The Golgi apparatus in the acinar cells of the developing embryonic pancreas: II. Localization of lectin‐binding sites

Abstract

The reaction patterns of the Golgi apparatus following staining with the lectins concanavalin A (ConA), Ricinus communis I agglutinin (RCA I), and Helix pomatia lectin (HPA) were studied in the pancreas acinar cells of rat embryos in the course of cell differentiation from day 13 through day 20 of gestation. The binding reactions were localized by means of pre-embedment incubation of 10-μm-thick cryosections of pancreas tissue, prefixed in a mixture of 4% formaldehyde/0.5% glutaraldehyde, using horseradish peroxidase for electron microscope visualization. ConA, which preferentially binds to α-D-mannosyl residues, consistently stained the cisternae of the cis Golgi side. The majority of the stacks also showed ConA staining of medial cisternae. The reaction of the trans side was variable; in each stage of development, the cisternae of the trans Golgi side either were devoid of labeling or appeared intensely stained. The reactions obtained with RCA I, which recognizes terminal β-D-galactosyl residues, changed in the course of cell differentiation; in the protodifferentiated and early differentiated states, the system of “rigid lamellae,” located at the trans side of the Golgi stacks, was intensely labeled, but became unreactive after production of secretion granules had started, the reaction then being restricted to the stacked saccules. In regard to the Golgi stacks in each of the developmental stages, RCA I binding sites either were confined to the trans cisternae, or, in addition, were found distributed across elements of the medial and cis compartments. Frequently, the reaction intensities increased in the cis-to-trans direction. The Golgi patterns apparent after the staining with HPA, which interacts with α-N-acetyl-D-galactosaminyl residues, were variable at all stages of differentiation; the reactions either were restricted to the cis and/or medial cisternae, or expanded across the entire stack of saccules. These results illustrate that the distribution across the Golgi stacks of binding sites for ConA, RCA I, and HPA is variable; none of the three lectins tested can be considered to be a probe for specifically labeling subsections of the Golgi stacks in developing pancreas acinar cells.