How many axons in the murine optic nerve?

#1

There are between 770,000 and 1,700,000 axons in the human optic nerve (http://www.iovs.org/content/33/6/2012.full.pdf). How many are there in the (normal) murine optic nerve? Also, how much of the difference between human and mouse is explained by retinal size, and how much by mousiness?

#2

I can’t say that I know the exact number of axons in the murine optic nerve, but if you’re interested in reading about the differences between the mouse and the human retina, this paper is definitely a good one to check out: http://www.ncbi.nlm.nih.gov/pubmed/21613480

Hope this helps! :) 
#3

The number of axons in the optic nerve is the number of ganglion cells in the retina, as all the ganglion cells, and only the ganglion cells among different types of cells in the retina, send axons to the brain. We can roughly estimate the number from our dataset, e2198. I count approximately 90~100 ganglion cells from an area of 3x10^-2 mm^2. As typical retina of adult mice has area of ~15 mm^2, the number will be around 50,000. 


It is generally accepted that the retina is composed of identical “units”. The units are, in turn, composed of a group of neurons, and they tile the retina in homogeneous manner. So I’d assume that the retina could perform its basic functions if there are a certain minimum number of such units in the retina. 

On the other hand, the differences in the cell types between different species are pretty big. “Rods are rods, cones are cones”, Richard Masland, a retina legend, once said in a meeting with MIT students. But as we go deeper into the retina to the bipolar cells, amacrine cells, and ganglion cells, a type of cell in one species cannot always be found in another. It is most extreme for the ganglion cells that different species seem to have intensively differentiated their ganglion cells for their own purposes. 



#4

Your estimate seems in good accordance with this study avoiding the problem of displaced cells by counting optic nerve axons
Cenni, M. C., Bonfanti, L., Martinou, J.-C., Ratto, G. M., Strettoi, E., & Maffei, L. B. (1996). Long-term Survival of Retina Optic Nerve Section in Adult Ganglion Cells Following bcl-2Transgenic Mice. The European Journal of Neuroscience, 8(8), 1735–1745.
They found the number of ganglion cells starts at around 112000 around birth and declines (natural cell death) to 45000 ±40000 in adults.

#5

45,000 ±40,000  Got to love it!  That’s almost within the error bounds of knowing nothing!

#6

Sorry one 0 too much:
45000 ±4000