The mucus present in the goblet cells of the colon is condensed, but when it is secreted, the material expands significantly. This process is very rapid. Studies indicating that mucus can increase 500 fold in volume in as little as 20 milliseconds. The expansion is indicative of a phase change that occurs as mucins move from secretory granules to the exterior of the cell via exocytosis. Inside the granules, the highly negative charge of mucins is neutralized by calcium ions. This calcium diffuses out of the granules as pores in the cellular membrane open in order to release the mucin. The sudden ionic change in the mucin environment along with hydration produce an almost instantaneous phase change and corresponding expansion of the substance.

The rat colon tissue section illustrated in the digital image above was labeled with wheat germ agglutinin (WGA) conjugated to Oregon Green 488. WGA, which selectively binds to N-acetylglucosamine and N-acetylneuraminic (sialic acid) residues, is well suited for staining the Golgi network in fixed cells and tissues since a number of proteins and lipids found in the Golgi apparatus are glycosylated. The specimen was also labeled with Alexa Fluor 568 conjugated to phalloidin (targeting F-actin) and Hoechst 33342 (targeting DNA in the nucleus). 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.