Fine structure of the secretory epithelium in the hypobranchial gland of the prosobranch gastropod mollusc Buccinum undatum L

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Abstract
The secretory epithelium in the hypobranchial gland of Buccinum undatum L. consists of a single layer of elongated cells resting upon a basement membrane which overlies layers of connective tissue and muscle. Three principal types of cell are present. Two of these participate in the actual production of the mucinous secretion while the third is a ciliated cell. The ciliated cells also bear numerous microvilli and contain abundant microtubules and microfilaments. Neuronal contacts with the ciliated cells can be detected.The secretion product of the gland is a loose association of acid mucopolysaccharide and glycoprotein and the cells secreting these two materials can be clearly identified. Those secreting acid mucopolysaccharide have been called acid mucin cells while those secreting glycoprotein have been called goblet cells because of their resemblance to intestinal goblet cells. The acid mucin cells are large and distended with an amorphous mass of electron luscent material. This material is strongly metachromatic in the light microscope when stained with toluidine blue. Newly secreted acid mucopolysaccharide is contained within membrane-bound vacuoles where it exhibits a whorl-like or lammelar form. This latter fine structure is lost as the vacuoles coalesce filling the cell. The goblet cells are smaller and contain discrete globular, membrane-bound, packets of electron dense material. These do not stain metachromatically. Both types of cell release their contents by rupture of the cell membrane at the epithelial surface.