What is this whitish blob? How to trace around it?

Questions
#1

Take a look at this movie (5 MB). There’s a branch running diagonally across at the beginning, from the bottom left to the middle. Concentrating on the middle, you can see that there’s a relatively whiter blob appearing that seems to obliterate the membrane between the branch and another one approaching on the left.


I suspect that the blob is not part of the branch, and should be colored around it, but the filling algorithm doesn’t seem to handle this white blob very well. I’ve aborted this task with the note “white blob makes things unclear”.

What do you think this is?
#2

Hi robertb, 

That whitish blob is a tricky one.  It’s a shame that the blob looks like it’s starting to differentiate itself so close to the edge of the volume–if we could see a bit more it’d be helpful.  I’d say you’re correct that the whitish blob is not part of the branch.  The problem is that there is a staining gap (part of the cell membrane is not stained, and it makes the boundary between two neurons difficult to see).  In this case the filling algorithm handles the differentiation between the two neurons poorly because it can’t actually see the difference.  When you come across stuff like this try to trace around the object that doesn’t belong, or as you did, abort the task and note the reason why.
#3

OK, I think I see. The whitish blob should have had a darker surround, but the stain just didn’t get in there, which makes segmentation fail.

#4

I’m far from an expert, but it kind of looks like an air bubble to me.  Could that be what it is?