Thank you for sending in your pictures.   After closely looking at the leaves and especially the section of the branch sample, it appears your Japanese maple could be infected with verticillium wilt.  Japanese maples and maple species are very susceptible to this disease.  Verticillium is a soil fungal disease that can cause a wilting or dieback of branches and can lead to the tree dying.  The wood of affected plants shows a gray to olive-green streaking if the bark is peeled from recently affected branches.  The sample cut you provided shows this streaking which leads me to the concern of verticillium wilt. 

From the MSU bulletin below……..

  “There are many other factors that can cause leaf wilt and branch dieback. In general, adverse site conditions and environmental stress such as girdling root injury, drought, flooding, compaction, deicing salts or gas leaks can cause symptoms similar to verticillium. In addition, canker-causing fungi that grow through the bark and cambium to almost girdle a branch can cause wilting. Bark beetles, wood boring insects or sapsuckers can attack selected branches in a tree and cause enough girdling to result in branch failure as well. The question of whether the cause is verticillium or something else is best answered by having the sample tested.”

  I would recommend a positive identification which can be made by MSU Diagnostic Services.  They can also be reached at<personal data hidden> for further information regarding how to send in your sample and how to fill out their forms.  It would also be beneficial to them to send in your pictures.

 Please read through this bulletin for more information and notice the pictures of cross section.

http://msue.anr.msu.edu/news/verticillium_wilt_refresher

If you have any further questions don’t hesitate to contact AaE again.