Figure 3. Gene expression in the developing hippocampus is dynamic.

There are many intrinsic and technical challenges to the study of critical periods and these are exaggerated in the case of sexual differentiation of the brain due to the short time course and the dynamic events during that period. We quantified mRNA levels of 20 candidate genes critical to proliferation in the developing dentate gyrus of the hippocampus in an attempt to gain insight into the mechanisms behind a 2-fold higher rate of neurogenesis in males (Bowers et al., 2013). Levels of mRNA were measured by qPCR on the day of birth and every 2 days thereafter until postnatal day 10. None of the genes exhibited robust sex differences but 19 of the 20 showed highly dynamic expression profiles across the short time span examined. These results highlight the challenges inherent in the use of more comprehensive approaches such as RNA-Seq or genome wide bisulfate sequencing (GWBS) which survey the entire genome but are not practically implemented in multiple samples over many days. As a result, most transcriptome and genome wide studies are of a single “snap shot” in time, which may or may not capture critical events.