Thyroid Hormone Deposition in Avian Eggs and Effects on Embryonic Development

Studies to date indicate that thyroid hormones are present inthe eggs of chickens and quail and that those hormones are primarilyin the yolk. Quail hens deposit thyroid hormones into eggs inproportion to their own thyroid status, but appear to show someregulation of this process. Indirect studies suggest that thyroidhormones are transferred into oocytes bound to lipoproteinsand transthyretin, both of which are taken up by receptor-mediatedprocesses. Thyroid hormones bound to yolk lipoproteins may enterembryos with yolk that is taken up by non-specific endocytosisor they may be transported into the embryo by specific carriers.To date most of these ideas about thyroid hormone transportinto egg yolk and from egg yolk into embryos have not been investigateddirectly. In quail, very high T₄ content of eggs is associatedwith accelerated differentiation and growth of embryonic pelviccartilage, a thyroid hormone-responsive tissue. We evaluatethese effects on embryonic tissues and the changes in yolk hormonecontent during incubation in relation to the timing of thyroiddevelopment and studies of the capability for tissue responsesto thyroid hormones during early embryonic life     

Peripheral Thyroid Hormone Dynamics in Precocial and Altricial Avian Development1

SYNOPSIS. Precocial and altricial birds have distinctly differentpatterns of general ontogeny and metabolic/thermoregulatorydevelopment. In my laboratory, we have studied the developmentof thyroid function in Japanese quail (Coturnix japonica) andRing doves (Streptopelia risoria) as examples of precocial andaltricial development, respectively. In this paper, I reviewthe literature and our work on the factors that influence peripheralhormone dynamics in birds. The first section of the paper describesadult peripheral thyroid function to set the stage for the developmentalpicture. Thyroid development is divided into two phases: PhaseI, in which the thyroid gland develops functional capacity butthere is low thyroid activity in the periphery, and Phase II,in which peripheral thyroid function increases and approachesadult levels. In precocial development Phase I occurs duringembryonic life and Phase II begins at the perinatal period.In altricial development, the pattern of thyroid functionaldevelopment is different and delayed. Peripheral thyroid hormonepatterns in both developmental modes are discussed with referenceto the factors that determine their dynamics: hormone availabilityfrom the thyroid gland, total serum hormone concentrations,the roles of serum binding proteins in regulating freehormoneconcentrations, hormone turnover and excretion, hormone receptorsand thyroid hormone metabolism, especially extrathyroidal productionof triiodothyronine from thyroxine.