The Mexican salamander, or axolotl (Ambystoma mexicanum), is a unique amphibian that has captured scientific attention due to its remarkable biological features. It remains in a perpetually juvenile or neotenic state, retaining external gills and living its entire life in water. This aquatic creature possesses an unusual biology that often leads to questions about its internal anatomy. The common curiosity regarding how many hearts this salamander possesses can be definitively answered by looking at its basic amphibian physiology.
The Simple Answer: Counting the Axolotl’s Hearts
The axolotl has a single, centrally located heart, just like all other salamanders and most vertebrates. This single organ is responsible for pumping blood throughout the entire body and gill system. The misconception that the axolotl has multiple hearts likely stems from its visible external anatomy, which includes three pairs of feathery gills, and the presence of other pulsing structures in its body.
A source of confusion comes from the axolotl’s lymphatic system, which is a network of vessels that returns fluid from the body’s tissues back to the blood. Like many amphibians, axolotls possess multiple pairs of lymphatic hearts. These are small, muscular sacs that actively pump lymph fluid. While these structures beat rhythmically to circulate lymph, they are entirely separate from the main circulatory system and are not true blood-pumping hearts.
Anatomy of the Amphibian Heart
The single axolotl heart is located behind the pectoral girdle and comprises the typical three chambers found in amphibians: two atria and one ventricle. The heart also includes a sinus venosus, which collects deoxygenated blood from the body, and a conus arteriosus, which is the final outflow tract. The unseptated ventricle is a highly trabeculated chamber, meaning it has a sponge-like appearance due to a network of muscle fibers, rather than the smooth walls found in mammals.
The circulatory process begins when deoxygenated blood from the body enters the right atrium via the sinus venosus. At the same time, oxygenated blood returning from the gills and lungs enters the left atrium. Both atria then contract simultaneously, pushing the blood into the single ventricle.
Although the ventricle is unseparated, the architecture of the muscular ridges helps to minimize the mixing of the oxygen-rich and oxygen-poor blood. The conus arteriosus, the muscular tube leading out of the ventricle, assists in shunting the blood. This structure helps direct the deoxygenated blood toward the gill arches for re-oxygenation and the relatively more oxygenated blood toward the rest of the body.
The Role of Circulation in Axolotl Regeneration
The axolotl’s circulatory system, despite its simple structure, plays a significant role in its most famous capability: the complete, scar-free regeneration of limbs, spinal cord, and even parts of the brain and heart. The heart itself can regenerate functional tissue after a partial amputation of the ventricle. This recovery is linked to the proliferation of existing heart muscle cells, or cardiomyocytes, which is a process rarely seen in adult mammals.
The blood vessels and circulating components are also instrumental in the initial phases of regrowth. The circulatory network delivers the necessary nutrients and signaling molecules to the injury site, forming a mass of undifferentiated cells known as the blastema. Immune cells, particularly macrophages, are an important factor in this process, arriving quickly to clear debris without triggering the scar formation that typically hinders regeneration in other animals.
The efficient, localized blood flow and the unique cellular response facilitated by the circulatory system support rapid tissue rebuilding. The axolotl’s ability to maintain a relatively low metabolic rate also contributes, reducing the immediate demand on the newly forming tissues. This coordinated effort between the single heart, the blood, and the immune system allows the axolotl to heal complex wounds and regrow structures that are permanently lost in most other vertebrates.