Ultrastructure of taste receptor cells in active and hibernating ground squirrels

Differences in taste bud ultrastructure between active and low-temperature hibernation states were investigated in the Siberian ground squirrel, Citel lus undulatus. Compared to active summer animals, taste bud volume and the morphology of the taste pore showed little change while an animal was hiber nating. However, impressive differences were observed in nuclei and cytopla smic organelles of all cells within taste buds. In low-temperature hibernat ion the nucleoli of elongated, non-dark cells had few, but large, fibrillar centres, a sign of inactivity. In the cytoplasm, the number of free polyri bosomes (polysomes) was sharply decreased; ribosomes were seen mainly as (n on-synthesizing) monosomes and more seldom as membrane-associated ribosomes . Profiles of endoplasmic reticulum (ER) were reduced, to the extent that o nly separate and rare cisternae of smooth and rough ER remained. The large vacuoles, which are typically found in many taste cells of active animals, were absent during hibernation, and the Golgi apparatus appeared to be disa ssembled into numerous vesicles. Of these, many had small diameters near 60 nm, while a few had larger diameters near 300 nm. Secretory organelles (de nse granules in dark cells and dense-cored vesicles in type III cells) were rare during hibernation, the lateral plasma membrane was smooth, and signs of exocytosis were not found. These changes suggest reduced protein synthe sis and reduced sensory function during hibernation.