Hi,
I recommend to apply a machine learning algorithm. In a first step you make the correct annotations on your own in a training sample. This means you draw the line for the nuclei.
In a second step you use this training sample and apply U-net as it is described here : UNET image segmentation in stem cells research
if the images are not too large (say smaller than 192x192 pixels), I can help you and apply U-net for you.
Best Wolfgang