The amazing and anomalous axolotls as scientific models

Abstract

Ambystoma mexicanum (axolotl) embryos and juveniles have been used as model organisms for developmental and regenerative research for many years. This neotenic aquatic species maintains the unique capability to regenerate most, if not all, of its tissues well into adulthood. With large externally developing embryos, axolotls were one of the original model species for developmental biology. However, increased access to, and use of, organisms with sequenced and annotated genomes, such as Xenopus laevis and tropicalis and Danio rerio, reduced the prevalence of axolotls as models in embryogenesis studies. Recent sequencing of the large axolotl genome opens up new possibilities for defining the recipes that drive the formation and regeneration of tissues like the limbs and spinal cord. However, to decode the large A. mexicanum genome will take a herculean effort, community resources, and the development of novel techniques. Here, we provide an updated axolotl-staging chart ranging from one-cell stage to immature adult, paired with a perspective on both historical and current axolotl research that spans from their use in early studies of development to the recent cutting-edge research, employment of transgenesis, high-resolution imaging, and study of mechanisms deployed in regeneration.

Keywords: Ambystoma mexicanum; axolotl; embryonic development; regeneration; staging.

FIGURE 1

Staging series for Ambystoma mexicanum from fertilization through maturation. Stages were grouped into eight different categories: cleavage-blastula (0–24 hours post laying, HPL), gastrula (24–54 hours HPL), neurula (54–72 hours HPL), early tailbud (72–83 hours HPL), tailbud (83–122 hours HPL), prehatched (122–342 + HPL), hatched (342 hours to 1–3 months postlaying/fertilization), and larva-adult. Hatched larva to sexually mature adult can take up to 18 months depending on tank density. At cleavage/blastula stages, both animal and lateral views are shown. Similar to Xenopus laevis and tropicalis, Ambystoma embryos have pigment differentials on the animal and vegetal poles, the animal poles being dark and the vegetal poles light colored and filled with yolk even in leucistic animals as shown. At gastrula stages, we show both dorsal and posterior views. As gastrulation proceeds, the blastopore closes and the animals begin to neurulate. At neurula stages, we show dorsal and anterior views. After neural tube closure, the embryos begin axis elongation throughout the early tailbud and tailbud stages. At these stages, we show lateral views. The gill arches become visible during tailbud stages and further develop in the prehatch tadpoles (lateral view). At these stages, gills become more pronounced and eyes gain pigment and become visible. Prehatch embryos remain in their thick jelly coats in natural settings. In hatched tadpoles, the dorsal fin becomes more transparent, the gills branch and grow outward (lateral view). We show a dorsal view of a larva-adult. This animal is approximately 1 year old, but is not yet fully grown or sexually mature