Secretion granule formation in the rabbit parotid gland after isoprenaline-induced secretion: stereological reconstructions of granule populations.

The development of the secretion granule population of rabbit parotid glands after isoprenaline (IPR)-induced degranulation has been analyzed at the E.M. level using stereological techniques. Young male New Zealand White rabbits were sacrificed 2, 4, 8, 12 and 16 hours after IPR administration and the granule population compared with those of starved controls. In control glands a third of cell volume was occupied by stored secretory material, and it was estimated that, on average, cells contained 386 granules. The granule population as a whole had a mean diameter of 0.94 micrometers, with a unimodal positively skewed size distribution. Two hours after IPR treatment overall granule volume density was only 15% of that of control glands, but there was evidence that the process of restitution had already begun. At 8 hours about a fifth of acinar cell volume was occupied by electron-dense granules with an estimated mean diameter of only 0.58 micrometers, and the population as a whole was more strongly skewed than in the controls. In the later stages of restitution (12 and 16 hours), the volume of stored secretory material continued to rise, mean granule diameter increased, the size-frequency distribution became less skewed and the estimated number of granules per cell fell to 277 by 16 hours, suggesting that some granule fusion occurs during development. The analyses are discussed in relation to the techniques employed, and the results are equated with other independent evidence of the mode of granule genesis.