Final answer:
Tight junctions are responsible for the impermeability of the epithelium lining the bladder and ducts in the pancreas, as they create watertight seals between adjacent cells.
Step-by-step explanation:
The impermeability of the epithelium lining the bladder and the ducts connecting the pancreas to the digestive tract is primarily the function of tight junctions. These junctions create a watertight seal between adjacent animal cells, ensuring that materials such as urine in the bladder do not leak out into the extracellular space. Tight junctions form a band that encircles an entire cell, attaching it to all surrounding cells, thereby making the layer of cells impermeable to the passage of substances in between them.