The axolotl, often called the Mexican walking fish, is a critically endangered amphibian native exclusively to the lake complex of Xochimilco near Mexico City. This unique animal retains its feathery external gills and aquatic lifestyle throughout its adult life. The axolotl does possess a skeleton made of both bone and cartilage, but the proportion of each is fundamentally different from nearly all other vertebrates.
The Axolotl’s Skeletal Composition
The axolotl’s skeleton is not purely cartilaginous, but it is substantially less ossified than that of most adult salamanders. When the animal is young, its skeletal elements are predominantly cartilage, a flexible connective tissue. As the axolotl grows and reaches sexual maturity, a slow process of bone formation, or ossification, begins.
Ossification starts at primary centers in the mid-shafts of the long bones, forming a core of cortical bone with a marrow cavity. Unlike mammals, the axolotl’s bone maturation progresses gradually throughout its life. The ends of the long bones, the epiphyses, remain cartilaginous even in a large adult axolotl, which is a distinct juvenile trait. This partial development means a large fraction of the adult skeleton remains the softer, more flexible cartilage characteristic of larval stages.
Neoteny: Why Axolotls Retain Cartilage
The retention of this largely cartilaginous skeleton is a direct result of neoteny, the retention of juvenile features into reproductive adulthood. In most amphibians, the transition from an aquatic larva to a terrestrial adult is triggered by a surge of thyroid hormones. This hormonal cascade causes widespread changes, including the replacement of cartilage with mature bone tissue.
Axolotls typically do not experience this necessary surge of thyroid hormones, specifically thyroxine, or they possess a reduced sensitivity to the hormone. This failure to activate the metamorphic process arrests their development at the larval stage while still allowing them to become reproductively mature. Because the signal for widespread ossification never fully occurs, the axolotl maintains the flexible, cartilaginous structure typical of an aquatic juvenile salamander. This mechanism preserves the fundamental structure of the skeleton, preventing the complete transformation seen in their close relatives, the tiger salamanders.
Skeletal Development and Regeneration
The unique composition of the axolotl’s skeleton is linked to its remarkable ability to regrow lost limbs and spinal cord sections perfectly. When an appendage is amputated, the remaining bone and cartilage structures are rapidly covered by a specialized mass of tissue known as the blastema. The blastema is a collection of progenitor cells capable of differentiating into all the necessary tissues for a new limb.
The process requires the rebuilding of both bone and cartilage, which the blastema manages by recapitulating the initial developmental steps. The blastema cells receive positional cues, such as a gradient of retinoic acid, which instructs them on what part of the limb to regenerate. The regenerating limb forms a cartilaginous model first, which is then progressively converted into bone tissue, ensuring the new structure is an exact replica. This ability contrasts sharply with the healing response in mammals, which involves fusing fractured bone and forming scar tissue at the injury site.
Changes Following Induced Metamorphosis
While axolotls naturally remain in their aquatic, cartilaginous state, they can be induced to undergo metamorphosis by administering thyroid hormones like thyroxine. This artificial induction causes rapid changes in the animal’s skeletal structure, providing a clear contrast to its neotenic state. The introduction of the hormone accelerates the ossification process throughout the body, converting much of the remaining cartilage into mature bone.
The resulting terrestrial salamander possesses a skeletal structure much closer to that of other land-dwelling amphibians. Long bones become shorter and the once-flexible cartilaginous regions decrease in cellularity, thickening the skeletal elements for a weight-bearing life on land. This transformation demonstrates the plasticity of the axolotl’s skeletal development, confirming that the genetic machinery for a fully ossified skeleton is present but is suppressed in its natural, neotenic state.