Androgen target cells in the pituitary of the chick embryo

Steroid receptors and their regulation in avian extraembryonic membranes provide a novel substrate for hormone mediated maternal effects

Exposure of the vertebrate embryo to maternal hormones can have long-lasting effects on its phenotype, which has been studied extensively by experimentally manipulating maternal steroids, mostly androgens, in bird eggs. Yet, there is a severe lack of understanding of how and when these effects are actually mediated, hampering both underlying proximate and ultimate explanations. Here we report a novel finding that the embryo expresses androgen receptor (AR) and estrogen receptor (ERα) mRNA in its extraembryonic membranes (EMs) as early as before its own hormone production starts, suggesting a novel substrate for action of maternal hormones on the offspring. We also report the first experimental evidence for steroid receptor regulation in the avian embryo in response to yolk steroid levels: the level of AR is dependent on yolk androgen levels only in the EMs but not in body tissues, suggesting embryonic adaptation to maternal hormones. The results also solve the problem of uptake of lipophilic steroids from the yolk, why they affect multiple traits, and how they could mediate maternal effects without affecting embryonic sexual differentiation.

Androgen receptors in the embryonic zebra finch hindbrain suggest a function for maternal androgens in perihatching survival

Bird embryos are exposed to maternal androgens deposited in the egg, but the role of these hormones in embryonic development and hatchling survival is unclear. To identify possible target organs, we used in situ hybridization to study the distribution of androgen receptor (AR) RNA in the developing zebra finch brain. The first brain expression domain of AR mRNA is in the hindbrain. From embryonic day 7 (E7) onward, when the hypoglossal motor nucleus (nXII) has just formed, there was AR mRNA expression in both its lingual (nXIIl) and its tracheosyringeal (nXIIts) parts, and this was the major site of hindbrain expression at all embryonic stages and in both sexes. From E8 onward, we also found AR mRNA in the supraspinal motor nucleus (nSSp), which innervates neck muscles. Furthermore, the syrinx, the target of the nXIIts, contained AR mRNA by E10, localized principally in the perichondria. Muscle was first evident in the syringeal region at E9, but no AR was detected in syringeal muscles until after hatching. The expression pattern of AR in the zebra finch embryo suggests that maternal androgens act via AR in the brainstem and syrinx to influence hatching as well as acoustic and visual components of food-begging behavior. Maternal androgens seem unlikely to function in the development of sexual dimorphisms in the zebra finch nXIIts and syrinx, insofar as these are not evident until between 10 and 20 days posthatching.