Regeneration enables the regrowth and restoration of missing body parts. It is a common phenomenon among animals. However, only some species exhibit remarkable regeneration capabilities and can regenerate organs such as limbs, lenses or hearts. Regeneration has been widely studied, thereby giving rise to new fields, such as regenerative medicine. Furthermore, regeneration has the potential to be applied to the human body. However, the molecular mechanisms governing this process should be elucidated first. Recent advancements in research methods have led to the identification of numerous signaling pathways involved in regeneration. One of them, the Wnt transduction pathway, is an ancient and evolutionarily conserved pathway that plays an important role in both embryonic development and regeneration. The Wnt pathway plays an important role during the regeneration process, as it is implicated in cell fate determination, cell migration, cell polarity and adult cell homeostasis. To date, two major Wnt pathways have been identified: the canonical (β-catenin dependent) pathway and the non-canonical pathway. The latter pathway can be further divided into planar cell polarity, the Wnt/Ca2+ pathway and the JNK pathway. In this review, we summarize the current state of knowledge regarding the Wnt signaling pathway and its role in regeneration, with a particular emphasis on key model species.

The eggs in most invertebrates are fertilized externally, and therefore their resulting embryos are exposed to an environment full of microbes, many of which are pathogens capable of killing other organisms. How the developing embryos of invertebrates defend themselves against pathogenic attacks is an intriguing question to biologists, and remains largely unknown. Here we clearly demonstrated that the egg cytosol prepared from the newly fertilized eggs of amphioxus Branchiostoma belcheri, an invertebrate chordate, was able to inhibit the growth of both the Gram-negative bacterium Vibrio anguillarum and the Gram-positive bacterium Staphylococcus aureus. All findings point to that it is the complement system operating via the alternative pathway that is attributable to the bacteriostatic activity. This appears to be the first report providing the evidence for the functional role of the maternal complement components in the eggs of invertebrate species, paving the way for the study of maternal immunity in other invertebrate organisms whose eggs are fertilized in vitro. It also supports the notion that the early developing embryos share some defense mechanisms common with the adult species.