The zebrafish presomitic mesoderm elongates through compaction-extension.

In vertebrate embryos the presomitic mesoderm becomes progressively segmented into somites at the anterior end while extending along the anterior-posterior axis. A commonly adopted model to explain how this tissue elongates is that of posterior growth, driven in part by the addition of new cells from uncommitted progenitor populations in the tailbud. However, in zebrafish, much of somitogenesis is associated with an absence of overall volume increase, and posterior progenitors do not contribute new cells until the final stages of somitogenesis. Here, we perform a comprehensive 3D morphometric analysis of the paraxial mesoderm and reveal that extension is linked to a volumetric decrease and an increase in cell density. We also find that individual cells decrease in volume over successive somite stages. Live cell tracking confirms that much of this tissue deformation occurs within the presomitic mesoderm progenitor zone and is associated with non-directional rearrangement. Taken together, we propose a compaction-extension mechanism of tissue elongation that highlights the need to better understand the role tissue intrinsic and extrinsic forces in regulating morphogenesis.

Reduced detection of positive expressions in major depression.

In patients with depression, negative biases have been reported in various cognitive domains, but few studies have examined whether even detection is affected, i.e. are depressed patients more likely to detect the presence of negative stimuli? This study compared detection of sad and happy faces in patients (n=17) and healthy participants (n=18) using an attentional blink task. Patients with depression detected significantly fewer happy faces than matched healthy participants, but for sad faces the group difference was non-significant. The results suggest that depression may affect the detection of positive stimuli.