Fine Structure of Mucus Granules in Rumen Epithelium

Rumen epithelium is a parakeratotic, stratified squamous epithelium which functions in metabolism, absorption, and protection . In previous light microscope histochemical studies (1, 3) and electron microscope studies (2) on this tissue, phenomena characteristic for both mucus-producing and keratinizing epithelia were described . The presence of mucus was demonstrated histochemically (1, 3) by the periodic acid-Schiff (PAS) reaction which exhibited a gradation in intensity ranging from weakly stained spinous cells to intensely stained horny cells . Electron microscope studies (2) described the differentiation products seen in both keratinizing and mucusproducing epithelia . Filaments are found in basal, spinous, granular, and horny cell types of rumen epithelium. Basal cells evolve into spinous cells characterized by the presence of membrane-bounded granules. These granules are filled with an amorphous granular material which appears similar to the content of mucus granules present in the epidermis of Rana pipiens (8) and the newborn mouse esophageal epithelium (9) . The mucus granules are responsible for the PAS-positive reaction exhibited by rumen epithelium . Spinous cells develop into granular cells containing both mucus granules and keratohyalin granules . The presence of these two structurally and chemically different differentiation products in the granular cell is a unique aspect of rumen epithelium . The horny cell is a parakeratotic, mucus-filled cell which contains remnants of various cell organelles . Continued studies on the mucus granules of rumen epithelium have revealed the presence of an internal substructure not previously observed in other mammalian mucus-producing tissues . In this note, these mucus granules will be described .