part_of starts_at ends_at outer layer of primitive bladder serosa of primitive bladder The serosa of primitive bladder (spbl) (TS:20-TS:21). The outermost layer of the bladder is comprised of either a serosa or an adventitia. The serosa (or serous membrane) of the primitive bladder, where the outer surface is covered by simple epithelium. Most of the bladder will be covered by serosa, but early when the primitive bladder is not developed and later in some regions (i.e. closer toward the urethra) there will instead be an adventitia. As the organs develop, more of the bladder is covered by serosa and less by an adventitia. anatomical_structure EMAPA:36094 TS20 TS21