs like that.
Nuclear blebs like yours have been described to be the predecessors for micronuclei. Check this figure for a scheme (http://www.jcb.org/content/vol140/issue6/images/large/JCB32976.f8.jpeg). In the paper containing the picture, the authors describe a way for the (cancer) cells to get rid of amplified DNA by incorporating it in these micronuclei. Quite an intriguing concept.
RBI,
This was an epi-fluorescence picture, 50msec exposure, so I'm pretty sure that the blebs were not caused by photodamage.
Thanks dpo - that's a very intriguing report - it may answer why, but it doesn't answer the mechanistic question of exactly how are these nuclear deformations are formed. That being said, sometimes digging deeper into one question, leeds to insights that are pertinant to a related question.
Could these blebs be due to some stress experienced by the cells?
Alex Palazzo looks like A PUFF
In the fish C.batrachus, oocytes,at the onset of vitelogenesis blebs have been seen to arise from the nuclear envelope. These blebe are DNase sensitive. The elimination of DNA from the nucleus may be a selective process to maintain the race.