Printed in Great Britain The effect of thyroid function on hemoglobin differentiation in the chick

The switching off of the synthesis of embryonic hemoglobin (EHb) and the turning on of adult hemoglobin (AHb) synthesis has served as one of several major examples of biochemical differentiation. However, the control mechanisms underlying this transformation remained elusive, except in the case of Rana catesbeiana (Moss & Ingram, 1965, 1968 a, b) where the classical thyroxine-induced metamorphosis effected a precocious shift from EHb to AHb. In the chick embryo such a dramatic metamorphosis does not occur, but a change from EHb to AHb can be detected electrophoretically on the sixth day of incubation (Hashimoto & Wilt, 1966; Manwell, Baker & Betz, 1966). Normal thyroid development can be seen on the second day in the chick embryo, appearing as a medial outgrowth from the pharyngeal floor. By the sixth day, when AHb is first recognized electrophoretically, the thyroid con-centrates 131I 500 times more than does muscle (Olonoski, Lengeman & Comar, 1960). At 7 days, but not before, TSH has been detected in vitro (Tixier-Vidal, 1956). On the eighth day, yolk-sac absorption can be inhibited by injection of 100 /ic of131!, and 50 JLLC of 131I will delay hatching and significantly reduce body and thyroid uptake of isotope (Maraz, 1966). By 8 days, anterior pituitary tissue placed on the chorioallantoic membrane causes an accelerated thyroid morphogenesis (Studitskii, 1938), indicating the presence of biochemical inter-relationships. Thus, in the chick, there is evidence that the thyroid-pituitary axis is anatomically developed and biochemically active during and shortly following the switch from EHb to AHb. Experimentally it has been shown that the chick embryo, hypophysectomized at 33-36 h of development, exhibited a normal switch from embryonic to adult hemoglobin (Manwell & Betz, 1966). It would appear that the lack of embryonic TSH does not affect the differentiation of hemoglobin. However, evidence is available that egg yolk has a thyroid stimula-ting effect that increases the weight, function and histological structure of the gland (Tsuji, 1922). In the present study, electrophoretic patterns of hemoglobin and erythrocyte morphology were investigated in the chick rendered hypo- or hyper-thyroid b