Matlab - Segment cell nucleus from similar background
You need to work with different color space. Try YCbCr for instance

oimg = imread('http://i.stack.imgur.com/aYhrS.png');
img = rgb2ycbcr(oimg); % conver RGB to YCbCr color space
msk = img(:,:,3)&gt;130;  % apply threshold on Cr channel to get segmentation mask


I picked up 130 as arbitrary threshold, you might need to adjust it a bit.
Showing the input image, the image region for which msk==1, and the background:


I'll leave it to you as an exercise to think why the Cr channel performed well for this image.