Postembryonic development and aging of the appendicular skeleton in Ambystoma mexicanum

Abstract

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.

Neural control of growth and size in the axolotl limb regenerate

“…Moreover, axolotl are an indeterminately growing species, and continue to grow in size throughout their life cycle. Thus, the size of the limb at the time of amputation is different from that once the limb has completed regeneration ( Riquelme‐Guzmán et al, 2021 ). This simple observation indicates that rather than having a ‘set-point’ of size, growth must be dynamically regulated throughout the process of limb regeneration in the axolotl.…”

“…We have also discovered that the regulation of limb size is not autonomously regulated by the limb nerves.Together, these observations show that the limb nerves provide essential cues to regulate ontogenetic allometric growth and the final size of the regenerating limb. Guzmán et al, 2021). This simple observation indicates that rather than having a "set-point" of size, growth must be dynamically regulated throughout the process of limb regeneration in the axolotl.Most of what is known about the factors that impact tetrapod limb size has been discovered by mutant analyses on determinately growing species, such as humans, mice, and chicken, which cease growing after they reach adulthood due to the closure of the epiphyseal growth plates.…”

“…Together, these observations show that the limb nerves provide essential cues to regulate ontogenetic allometric growth and the final size of the regenerating limb. Guzmán et al, 2021). This simple observation indicates that rather than having a "set-point" of size, growth must be dynamically regulated throughout the process of limb regeneration in the axolotl.…”

The Regulation of Growth in Developing, Homeostatic, and Regenerating Tetrapod Limbs: A Minireview

“…For example, altered nerve signaling in the limbs of pre-adult humans can result in a phenomenon known as macrodactyly, where one or more digits grows disproportionally larger than the other ( Tsuge and Ikuta, 1973 ; Frykman and Wood, 1978 ; Razzaghi and Anastakis, 2005 ). In contrast, indeterminately growing species grow throughout their entire lives, and thus maintain active growth plates as adults ( Riquelme-Guzmán et al, 2021 ). This indicates that growth plate activity must be continuously regulated in these limbs to maintain a size that is proportionally appropriate.…”

In vivoassessment of mechanical properties during axolotl development and regeneration using confocal Brillouin microscopy

“…Axolotls were kept and bred in the axolotl facility of the Center for Regenerative Therapies Dresden (CRTD) of the Technische Universität Dresden (TUD). A full description of the husbandry conditions was recently published (Riquelme-Guzmán et al, 2021).…”