Differential Growth in the Telencephalon of Developing Chick – The Olfactory Bulb in Ontogeny and Phylogeny

Abstract

The avian crown group is among the most successful of extant vertebrate radiations, in terms of taxonomic, ecologic, and morphologic diversity. Two of the many features that distinguish crown birds from the other living reptile groups are the highly expanded nature of their cerebrum and the relatively diminutive condition of their olfactory bulbs. The rapid proliferation of computed tomography as a research tool in vertebrate paleontology has prompted considerable interest and activity on the deep history of the avian brain — as that history is revealed in the endocranial casts of non‐avian dinosaurs (i.e., stem birds). Patterns emerging from that work indicate that cerebral expansion began early in theropod dinosaurs, with multiple pulses of evolutionary growth and considerable homoplasy, especially in the area of the tree defined by troodontids, dromaeosaurs, and early avialans such as Archaeopteryx. Although there has been less of a concerted effort to understand the evolutionary origin of the olfactory bulbs, their stem history also appears marked by considerable complexity, including multiple expansions and retractions in volumetric size. Perhaps surprisingly, these phylogenetic patterns are complemented by relatively few comparative data on the morphogenesis, spatial connectivity, and volumetric growth of this region during ontogeny. Such data are likely imperative if we ever hope to approach a comprehensive understanding of the tempo, mode, and mechanisms of sensory evolution in crown and stem birds. As a first step in this process, we examined volumetric growth in the cerebrum and olfactory bulbs with respect to total brain size in a developmental series of the chick Gallus gallus. Examined stages ranged from Day 5 to adult. All specimens were subjected to iodine staining protocols as a means to enhance tissue contrasts and then scanned using μCT. Brain regions were examined and segmented, and 3D volumes created and measured using Amira software. Results indicate highly modular growth trajectories for the cerebrum and olfactory bulbs. The olfactory bulbs exhibit rapid early growth — expanding volumetrically by 368% between embryonic days 9 and 12, with a further increase of 58% between days 12 and 15 — preceding a period of comparatively slow growth through to six weeks post hatching. Considered as a percentage of the total brain and forebrain, the olfactory bulbs hit a volumetric peak on embryonic day 12 and then undergo a precipitous reduction as the cerebrum begins to rapidly expand. This relative growth pattern is highly congruent with the phylogenetic pattern of volumetric change in these regions along the avian stem, suggesting that more detailed ontogeny/phylogeny comparisons of these regions may well prove fruitful.Support or Funding InformationFunding sources: NSF‐DEB‐1457181, NSF DEB‐1406849This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.