EARLY GILL DEVELOPMENT IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

The morphology of the gills of rainbow trout (Oncorhynchus mykiss) emb ryos was studied by light and electron microscopy at four different de velopmental stages. Embryos were examined during gill arch formation ( stages 22-24, Vernier, '69), development of the gill filaments (hatchi ng stage), and formation of gill lamellae (stage 36). Our observations showed that the gill arches are covered by endoderm of the pharyngeal pouches and ectoderm of the gill furrows, which meet to form a closin g plate. Ultrastructural evidence indicates that secretion of mature h atching gland cells around the closing plates is the mechanism respons ible for their perforation and is consistent with the hypothesis of Mi ller et al. ('93) that cellular reorganization is a major mechanism of initial perforation of closing plates. Epithelia of the gill filament s and gill lamellae are composed of undifferentiated basal epithelial cells, outer pavement cells, and specialized chloride cells and goblet cells. The appearance of these cells and their ultrastructural featur es during embryonic development are correlated with gill function. It is not until stage 36 that gill epithelia exhibit adultlike ultrastruc tural features. (C) 1996 Wiley-Liss, Inc.