Membrane particle distribution in the sternal epithelia of the terrestrialisopod Porcellio scaber latr. (Crustacea, Oniscidea) during CaCO3 deposit formation and resorption, a freeze-etch analysis

The anterior sternal epithelium of terrestrial isopods transports cuticular Ca2+ to and from large sternal CaCO3 deposits. We analyzed the anterior an d posterior sternal epithelium by the means of the freeze-etch technique an d measured the size distribution and density of intramembrane particles (IM Ps) during three different molting stages. At least three IMP size classes around 4.5, 7.7, and 9.4 nm can be distinguished on the P-face of the apica l and basolateral plasma membrane. An additional size class of around 12.8 nm is restricted to the apical compartment. In the anterior sternal epithel ium, the density of these large particles changes by a factor of 1.9 during the molt cycle, suggesting a role in CaCO3 formation and/or resorption. Th e density of the smaller IMPs rises transiently by a factor of 1.3 in the p osterior sternal epithelium only. The IMP density of the basolateral plasma membrane increases significantly by a factor of 1.4 and 1.3 in the anterio r and posterior sternal epithelia, respectively. The results indicate that increases in the IMP density contribute to the differentiation to an increa sed transport activity during the cyclic enlargements of the plasma membran e surface area in the anterior sternal epithelium. (C) 1999 Academic Press.