Citation

The hyperpallium densocellulare (HD) is an avian cerebral structure, the origin of which remains
unclear and considered to be critical to the understanding of the non-mammalian forebrain evolution. The
main goal of this investigation was to clarify the anatomical characteristics and behavioral implications of
HD, which had been assumed to be a homogeneous unitary entity. This project had three specific aims: 1)
To identify possible subdivisions of HD in terms of cytoarchitecture, Substance P distribution, and
connections based on unpublished (Aim 1a) as well as published data (Aim 1b); 2) To identify the
unknown connections of these subdivisions by using anatomical tract-tracing methods; and 3) To identify
the behavioral implications of these subdivisions by examining the expression of an immediate early gene
EGR-1 (early growth response protein 1) using immunohistochemical methods. Based on the present
study, four subdivisions of HD (rdHD, cdHD, rvHD, and cvHD) were identified in terms of
cytoarchitecture, hodology, and EGR-1 immunoreactivity. In general, the results showed that the dorsal
regions of HD (rdHD and cdHD) are closely connected to each other and involved in networks linking
sensory and limbic functions. The ventral regions of HD (rvHD and cvHD) also have close connections
between them, but also have direct associations with many limbic forebrain structures. In terms of
behavioral implications, the EGR-1 study showed that only one subdivision, rvHD, had significant
changes in EGR-1 immunoreactivity when animals were exposed to live conspecifics. These findings
suggest that this area is uniquely involved in reactions to external stimuli. These results further provide an
important insight on the organization and evolution of the avian forebrain, the cerebrum in particular.