Very early development of nucleus taeniae of the amygdala.

The avian nucleus taeniae of the amygdala (TnA) corresponds to part of the mammalian medial amygdala. Like its mammalian counterpart, it has been shown to be involved in the control of social function. According to behavioral observations, such control is already necessary early in the ontogenetic development of a bird. If so, TnA should be one of the earliest differentiating brain structures of the telencephalon. Our anatomical study shows that TnA can already be delineated at posthatching day one. The volume of TnA exhibits a growth spurt between days 1 and 8 posthatch, developing at a faster rate than the entire telencephalon. Our results suggest that between days 1 and 8 the growth of neuropil exceeds the enhancement of neuron number (leading to a decrease of cell density), and an addition at the same pace of new neurons and neuropil thereafter. A plateau is reached at posthatch day 30. The development of TnA precedes that of the song control nuclei and is similar to the early growth of thalamic and telencephalic sensory areas. This adds to the idea that this structure may already be involved in social control at the time of hatching. A proximate cause of the early development of TnA might be the direct afference from the olfactory bulb.