Thursday, January 29, 2015

Where's Bregma?

OR: A fun game for every surgery!

If you're interested in any of the important parts of the brain (i.e. not cortex), you're going to need do to stereotaxic surgery to target the area you're interested in. And to do stereotaxic surgery, you need fiducial coordinates on the skull, which canonically are bregma and lambda.

You would think that the definitions of bregma and lambda are well known, but I found out last year that I had a completely wrong definition of lambda. So in the interest of clarity, here's a diagram of a mouse skull with four possible locations for bregma and lambda (nose to the bottom). Without looking at the caption, can you, dear reader, identify the correct locations?
Diagram of mouse skull (not to scale). The bregma, for mice, is where the coronal suture intersects the midline, point B. Lambda is defined as the intersection of the midpoint and a curve fitting the "lamboid structure" (whatever that is). Basically, it's where the rear sinus would intersect the midline if the rear sinus didn't curve, point D. It is NOT point C, as any reasonable person would guess.
Now that you know the actual definitions of bregma and lambda, you can impress your professors at happy hour, and do accurate surgeries! Except bregma, is almost NEVER that simple in practice. The coronal suture curves this way and that; the left and right sides almost never meet at the same place on the midline. Sometimes the midline curves too!

Just for fun, I've diagrammed some common bregmas you might encounter during surgery. Which of the points listed below do you think are bregma? My opinion is in the caption.

Realistic bregmas (not to scale).
Top left: bregma where the two sides do not meet at the midline. The bregma here is at point B.
Top right: bregma where the mouse's right coronal suture veers. Here I'd say Bregmas is at C.
Bottom left: bregma where the midline curves. This is a bit trickier, but I'd say point B.
Bottom right: bregma where the midline shifts. Here there is actually not enough information! I would follow the midline all the way to lambda to see if it shifts back.
Disagree? Tell me in the comments!

4 comments:

  1. Thanks for this helpful post! One important thing to point out, though, is that Paxinos defines bregma as:
    "No atlas or stereotaxic instrument will compensate for using bregma and lambdoid points
    inappropriately. These reference skull marks are the midpoints of the curve of best fit along the coronal and the lambdoid suture, respectively. They are not necessarily the points of intersection of these sutures with the midline suture."

    You apply this principle in the top-right image on the bottom, but in the text its written as the intersection. I think your top diagram is a little misleading, because you have such perfectly curved sutures that the projected line is virtually the same. I usually see sutures that look more like what I have in the note linked below, in which case the projected line is more accurate than the intersection. This is what I've seen as a realistic ideal bregma, not an unusual exception.


    http://www.evernote.com/l/AArFfFxnqxREa6eZm6FAU3jAOPRsoPpvJ3c/

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    1. Thanks for the link! It seems I still don't have the exact definition of bregma. I agree that the illustrations were idealized. I wish there was a resource with real pictures of mouse bregma that showed how to deal with non-ideal situations. I tried taking pictures myself, but couldn't get clean, high magnification images.

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  2. What do you do if you have a frozen brain that is not in the skull?

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    1. I'm not sure I understand the question. If there's no skull, there's no Bregma to worry about! If you're sectioning brains, and you're not sure where to start, you can start a few hundred microns away from where you think you should, and just make more sections.

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