The development of embryos and larvae of the Atlantic cod, Gadus morhua, with particular emphasis on the ontogeny of chloride cells

Valerio, Paul F.(Paul Federico) (1995) The development of embryos and larvae of the Atlantic cod, Gadus morhua, with particular emphasis on the ontogeny of chloride cells. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Chloride cells are mitochondria-rich ion-transporting cells (ionocytes) that secrete chloride from epithelial surfaces. The origin and differentiation of presumptive chloride-secreting cells in Atlantic cod (Gadus morhua) were studied by a combination of light and electron microscopy. -- By examination of available literature data, in conjunction with data collected from the present study, estimates of the effect of temperature on hatching time in the Atlantic cod were made. Subsequently, methods for the localization of chloride cells were developed, and compared with classical methods of staining ionocytes. Ionocytes were stained using conventional mitochondrial dyes, potential-sensitive fluorescent mitochondrial dyes (including styryl and rhodamine dyes, and a novel aldehyde-fixable rhodamine derivative), a fluorescent probe for the Na⁺, K⁺-ATPase (the ouabain derivative, anthroyl-ouabain), a cardiolipin-specific probe (nonyl-acridine orange), tetracycline, a lipophilic membrane probe (DiOC₆(3)), and conventional precipitation methods for ion localization. -- Staining methods were used to examine the gradual differentiation of ionocytes from mitochondria-rich precursor cells. This information was applied to the analysis of chloride cell differentiation from the embryonic stages through to maturity. -- The development of Atlantic cod embryos and larvae was then observed under various salinity and temperature conditions, and in the presence of hormone or vitamin-supplemented media, to determine the potential role of exogenous factors on the timing of chloride cell differentiation. -- A variety of secretory cells, including sacciform, goblet and ionocytes, developed in the epidermis of cod embryos and larvae. In the embryo, ionocytes first appeared on the yolk sac and buccal/gill (oropharyngeal) cavity epithelium. Such cells were lenticular, and often had a large multilobed nucleus. Some appeared to be binucleate. Beyond yolk sac absorption, ionocytes appeared on the gill filament epithelium, and were retained on the oropharyngeal epithelia, but not the abdominal region. In postmetamorphic stages, oropharyngeal ionocytes become elongate and traversed epithelia that were several cells thick. -- Although some experimental treatments delayed or accelerated rates of embryo development, yolk sac chloride cell differentiation appeared to occur after epiboly completion in all cases. The pattern of distribution of chloride cells in the later embryonic and larval stages appeared to be a result of migration of undifferentiated cells over the surface of the yolk sac and into the pharyngeal cavity. This migration probably occurs through pharyngeal 'pores', the embryonic precursors of the openings to the adult gill chambers, and is ultimately responsible for the recruitment of epithelial cells (including chloride cells) by the developing gill filaments.

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