High population density of juvenile chum salmon decreased the number and sizes of growth hormone cells in the pituitary

Juveniles of chum salmon (Oncorhynchus keta) held at high population densit y were apparently smaller than those held at medium and low population dens ities. The effects of high population density on pituitary growth hormone ( GH) cells in juvenile chum salmon were examined using immunocytochemical an d in situ hybridization techniques. The ratio of GH-immunoreactive (ir) are a to the whole pituitary was almost constant in all of the high, medium and low population density groups, although the number and sizes of GH-ir cell s were decreased in the high population density group. Image-analysis of GH -ir cells indicated the presence of a population of heterogenous cells, in which medium or rather strongly stained smaller cells and as extreme weakly stained larger cells. The medium or rather strongly stained smaller cells predominated in the high population density group, while weakly stained lar ger cells in the low population density group. In situ hybridization study showed somewhat different distributions and intensities of hybridization si gnals for mRNAs encoding GH I and II precursors. The area showing signals f or GH II mRNA in the high population density group was significantly smalle r than those in the medium and low population density groups. In contrast, the sizes of areas showing signals for GH I mRNA did not differ among the g roups, although the intensity was slightly higher in the high population de nsity group. These results indicate that high population density decreased the number of weakly immunoreactive larger GH cells, and also suppressed ex pression of the gene encoding GH II precursor, which may result in retarded somatic growth.