Presence of thyroxine in eggs and changes in its content during early development of chum salmon, Oncorhynchus keta

In order to examine the role of thyroid hormones during salmonid development, techniques were developed for quantitative extraction of thyroxine from eggs, whole embryos, and alevins of chum salmon (Oncorhynchus keta) at various stages of development. Frozen eggs, embryos, alevins, or fry were homogenized in ice-cold methanol. The homogenate was centrifuged, and the supernatant was washed with a mixture of chloroform and 0.05% CaCl2. The aqueous layer was lyophilized, and the residue was redissolved in barbital buffer for thyroxine radioimmunoassay (RIA). Serial dilutions of the egg or tissue extracts gave inhibition slopes that were parallel to that of the thyroxine standard in the RIA. Immunoreactivity of the extracts coeluted with thyroxine standard in reverse-phase HPLC on an ODS column. Recovery of thyroxine from egg and tissue extracts was estimated from the recovery of 125I-labeled thyroxine added to the initial homogenates. Thyroxine content of eggs just after fertilization was 4-5 ng/egg, and this level was maintained until hatching. A decrease in thyroxine content was seen during yolk absorption. Total thyroxine increased to about 10 ng/fish, a level higher than that in the unfertilized egg, at the time of complete yolk absorption, and decreased within 10 days to a low level of 1 ng/fish. These findings are discussed in relation to the role of maternal thyroid hormones during early development and also to the onset of larval thyroid function.