Evolutionary origin of endochondral ossification: the transdifferentiation hypothesis.

Area of Science:

• Evolutionary Developmental Biology
• Vertebrate Paleontology
• Skeletal Biology

Background:

• The vertebrate endoskeleton forms from diverse calcified tissues, including bone and cartilage.
• Understanding the origin and diversification of skeletal tissues is a significant challenge.
• Hybrid cell types with intermediate osteoblast and chondrocyte phenotypes contribute to skeletal development.

Purpose of the Study:

• To interpret recent lineage tracing data on chondrocyte-to-osteoblast transdifferentiation within a broad phylogenetic context.
• To propose a scenario for the evolutionary emergence of endochondral ossification.
• To explore the role of skeletal cell plasticity in the diversification of skeletal tissues.

Main Methods:

• Integration of fossil evidence.
• Histological analysis of skeletal tissues.
• Cellular and genetic data from diverse vertebrate species.
• Phylogenetic comparative analysis.

Main Results:

• Lineage tracing in mice demonstrates chondrocytes transdifferentiating into osteoblasts at the chondro-osseous junction.
• This transdifferentiation process is proposed as a fundamental mechanism in the evolution of endochondral ossification.
• Skeletal cell plasticity, potentially driven by similar gene regulatory networks, underlies diverse skeletal tissue types.

Conclusions:

• Transdifferentiation is a key factor in the evolutionary origin of endochondral ossification, an osteichthyan novelty.
• Skeletal cell plasticity provides a unifying mechanism for both complete transdifferentiation and partial cell type conversions.
• This plasticity explains the existence of intermediate skeletal tissues like chondroid bone and globuli ossei.