The axolotl (Ambystoma mexicanum) is a unique salamander known for neoteny, meaning it retains its larval, juvenile features throughout its adult life. Unlike most amphibians that undergo metamorphosis to become terrestrial adults, the axolotl remains fully aquatic, keeping its feathery external gills and finned tail. This retention of juvenile form deeply affects its anatomy, particularly its skeletal structure. Understanding the number of bones an axolotl possesses requires exploring how this unique life history influences its skeletal development.
Defining the Axolotl Skeletal Count
Pinpointing a single, fixed number of bones in an axolotl is not possible, a reality that reflects its unusual biology. The difficulty stems from the fact that its skeleton is not fully ossified, or hardened into bone, even in adulthood. This makes it challenging to define where cartilage ends and bone begins. Researchers using non-invasive methods like radiography have observed significant individual variation in the number of certain skeletal elements, even among animals of the same age. For instance, the number of trunk vertebrae in adult axolotls can vary slightly between 14 and 16 segments.
The number of small bones in the limbs also shows inconsistency, particularly in the feet. The number of phalanges, or toe bones, can vary widely on each foot, typically ranging from two to four per digit. Subadult axolotls present an even greater counting challenge, as their developing skeletal tissue is less dense and harder to visualize clearly on X-rays. The resulting count is not a static figure like the 206 bones in a human, but rather a variable estimate influenced by the animal’s continuous, incomplete development.
The Unique Cartilaginous Structure
The primary reason for the variable bone count is the axolotl’s neotenic skeleton, which is composed of a much higher proportion of cartilage than that of a typical adult salamander. While other salamanders replace their cartilage with hard bone during metamorphosis, the axolotl largely bypasses this process. Their aquatic lifestyle removes the need for dense, load-bearing bones required for movement on land, allowing the more flexible, lighter cartilage to persist.
This retention of juvenile features is linked to an altered endocrine system that does not produce the necessary hormones to trigger full metamorphosis. Skeletal elements typically cartilaginous in a larva, such as the branchial arches that support the gills, remain cartilaginous in the adult axolotl. Even the skull retains significant amounts of cartilage, including Meckel’s cartilage in the lower jaw. The long bones of the limbs do begin to ossify, but this process is less complete and occurs much later in life than in a metamorphosing amphibian.
Skeletal Role in Limb Regeneration
The axolotl’s unique skeletal structure supports its unparalleled ability to regenerate lost limbs. When an axolotl loses a limb, the first step involves the active resorption of the existing bone and cartilage near the site of amputation. Specialized cells called osteoclasts break down the mineralized tissue, which clears the way for the regenerative process. This resorption is considered a necessary step for the successful integration of the new skeletal structures.
Following the injury, a mass of specialized cells called the blastema forms at the stump. The skeletal components of the stump, particularly the periosteum—the membrane surrounding the bone—provide some of the cells that populate this blastema. The blastema then proceeds to rebuild the missing limb, including the new bone and cartilage, in a process that essentially recapitulates embryonic development. This regenerative capacity is a direct functional outcome of its neotenic state, maintaining the cellular plasticity required for rebuilding complex structures throughout its life.