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Unless otherwise mandated, neuroscientific research on mice is done with ~7-week old animals. There is a sort-of mantra that at this age their skulls stop growing.

However, recently, I noticed a rather robust difference in how I have to place ~4-month old mice in a scanner versus 7-week old ones. I am pretty sure this is due to skull size, since I am fixing all my animals relative to the ear canal, and there should be no fat/hair accumulation in the inner ear. Also, if there would be, I am quite certain I would feel it.

This made me try to look up the source of the 7-week recommendation, but up until now I found nothing. Do you know whether it is correct that the mouse skull "stops growing" at 7 weeks of age, and if so, what studies support this observation?

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This doesn't fully answer the question, but for those mouse skull sutures that do fuse, fusion is complete by about 45 days of age, according to Studies in cranial suture biology: IV. Temporal sequence of posterior frontal cranial suture fusion in the mouse. That suggests that most cranial expansion is more or less complete by about 6 weeks of age. I think there is still the potential for remodeling of the skull, especially bone thickness and so on, but since your interest is on the internal size of the cavity, this may help address it.

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Apparently there's a really nice paper with some very informative figures on Cranial Base and Craniofacial Development in Mice. They measure the distance between key landmarks of the cranium over a period of 112 days.

The paper does not specifically look for landmarks that allow a good positioning of the brain relative to the ear canal, but from their data on Cranial Base length, one may conclude that:

  • The position of the brain relative to the ear canal is highly variable during the first 28 postnatal days.
  • The position of the brain relative to the ear canal, along with most measures pertaining to the neurocranium become more stable after about 56 days of age.
  • There are small but significant changes in the neurocranium between 56 and 112 days of age. The position of the brain relative to the ear canal could for instance become around 0.5 mm more rostral between 56 and 112 days of age.

My observation indicates the same tendency, though I would say the shift in position was greater (perhaps 2mm) - but since they did not choose the ear canal proper as a landmark, it is difficult to compare.

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