Settings for detection structures with lumen with Macro

Hi there,

I am relatively new to image analysis in ImageJ, but I am trying to analyze images with structures that contain a lumen. I’m interested in obtaining the area of all of the structures including the lumen.
However, I’m having troubles setting thresholds because the lumen is difficult to discriminate from the background for example by brightness.
I realize I could threshold the borders than -Close and Fill Holes, but this way I will also include the space between different structures that are close together.

Any suggestions on what would be a good way to start?
The image I will upload in second post is a Tiff, where I already subtracted background and improved brightness/contrast (auto).

Many Thanks!

Marjolein

Example-structures-I-want-to-obtain-area-of.tif (768.3 KB)
Please find an example here. There are many structures in this image, I would like to obtain the total area of all the structures (including their lumen). Preferably without the space in between the structures if they are close together.