Wang,

I am not real sure what the dark material is. This cell is obviously manufacturing something, but this view of the cell does not indicate the dark material is being extruded. The dark shade indicates it is quite electron dense. Have you considered the dark material (may) be a waste material, or possibly something that isn’t being extruded? The dark material is not the right shape for being a malonocite.

Thank you very much for your answer, Professor. This structure is found in the secretory cells of the salivary gland. Secretory cells have a very strong secretion function, so it is quite possible that this cell is secreting something important. I also had a guess that it might be an autophagosome, but it doesn't seem quite right.