Hypothalamo-adenohypophyseal-thyroid interrelationships in the chick embryo. IV. Immunocytochemical demonstration of TSH in the hypophyseal pars distalis.

The cells that synthesize thyroid-stimulating hormone (TSH) in the pars distalis of the chick embryo were identified immunocytochemically (immunoperoxidase and immunofluorescence) using anti-bovine TSH-beta and anti-human TSH-beta sera. TSH cells are first demonstrable on Day 6.5 of incubation. By Day 11.5, when the two lobes (rostral and caudal) of the pars distalis are easily recognized, TSH cells are confined exclusively to the rostral lobe. TSH cells identified by means of immunofluorescence were stained with the periodic acid-Schiff component of the performic acid-Alcian blue periodic acid-Schiff's Orange G stain. Immunoreactive TSH cells in the pares distales of Day 13.5 chick embryos, injected at 5.5 days of incubation with thiourea, were more intensively stained than their normal counterparts. The marked change in immunocytochemically demonstrable TSH on Day 11.5 corresponds with physiological and morphological events occurring within the hypothalamus, adenohypophysis, and the thyroid gland of the developing chick during this midincubational (midgestational) period. The data suggest that not only is hypophyseal TSH present in greater quantities after Day 10.5, but that adenohypophyseal synthesis and secretion of TSH may be stimulated by another factor (hypothalamic TRH) at this time, signaling functional maturation of the hypothalamo-adenohypophyseal-thyroid axis.