The bladder is a collapsible, thick-walled muscular sac surrounded by a layer of fat that serves as a storage site for the urine produced by the kidneys. Urine is transported to the bladder via the tube-like ureters, each of which is linked to a kidney, and the fluid exits through the urethra.

Depending on how much fluid it contains, the bladder may be expanded or contracted. Receptors located inside of the bladder indicate when the organ is sufficiently distended for urination to occur and activate a reflex pattern that causes the contraction of the muscular wall of the organ and the relaxation of the internal urinary sphincter, allowing urine to enter the upper portion of the urethra. A second sphincter under voluntary control generally keeps the urine from being passed from the body until directed.

The bladders of sheep have been utilized by humans in a variety of ways throughout history. Sheep bladders have, for instance, been used as holders for water and other beverages, as musical instruments, and even as primitive footballs. Also, in modern times, sheep bladders have become a subject for scientific study. Indeed, the organs of the ruminants are often employed to gain new knowledge of the bladder that could eventually lead to ways to prevent or treat human bladder problems.

The filamentous actin present in a sample of sheep bladder tissue (illustrated above) was targeted with Alexa Fluor 350 conjugated to a phallotoxin, phalloidin. In addition, the specimen was stained with Oregon Green 488 conjugated to wheat germ agglutinin, a lectin that selectively binds N-acetylglucosamine and N-acetylneuraminic (sialic acid) residues. SYTOX Orange was employed to stain nucleic acids. Images were recorded in grayscale with a 12-bit digital camera coupled to either a Nikon E-600 or Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.