Ultrastructural localization of epithelial mucin core proteins in colorectal tissues

Mucins are high molecular weight glycoproteins with a variety of postulated biological functions, including physicochemical protection from toxins and mutagens, adhesion modulation, signal transduction, and regulation of cell growth. Mucins are widely and differentially expressed in the gastrointest inal tract. To date, studies of cellular expression have relied on light mi croscopy using in situ hybridization and immunohistochemistry. Although inf ormative, it has been difficult with these techniques to ascertain exactly which cell types are producing a given mucin. We studied expression of MUC1 , MUC2, and MUC4 apomucins in a series of normal colon biopsies using a com bination of immunoelectron microscopy and light microscopy. MUC1 mucin was localized to both goblet and columnar cells, where it was seen in secretory vesicles, microvilli, and in cytoplasmic remnants in goblet cell thecae. M UC2 expression was restricted to goblet cells, in which reactivity was conc entrated in the rough endoplasmic reticulum (RER). MUC4 expression was seen in both columnar and goblet cells, localized to the RER. The inability to detect MUC2 and MUC4 apomucins in the Golgi complex and the mature mucous g el probably represents masking of peptide epitopes following O-glycosylatio n. This study has helped clarify lineage-specific mucin synthesis in the no rmal colon.