Keratin 20 Expressed in the Endocrine and Exocrine Cells of the Rabbit Duodenum

The expression of intermediate filaments is sensitively reflected in cell function. To examine the involvement of keratin in a secretory function, 15 kinds of keratin (keratin-2, 3, 4, 5, 6, 7, 8, 10, 13, 14, 16, 17, 18, 19, 20) were detected immunohistochemically and immunoelectron microscopically in the rabbit duodenum. Four types of secretory cells existed in the rabbit duodenum: enteroendocrine cells and goblet cells in the epithelium and mucous and serous cells in the duodenal glands. Among the 15 kinds of keratin, keratin 20 was selectively expressed in all these secretory cells. However, localization of keratin 20 in the endocrine cells differed from that in three types of exocrine cells. In the enteroendocrine cells, keratin 20-containing filaments formed a juxtanuclear network from which they extended to the apical cell membrane. These filaments may play a role in intracellular signal transduction, since the apical cell membrane contains some receptors for binding a specific extracellular signal. In the exocrine cells, on the other hand, keratin 20-containing filaments existed just beneath the cell membrane. These filaments may play some role in maintaining cell shape, which is remarkably changed during the secretory cycle.