High temperature limits on developmental canalization in the ascidian Ciona intestinalis.

The normal embryogenesis of marine animals is typically confined to a species-specific range of temperatures. Within that temperature range development results in a consistent, or canalized, phenotype, whereas above and below the range abnormal phenotypes are produced. This study reveals a high temperature threshold, occurring over a 1-2 °C range, for normal embryonic development in C. intestinalis. Above that threshold the prevalence of morphological abnormalities increases significantly, beginning with cleavage and gastrula stages, and becoming more pronounced as embryogenesis proceeds. However, even in highly morphologically abnormal temperature disrupted (TD) embryos, muscle, endoderm, notochord, epidermis, and sensory pigment cells are recognizable, as evidenced by histochemical markers or morphology. On the other hand, morphogenesis of the notochord and other structures is dependent on precise cell movement and shape changes after the gastrula stage, which are disrupted above the high temperature threshold. These findings suggest that morphogenetic processes may be more sensitive to high temperature than cell type specification events. They also point to avenues for investigation of the limiting factors to developmental canalization in marine invertebrates.

Contextual and auditory fear conditioning continue to emerge during the periweaning period in rats.

Anxiety disorders often emerge during childhood. Rodent models using classical fear conditioning have shown that different types of fear depend upon different neural structures and may emerge at different stages of development. For example, some work has suggested that contextual fear conditioning generally emerges later in development (postnatal day 23-24) than explicitly cued fear conditioning (postnatal day 15-17) in rats. This has been attributed to an inability of younger subjects to form a representation of the context due to an immature hippocampus. However, evidence that contextual fear can be observed in postnatal day 17 subjects and that cued fear conditioning continues to emerge past this age raises questions about the nature of this deficit. The current studies examine this question using both the context pre-exposure facilitation effect for immediate single-shock contextual fear conditioning and traditional cued fear conditioning using Sprague-Dawley rats. The data suggest that both cued and contextual fear conditioning are continuing to develop between PD 17 and 24, consistent with development occurring the in essential fear conditioning circuit.