Abstract
The brains of sengis (elephant shrews, order Macroscelidae) have long been known to contain a hippocampus that in terms of allometric progression indices is larger than that of most primates and equal in size to that of humans. In this report, we provide descriptions of hippocampal cytoarchitecture in the eastern rock sengi (Elephantulus myurus), of the distributions of hippocampal calretinin, calbindin, parvalbumin, and somatostatin, of principal neuron numbers, and of cell numbers related to proliferation and neuronal differentiation in adult hippocampal neurogenesis. Sengi hippocampal cytoarchitecture is an amalgamation of characters that are found in CA1 of, e.g., guinea pig and rabbits and in CA3 and dentate gyrus of primates. Correspondence analysis of total cell numbers and quantitative relations between principal cell populations relate this sengi to macaque monkeys and domestic pigs, and distinguish the sengi from distinct patterns of relations found in humans, dogs, and murine rodents. Calretinin and calbindin are present in some cell populations that also express these proteins in other species, e.g., interneurons at the stratum oriens/alveus border or temporal hilar mossy cells, but neurons expressing these markers are often scarce or absent in other layers. The distributions of parvalbumin and somatostatin resemble those in other species. Normalized numbers of PCNA+ proliferating cells and doublecortin-positive (DCX+) differentiating cells of neuronal lineage fall within the overall ranges of murid rodents, but differed from three murid species captured in the same habitat in that fewer DCX+ cells relative to PCNA+ were observed. The large and well-differentiated sengi hippocampus is not accompanied by correspondingly sized cortical and subcortical limbic areas that are the main hippocampal sources of afferents and targets of efferents. This points to intrinsic hippocampal information processing as the selective advantage of the large sengi hippocampus.
Highlights
The small family of sengis or elephant shrews (Macroscelididae) was long placed within the order Insectivora, but is considered to form an order by itself, the Macroscelidea (Rathbun, 2009)
We provide descriptions of hippocampal cytoarchitecture in the eastern rock sengi (Elephantulus myurus), of the distributions of hippocampal calretinin, calbindin, parvalbumin, and somatostatin, of principal neuron numbers, and of cell numbers related to proliferation and neuronal differentiation in adult hippocampal neurogenesis
Pyramidal-shaped cells are embedded in the lower margin of the gcl (1, Figure 3A), while ovoid cells slightly larger than granule cells and with a larger cytoplasm are found at the upper gcl border (2, Figure 3A)
Summary
The small family of sengis or elephant shrews (Macroscelididae) was long placed within the order Insectivora, but is considered to form an order by itself, the Macroscelidea (Rathbun, 2009). They have been positioned with the Anagalida (rodents and lagomorphs) or even been taken as living representatives of Condylarths, the predecessors of hoofed mammals (Holroyd and Mussell, 2005). Sengis are generally small sized mammals and, with the exception of one species, they inhabit woods, bushland and dry boulder or gravel plains of the southern part of the continent (Rathbun, 2005, 2009). Sengis live in facultative monogamy, a rare trait in mammals, and give birth to highly precocial young
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