title: Neocortical neuron types in Xenarthra and Afrotheria: implications for brain evolution in mammals.
abstract: Interpreting the evolution of neuronal types in the cerebral cortex of mammals requires information from a diversity of species. However, there is currently a paucity of data from the Xenarthra and Afrotheria, two major phylogenetic groups that diverged close to the base of the eutherian mammal adaptive radiation. In this study, we used immunohistochemistry to examine the distribution and morphology of neocortical neurons stained for nonphosphorylated neurofilament protein, calbindin, calretinin, parvalbumin, and neuropeptide Y in three xenarthran species-the giant anteater (Myrmecophaga tridactyla), the lesser anteater (Tamandua tetradactyla), and the two-toed sloth (Choloepus didactylus)-and two afrotherian species-the rock hyrax (Procavia capensis) and the black and rufous giant elephant shrew (Rhynchocyon petersi). We also studied the distribution and morphology of astrocytes using glial fibrillary acidic protein as a marker. In all of these species, nonphosphorylated neurofilament protein-immunoreactive neurons predominated in layer V. These neurons exhibited diverse morphologies with regional variation. Specifically, high proportions of atypical neurofilament-enriched neuron classes were observed, including extraverted neurons, inverted pyramidal neurons, fusiform neurons, and other multipolar types. In addition, many projection neurons in layers II-III were found to contain calbindin. Among interneurons, parvalbumin- and calbindin-expressing cells were generally denser compared to calretinin-immunoreactive cells. We traced the evolution of certain cortical architectural traits using phylogenetic analysis. Based on our reconstruction of character evolution, we found that the living xenarthrans and afrotherians show many similarities to the stem eutherian mammal, whereas other eutherian lineages display a greater number of derived traits.
authors.affiliation: Department of Anthropology, The George Washington University, 2110 G Street, NW, Washington, DC 20052, USA. sherwood@gwu.edu
authors.forename: Chet C
authors.initials: CC
authors.lastname: Sherwood
authors.forename: Cheryl D
authors.initials: CD
authors.lastname: Stimpson
authors.forename: Camilla
authors.initials: C
authors.lastname: Butti
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Bonar
authors.forename: Alisa L
authors.initials: AL
authors.lastname: Newton
authors.forename: John M
authors.initials: JM
authors.lastname: Allman
authors.forename: Patrick R
authors.initials: PR
authors.lastname: Hof
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001835:Body Weight; D064032:Calbindin 2; D064026:Calbindins; D005260:Female; D005904:Glial Fibrillary Acidic Protein / Q000032:analysis; D007043:Hyraxes; D008297:Male; D008322:Mammals / Q000033:anatomy & histology* / Q000145:classification / Q000235:genetics; D019579:Neocortex / Q000166:cytology*; D016900:Neurofilament Proteins / Q000032:analysis; D009474:Neurons / Q000737:chemistry / Q000145:classification* / Q000166:cytology; D009478:Neuropeptide Y / Q000032:analysis; D009929:Organ Size; D010320:Parvalbumins / Q000032:analysis; D010784:Photomicrography; D010802:Phylogeny; D012044:Regression Analysis; D064030:S100 Calcium Binding Protein G / Q000032:analysis; D012788:Shrews; D004490:Xenarthra / Q000033:anatomy & histology* / Q000145:classification / Q000235:genetics
chemical_list: D064032:Calbindin 2; D064026:Calbindins; D005904:Glial Fibrillary Acidic Protein; D016900:Neurofilament Proteins; D009478:Neuropeptide Y; D010320:Parvalbumins; D064030:S100 Calcium Binding Protein G