to get your started you can see that the tissue and the shadow area have similar values in all the color channels. you can do a bit of comparison of each of the color channels to generate a mask of those which are not gray and white:
img = imread('https://www.mathworks.com/matlabcentral/answers/uploaded_files/645980/image.jpeg');
mtemp = double(img)./median(double(img),3);
mtemp2 = abs(mtemp)<=.30;
which you can use a bit of imerode, bwfill, etc to edit the mask or generate something different.