The stomach’s chief cells synthesize the enzyme chymosin, also known as rennin, which is important in the digestion of milk. Most liquids pass very quickly through the stomach to the small intestine because they do not need to be broken down to the same extent as solids. Milk, however, is partially coagulated in the stomach due to the proteolytic action of chymosin. In its semisolid form, milk is retained in the stomach for a longer period of time than it would be as a liquid. Consequently, there is greater opportunity for proteins in milk to be digested in the stomach. This process is most significant in mammalian young, which depend on milk as their sole source of nutrients. Chymosin is most abundant in infants, and is largely replaced by the gastric protease pepsin later in life.

In order to localize a green fluorescent tag to filamentous actin in the sample of rat stomach tissue presented in the digital image above, the specimen was labeled with Alexa Fluor 488 conjugated to a phalloidin. Texas Red conjugated to the lectin wheat germ agglutinin, which selectively binds to N-acetylglucosamine and N-acetylneuraminic residues, was also applied to the tissue sample, as was the nuclear counterstain Hoechst 33342. Images were recorded in grayscale with a 12-bit digital camera coupled to a Nikon 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.