Postembryonic development and aging of the appendicular skeleton in Ambystoma mexicanum.

Background: The axolotl is a key model to study appendicular regeneration. The limb complexity resembles that of humans in structure and tissue components; however, axolotl limbs develop postembryonically. In this work, we evaluated the postembryonic development of the appendicular skeleton and its changes with aging. Results: The juvenile limb skeleton is formed mostly by Sox9/Col1a2 cartilage cells. Ossification of the appendicular skeleton starts when animals reach a length of 10 cm, and cartilage cells are replaced by a primary ossification center, consisting of cortical bone and an adipocyte‐filled marrow cavity. Vascularization is associated with the ossification center and the marrow cavity formation. We identified the contribution of Col1a2‐descendants to bone and adipocytes. Moreover, ossification progresses with age toward the epiphyses of long bones. Axolotls are neotenic salamanders, and still ossification remains responsive to l‐thyroxine, increasing the rate of bone formation. Conclusions: In axolotls, bone maturation is a continuous process that extends throughout their life. Ossification of the appendicular bones is slow and continues until the complete element is ossified. The cellular components of the appendicular skeleton change accordingly during ossification, creating a heterogenous landscape in each element. The continuous maturation of the bone is accompanied by a continuous body growth. Key Findings: The juvenile appendicular skeleton is formed mostly by Sox9/Col1a2 cartilage cells, transitioning to a slow and life‐long ossification that starts around sexual maturation.Axolotl growth is biphasic, defined by an explosive growth of axolotls in approximately the first 20 months of their life, after which growth velocity dramatically diminishes, however never reaching zero.A hybrid population of hypertrophic chondrocytes, expressing the chondrogenic marker SOX9 and the osteoblast marker OCN, resides next to the ossification center.Col1a2‐descendants contribute to bone and adipocytes during ossification. [ABSTRACT FROM AUTHOR]