Ontogenetic development of thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of the mallard embryo.

Developmental changes of thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of mallard embryos were studied by means of immunocytochemistry (PAP technique). The primary antibody was generated against synthetic TRH. Immunoreactive neurons were first detected in the hypothalamus of 14-day-old embryos. By day 20, increasing numbers of immunoreactive perikarya were observed in the paraventricular nucleus, anterior preoptic region and supraoptic region. Immunoreactive fiber projections were seen in the median eminence as early as embryonic day 20; they occurred also in some extrahypothalamic regions (lateral septum, accumbens nucleus). The number and staining intensity of the cell bodies increased up to hatching, and continued to increase during the first week after hatching.