The number of mammary glands characteristic of a mammalian species varies, with larger numbers of pairs generally belonging to animals that produce large litters of young. In females, the mammary glands develop in response to sex hormones that begin to be released into the body by the ovaries in significant amounts at the onset of sexual maturity. When a female becomes pregnant, the mammary glands undergo further changes in order to be able to provide milk to the newborn soon after its birth. In addition to nourishment, the milk supplies antibodies to the infant, which aid in the fighting off of infections.

The rat mammary gland 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 often used to target 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.