Brawn before brains in placental mammals after the end-Cretaceous extinction.

Mammals are the most encephalized vertebrates, with the largest brains relative to body size. Placental mammals have particularly enlarged brains, with expanded neocortices for sensory integration, the origins of which are unclear. We used computed tomography scans of newly discovered Paleocene fossils to show that contrary to the convention that mammal brains have steadily enlarged over time, early placentals initially decreased their relative brain sizes because body mass increased at a faster rate. Later in the Eocene, multiple crown lineages independently acquired highly encephalized brains through marked growth in sensory regions. We argue that the placental radiation initially emphasized increases in body size as extinction survivors filled vacant niches. Brains eventually became larger as ecosystems saturated and competition intensified.

Divergent evolution of developmental timing in the neocortex revealed by marsupial and eutherian transcriptomes.

Only mammals evolved a neocortex, which integrates sensory-motor and cognitive functions. Significant diversifications in the cellular composition and connectivity of the neocortex occurred between the two main therian groups: marsupials and eutherians. However, the developmental mechanisms underlying these diversifications are largely unknown. Here, we compared the neocortical transcriptomes of Sminthopsis crassicaudata, a mouse-sized marsupial, with those of eutherian mice at two developmentally equivalent time points corresponding to deeper and upper layer neuron generation. Enrichment analyses revealed more mature gene networks in marsupials at the early stage, which reverted at the later stage, suggesting a more precocious but protracted neuronal maturation program relative to birth timing of cortical layers. We ranked genes expressed in different species and identified important differences in gene expression rankings between species. For example, genes known to be enriched in upper-layer cortical projection neuron subtypes, such as Cux1, Lhx2 and Satb2, likely relate to corpus callosum emergence in eutherians. These results show molecular heterochronies of neocortical development in Theria, and highlight changes in gene expression and cell type composition that may underlie neocortical evolution and diversification. This article has an associated 'The people behind the papers' interview.

Hans Bluntschli in Berne: Researching reproduction in hedgehog tenrecs (Afrosoricida, Tenrecidae).

The Swiss anatomist Hans Bluntschli is best known as a primatologist. Yet, his focus during his later years in Berne was on reproduction in Malagasy tenrecs. This research was done with two graduate students, Robert Goetz and Fritz Strauss; all three had been obliged to leave Germany after the National Socialists came to power. Unique features of reproduction in tenrecs included nonantral follicles, intrafollicular fertilization, eversion of the corpus luteum, and polyovulation. The fertilized egg formed a blastula that developed into a blastocyst; there was no morula stage. A false placental cushion developed in the endometrium opposite the implantation site. Placentation was complex and included development of a prominent haemophagous organ. These findings are discussed in relation to current concepts of mammalian phylogeny that place tenrecs and golden moles in the same order and as close relatives to elephant shrews and the aardvark.

What is a placental mammal anyway?

Many developmental functions in marsupials and eutherian mammals are accomplished by different tissues, but similar genes.

Astroglial-mediated remodeling of the interhemispheric midline during telencephalic development is exclusive to eutherian mammals.

The corpus callosum forms the major interhemispheric connection in the human brain and is unique to eutherian (or placental) mammals. The developmental events associated with the evolutionary emergence of this structure, however, remain poorly understood. A key step in callosal formation is the prior remodeling of the interhemispheric fissure by embryonic astroglial cells, which then subsequently act as a permissive substrate for callosal axons, enabling them to cross the interhemispheric midline. However, whether astroglial-mediated interhemispheric remodeling is unique to eutherian mammals, and thus possibly associated with the phylogenetic origin of the corpus callosum, or instead is a general feature of mammalian brain development, is not yet known. To investigate this, we performed a comparative analysis of interhemispheric remodeling in eutherian and non-eutherian mammals, whose lineages branched off before the evolution of the corpus callosum. Whole brain MRI analyses revealed that the interhemispheric fissure is retained into adulthood in marsupials and monotremes, in contrast to eutherians (mice), in which the fissure is significantly remodeled throughout development. Histological analyses further demonstrated that, while midline astroglia are present in developing marsupials, these cells do not intercalate with one another through the intervening interhemispheric fissure, as they do in developing mice. Thus, developing marsupials do not undergo astroglial-mediated interhemispheric remodeling. As remodeling of the interhemispheric fissure is essential for the subsequent formation of the corpus callosum in eutherians, our data highlight the role of astroglial-mediated interhemispheric remodeling in the evolutionary origin of the corpus callosum.

Relação entre a quantidade de AgNORS, atividade proliferativa e o estágio de desenvolvimento placentário em equinos / Relationship between the amount of AgNORs, proliferative activity and stage of placental development in horses

As Regiões Organizadoras de Nucléolo (NORs - nucleolar organizer regions) são utilizadas para descrever regiões de cromatina coradas por Nitrato de Prata e estão relacionados com a atividade de síntese de RNAr e com a agilidade e rapidez na proliferação celular nos tecidos estudados. O objetivo deste trabalho foi relacionar a quantidade de AgNORs, a atividade proliferativa e o estágio da gestação em equinos, utilizando a coloração de Nitrato de Prata. Os anexos embrionários foram coletados, fixados em solução de formaldeído tamponado 10%, emblocadas em paraplast e submetidos à coloração de Nitrato de Prata. Os grupos foram determinados de acordo com a idade gestacional. A quantidade de NORs encontrada no cório no começo da gestação indica início da atividade celular e na medida em que a gestação avança, a quantidade de NORs aumenta, sugerindo maior atividade de síntese e aumento da sua importância na manutenção do feto. Ao contrário do que ocorre no cório, a quantificação das NORs foram maiores no final da gestação do que no inicio, sugerindo a estabilização destas membranas no final da gestação. A cinta coriônica e o saco vitelino foram encontrados no início da gestação e apresentaram grande quantidade de NORs, sugerindo função de síntese e proliferação no inicio da gestação, visto que suas funções é manutenção do embrião até a formação completa da placenta verdadeira (cório-alantoide). Concluímos que as membranas que se desenvolvem de maneira progressiva de acordo com o crescimento embrionário/fetal (cório, alantoide e âmnio) têm aumento no número de NORs e as membranas que involuem após a formação do embrião/feto (saco vitelino e cinta coriônica) têm um decréscimo neste número, sugerindo a diminuição da atividade proliferativa nestas membranas.

The Nucleolar Organizer Regions (NORs - nucleolar organizer regions) are used to describe regions of chromatin stained with silver nitrate and are related to the activity of rRNA synthesis and to the agility and speed of cell proliferation in the tissues studied. The objective of this study was to relate the amount of AgNORs, proliferative activity and stage of pregnancy in horses, using the coloring of Silver Nitrate. The embryonic attachments were collected, fixed in 10% buffered formaldehyde, embedded in paraplast and stained by silver nitrate. The groups were determined according to the gestational age. The amount of the corium NOR found in early pregnancy indicates the onset of cell activity, and in that the pregnancy progresses, the amount of NOR increases, suggesting higher activity and increased synthesis of their importance in maintaining the fetus. Contrary to what occurs in the corium, the quantification of NORs was higher in late pregnancy than in the beginning, suggesting the stabilization of these membranes in late pregnancy. The chorionic girdle and the yolk sac were found in early pregnancy and had lots of NORs, suggesting synthesis function and proliferation in early pregnancy, since their functions is maintenance of the embryo until the complete formation of the true placenta (chorio-allantoic membranes). We conclude that the membranes that develop in a progressive manner in accordance with the growing embryo/fetal (chorion, amnion and allantoic membranes) have an increased number of NORs and the membranes that involute after the formation of the embryo/fetus (yolk sac and chorionic girdle) have a decrease in number, suggesting a reduction in proliferative activity in these membranes.