Firstly you can convert the RGB image to a grayscake image using the method rg2gray/ After this, you can use imbinarize with a threshold for converting grayscale image to binary image. The code snippet below illustrates how to obtain the cell nucleus:
cell_img = imread("cell_nucleus.png");
cell_img_gray = rgb2gray(cell_img);
BW = imbinarize(cell_img_gray,0.35);
Hope this helps!