Emergence of production of the brain peptides

The onset of the synthesis of the releasing and inhibiting hormones, associated with their identified primary structures (LHRH, somatostatin), has been studied successfully in some vertebrates. It is known from radioimmunoassay and immunohistological studies that the synthesis of LHRH, TRH and somatostatin begins in the brain of different mammalian species (rat, mouse, guinea pig, man) during embryonic life. Much less is known about this phenomenon in birds. According to very scarce immunohistological data, the first traces of identifiable LHRH appear in the brain of the chicken on day 5 1/2 of embryonic life, while somatostatin appears on the 12th embryonic day. It is remarkable, both in mammals and birds, that the onset of trophic hormone secretion usually proceeds that of the releasing and inhibiting hormones. This would indicate that the releasing and inhibiting hormones do not play a significant role in the embryonic differentiation and induction of hormone secretion of the fetal adenohypophysis.