The gallbladder has a simple columnar epithelial lining characterized by recesses called Aschoff's recesses, which are pouches inside the lining. Under the epithelium there's a layer of connective tissue (lamina propria). Beneath the connective tissue's a wall of smooth muscle (muscularis externa) that contracts in response to cholecystokinin, a peptide hormone secreted by the duodenum. There's essentially no submucosa separating the connective tissue from serosa and adventitia, but there's… (More on Gallbladder)