The developmental hourglass model and recapitulation: An attempt to integrate the two models.

Recapitulation is a hypothetical concept that assumes embryogenesis of an animal parallels its own phylogenetic history, sequentially developing from more ancestral features to more derived ones. This concept predicts that the earliest developmental stage of various animals should represent the most evolutionarily conserved patterns. Recent transcriptome-based studies, on the other hand, have reported that mid-embryonic, organogenetic periods show the highest level of conservation (the developmental hourglass model). This, however, does not rule out the possibility that recapitulation would still be detected after the mid-embryonic period. In accordance with this, recapitulation-like morphological features are enriched in late developmental stages. Moreover, our recent chromatin accessibility-based study provided molecular evidence for recapitulation in the mid-to-late embryogenesis of vertebrates, as newly evolved gene regulatory elements tended to be activated at late embryonic stages. In this review, we revisit the recapitulation hypothesis, together with recent molecular-based studies that support the developmental hourglass model. We contend that the recapitulation hypothesis does not entirely contradict the developmental hourglass model and that these two may even coexist in later embryonic stages of vertebrates. Finally, we review possible mechanisms underlying the recapitulation pattern of chromatin accessibility together with the hourglass-like evolutionary conservation in vertebrate embryogenesis.