MORPHOLOGICAL AND DISTRIBUTIONAL CHANGES IN THE EOSINOPHILIC GRANULE CELL (EGC) POPULATION OF THE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS WALBAUM) INTESTINE FOLLOWING SYSTEMIC ADMINISTRATION OF CAPSAICIN AND SUBSTANCE-P

Intestinal eosinophilic granule cells (EGCs) of the rainbow trout (Oncorhynchus mykiss) have been likened to mammalian mast cells and degranulate in response to stimulation by capsaicin and substance P. This investigation was conducted to examine the effects of capsaicin and substance P on the trout intestinal EGC population and to quantify the different morphologies following systemic administration. Rainbow trout were injected intraperitoneally with capsaicin, substance P, serotonin, or vehicle controls (0.5 mug/g body weight). Fish were killed at timed intervals and the mid intestine was processed for light and electron microscopy. The number of EGCs which could be observed in the stratum compactum was quantified for each treatment over the time course of the experiment. EGCs could be classified ultrastructurally into 1 of 5 categories based on their granule morphology. The cell frequencies and relative proportion of each cell class were analyzed statistically. The frequency of EGCs in the stratum compactum of fish injected with capsaicin or substance P significantly decreased post-injection (P < 0.05) compared to controls (saline- and BSA-injected fish). Serotonin had no effect on EGC frequency, morphology, or distribution as compared with saline. Stimulation with capsaicin and substance P resulted in time-dependent changes in both EGC granule morphology and distribution within the intestinal mucosa. Following an apparent migration of EGCs to the lamina propria and degranulation, small granuled EGC-like cells appeared first in the lamina propria and then later in the stratum compactum. The significance of the stratum compactum as a depot for intestinal EGCs and the site for EGC maturation is discussed.