Gall bladder. Coloured scanning electron micrograph (SEM) of the surface of a gall bladder. This mucosa lining is made up of columnar epithelial cells (pink). Each cell of the gall bladder lining has microvilli (tiny projections) that increase its surface area and aid water uptake. Connective tissue (green) is seen below the lining where some of the cells have come away. The gall bladder is a sac that concentrates and stores bile produced by the liver and releases it into the duodenum (small intestine), where it aids the digestion of fats.