Scanning electron microscopy of development of the olfactory organ in ontogeny of grass carp Ctenopharyngodon idella

Abstract

Cytoand morphogenesis of the olfactory organ of grass carp Ctenopharyngodon idella are followed using the method of electron scanning microscopy. Dates of formation of olfactory placode and olfactory pit, olfactory sensory neurons, ciliated nonsensory cells, and other types of cells of olfactory mucosa are determined. In embryos, ciliated olfactory sensory neurons appear, then microvillous olfactory sensory neurons; the ratio of their numbers (approximately 2: 1) is retained throughout the entire fish ontogeny. In embryos, rod-bearing olfactory sensory neurons were found, whose numbers increase and then successively decrease, and they almost completely disappear at the beginning of the fry period. Towards this age, olfactory sensory epithelium is transformed from continuous into discontinuous at the expense of appearance of a large number of sites of ciliated nonsensory epithelium. With formation of olfactory rosette, sensory epithelium covers only lateral surfaces of olfactory lamellae. Numerous ciliated nonsensory cells appear in larvae on the head around the olfactory opening and rapidly disappear in fry with formation of the nasal bridge and entry and exit olfactory nares. The rate of growth of cilia and microvilli of olfactory sensory neurons and kinocilia of ciliated nonsensory cells was determined. In fish of all age groups, the density of distribution of olfactory sensory neurons in the olfactory epithelium is similar and comprises approximately 51000 cells/mm2. The amount of olfactory sensory neurons in the olfactory organ in fish of different age was assessed; in mature fish their number is approximately 40 millions. The relationship between olfactory sensory equipping and development of other structures of the olfactory organ and olfactory sensitivity of fish to odors was traced.