Early events of secretory granule formation in the rat parotid acinar cellunder the influence of isoproterenol. An ultrastructural and lectin cytochemical study

The events involved in the maturation process of acinar secretory granules of rat parotid gland were investigated ultrastructurally and cytochemically by using a battery of four lectins [Triticum vulgaris agglutinin (WGA), Ul ex europaeus agglutinin I (UEA-I), Glycine max agglutinin (SBA), Arachys hy pogaea agglutinin (PNA)I. In order to facilitate the study, parotid glands were chronically stimulated with isoproterenol to induce secretion. Specime ns were embedded in the Lowicryl K4M resin. The trans-Golgi network (TGN) derived secretory granules, which we refer to as immature secretory granules, were found to be intermediate structures i n the biogenesis process of the secretory granules in the rat parotid acina r cell. These early structures do not seem to be the immediate precursor of the mature secretory granules: in fact, a subsequent interaction process b etween these early immature granule forms and TGN elements seems to occur, leading, finally, to the mature granules. These findings could explain the origin of the polymorphic subpopulations of the secretory granules in the n ormal acinar cells of the rat parotid gland. The lectin staining patterns were characteristic of each lectin. Immature a nd mature secretory granules were labelled with WGA, SEA, PNA, and lightly with UEA-I. Cis and intermediate cisternae of the Golgi apparatus were labe lled with WGA, and trans cisternae with WGA and SBA.