The axolotl, \(Ambystoma\ mexicanum\), is a species of salamander native exclusively to the remnants of Lakes Xochimilco and Chalco near Mexico City. The study of its anatomy reveals why this amphibian is distinct among vertebrates. Unlike most of its relatives, the axolotl retains traits typically seen only in the larval stage, a phenomenon known as neoteny. This permanent aquatic existence means the axolotl’s body plan is a unique blend of juvenile and mature features. This unusual anatomy allows for remarkable biological capabilities that are currently the subject of intense scientific study.
Distinct External Features
The most immediately recognizable external feature of the axolotl is the set of three pairs of feathery, branching gills that project from the sides of its wide, flat head. These external gills, known as the ramus, are lined with vascular filaments called fimbriae, which provide a large surface area for gas exchange in the water. The axolotl can actively “flick” these structures to enhance water movement and oxygen uptake, though it also possesses internal gill slits behind the external gills.
The head hosts small, lidless eyes that are poorly adapted for vision, causing the animal to be highly sensitive to bright light. Its mouth is structured with small, soft, pedicellate teeth that are not used for chewing but for gripping prey before swallowing it whole using suction. The axolotl relies far more heavily on its sense of smell to navigate its environment and locate food than on sight.
The body is supported by four limbs, with the forelimbs possessing four digits and the hind limbs having five, which is typical for salamanders. The body is covered in smooth, permeable skin that lacks scales and also contributes to respiration through cutaneous gas exchange. A finned tail extends from the trunk, aiding in aquatic mobility, and color morphs range from the wild-type dark brown or gray to laboratory-bred variations.
Internal Organ Systems and Neoteny
The axolotl’s internal structure is fundamentally shaped by neoteny, which is the retention of juvenile physical characteristics into sexual maturity. This phenomenon occurs because the axolotl typically fails to produce the necessary thyroid-stimulating hormone or thyroxine required to initiate metamorphosis. Remaining permanently aquatic allows the species to reproduce while still exhibiting larval traits.
The skeletal system remains largely cartilaginous throughout its life, unlike in most adult salamanders where the cartilage is replaced by bone through ossification. While the skeleton does progressively ossify after sexual maturity, it retains a more primitive composition compared to fully metamorphosed amphibians. The structure of the vertebral column originates entirely from the somites, which is characteristic of the primitive tetrapod spine.
The circulatory system features a simple, three-chambered heart, a design common to amphibians. The digestive system is adapted for its carnivorous diet, structured to manage the suction-feeding technique used to capture small invertebrates and fish. Gas exchange relies on the external gills, the permeable skin, and small, rudimentary lungs. The lungs are used for occasional surface gulping, a process called buccal pumping, which allows the animal to supplement its oxygen intake when water quality is low.
The Cellular Basis of Regeneration
The most notable aspect of the axolotl’s anatomy is its capacity for regeneration, which extends far beyond simple wound healing. It can perfectly regrow entire limbs, parts of the spinal cord, portions of the brain, heart tissue, and jaws without forming scar tissue. This ability is rooted in a sophisticated cellular mechanism that is highly distinct from typical vertebrate healing processes.
When an amputation occurs, the wound is quickly covered by a specialized layer of cells called the wound epidermis. Beneath this cap, a mass of specialized, undifferentiated cells called the blastema begins to form. The blastema is the progenitor structure from which the new limb or organ will develop.
The formation of the blastema is enabled by a process known as cellular dedifferentiation. Specialized cells, such as those from the bone, muscle, and dermis near the injury site, revert to a stem-cell-like state. This involves the breakdown of the existing tissue matrix, known as histolysis, and the migration of these reprogrammed cells into the blastema.
These dedifferentiated cells accumulate under the wound epidermis and then begin to proliferate rapidly. Proteins like EVI5 help regulate this process by preventing the cells from entering the mitotic phase prematurely, ensuring a proper accumulation of cells before growth begins. Once sufficient cells are gathered, the blastema cells redifferentiate, following the correct genetic blueprint to rebuild the missing structure, resulting in functional, scar-free tissue.