Challenges of engineering a functional growth plate in vitro

Abstract

Several cartilage and bone organoids have been developed in vitro and in vivo using adult mesenchymal stromal/stem cells (MSCs) or pluripotent stem cells (PSCs) to mimic different phases of endochondral ossification (ECO), as one of the main processes driving skeletal development and growth. While cellular and molecular features of growth plate-like structures have been observed through the generation and in vivo implantation of hypertrophic cartilage tissues, no functional analogue or model of the growth plate has yet been engineered. Herein, after a brief introduction about the growth plate architecture and function, we summarize the recent progress in dissecting the biology of the growth plate and indicate the knowledge gaps to better understand the mechanisms of its development and maintenance. We then discuss how this knowledge could be integrated with state-of-art bioengineering approaches to generate a functional in vitro growth plate model.

Keywords: bioengineering; chondrocyte; endochondral ossification; growth plate; organoids; skeletal stem cell; stem cell niche.

FIGURE 1

(A) Growth plate (GP) architecture. The graphics highlights the key molecular pathways and feedback loops involved in the different zones of the GP, as well as the regulatory molecules involved in the establishment and maintenance of an epiphyseal stem cell niche. The different cell morphologies represent cell populations (i.e., mesenchymal stromal cells, endothelial cells and haematopoietic cells) within the secondary ossification center (SOC). Red arrows indicate the lineage of the skeletal stem cells (SSCs) within the resting zone of GP. (B) Engineering the epiphyseal stem cell niche. The graphics highlights that the recapitulation of the functional features of the GP requires the formation of a specific niche. Self-renewing stem cells and accessory cell populations (i.e., mesenchymal stromal cells, endothelial cells and haematopoietic cells) are in charge of an orchestrated integration of biochemical and biophysical signals (e.g., extracellular matrix (ECM) molecules and stiffness, cytokine gradients). (C) Generation of biological knowledge for GP modelling. Deeper understanding of molecular and cellular mechanisms underlying GP development by utilizing traditional and innovative tools (e.g., genetic mutants, lineage tracing, gene-editing, organoid systems, and transcriptomic/proteomic) will be key to recapitulate the epiphyseal stem cell niche and thus to engineer models of a functional GP. (D) Adoption of bioengineering strategies for GP modelling. Bioengineering tools, such as biomimetic materials, controlled-release systems, 3D printing, and Organ-on-chip, hold the potential to control stem cell behavior and replicate the epiphyseal stem cell niche, towards modelling a functional GP. (E) Potential applications of engineered growth plate. The development of a functional in vitro GP has several potential applications. For example, it could provide a platform for drug testing and discovery as well as studying organogenesis and pathogenesis of GP development. Additionally, it could serve as a graft which could be implanted to treat growth defects. Furthermore, it will provide a paradigm for generating growing tissues and open multiple avenues in paediatric regenerative medicine.