Though the general appearance and the cellular composition of the pancreatic islets of the Mongolian gerbil (Meriones unguiculatus) conformed to those of most other rodent species, some peculiar ultrastructural details were found. Thus there were diversiform, mainly vesicular particles with varying electron opacity in these islets. The vesicular particles showed a clear association to cilia which seemed to possess a basic fiber pattern of 9 + 0. The basal bodies were localized in the cytoplasm of the islet parenchymal cells, most often in the ß-cells, and the vesicular particles occurred in the portions of cilia that were protruding into intercellular spaces. The cilia were often swollen, and the vesicular particles were mainly found in the space between the ciliary membranes and the longitudinal fibers. A few vesicular particles could be seen inside and sometimes seemingly in contact with these fibers. Occasionally, there were morphologically similar structures in the cytoplasm of adjacent ß-cells. The vesicular particles were differentiated from the vesicles occurring in nerve structures by their larger size, as well as by their heterogeneous shape and electron opacity. The nature of the vesicular particles and the significance of their presence in cilia and in the cytoplasm of some of the islet cells remain unknown. Among other possibilities, it is, however, suggested that the vesicular particles may represent secretory material.

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