Post-embryonic development and aging of the appendicular skeleton in Ambystoma mexicanum

BackgroundThe axolotl is a key model organism for studying appendicular regeneration in vertebrates. The axolotl limb complexity resembles that of humans in structure and tissue components; however, axolotl limbs develop post-embryonically. In this work, we evaluated the post-embryonic development of the appendicular skeleton and its changes with aging. ResultsJuvenile skeletal elements are formed mostly by SOX9+/COL1A2+ cartilage cells. Ossification of the appendicular skeleton starts when animals reach a total 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 to the ossification center and to the formation of the marrow cavity. We identified the contribution of Col1a2+ cells to bone and adipocytes. Moreover, ossification progresses with age towards the epiphyses of long bones. Even though axolotls are neotenic salamanders, ossification remains responsive to L-thyroxine, increasing the rate of bone formation. ConclusionsBone maturation is a continuous process in axolotls that extends throughout the life span of the axolotls. 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.