Thyroid Hormone Deposition in Avian Eggs and Effects on Embryonic Development

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Abstract
Studies to date indicate that thyroid hormones are present in the eggs of chickens and quail and that those hormones are primarily in the yolk. Quail hens deposit thyroid hormones into eggs in proportion to their own thyroid status, but appear to show some regulation of this process. Indirect studies suggest that thyroid hormones are transferred into oocytes bound to lipoproteins and transthyretin, both of which are taken up by receptor-mediated processes. Thyroid hormones bound to yolk lipoproteins may enter embryos with yolk that is taken up by non-specific endocytosis or they may be transported into the embryo by specific carriers. To date most of these ideas about thyroid hormone transport into egg yolk and from egg yolk into embryos have not been investigated directly. In quail, very high T4 content of eggs is associated with accelerated differentiation and growth of embryonic pelvic cartilage, a thyroid hormone-responsive tissue. We evaluate these effects on embryonic tissues and the changes in yolk hormone content during incubation in relation to the timing of thyroid development and studies of the capability for tissue responses to thyroid hormones during early embryonic life

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