― The characteristics of the nuclear T 3 receptor in the liver of the chick embryo were stud- ied from incubation day 18 until day 1 posthatching. Treatment of the nuclei with 3 mol.l-1 MgC' 21 which removed the endogenously bound hormone, was used in order to determine the total amount of receptors. The affinity constant K a decreased between incubation day 18 (0.996 0.2761 O 9 M-! ) and day 19 (0.247 0.072 ' IQ S M-!), remained the same thereafter until hatching and increased again on day 1 posthatching (1.846 0.92810 9 M-1 ). The total amount of receptors tended to increase from in- cubation day 18 to day 20 non-pipping (np) (from 4.40 to 11.55 fmol/ffg DNA) and decreased thereafter to 2.38 fmol/ug DNA on day 1 posthatching. The amount of free binding sites reached a maximum on day 19 (6.91 fmol/pg DNA) and then decreased drastically until posthatching (0.19 9 fmol/og DNA). The maximal specific binding was found on day 20 (np), just prior to penetration of the air chamber. During the time at which the level of T 3 remains high in the plasma, a reduction in the amount of receptor was observed, which may be the consequence of a down-regulation by T 3 itself. chick embryo / T 3 -receptor I ontogeny Rsum ― Ontognie des rcepteurs nuclaires du foie la T 3 pendant les derniers jours d'incubation de l'embryon de poulet. Les caractristiques des rcepteurs nuclaires de la T 3 dans le foie de l'embryon de poulet ont t tudies du 1 s e j d'incubation jusqu'au jour suivant l'closion.
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Dewil et al. (1992) studied this question.