Advanced models of human skeletal muscle differentiation, development and disease: Three-dimensional cultures, organoids and beyond

Abstract

Advanced in vitro models of human skeletal muscle tissue are increasingly needed to model complex developmental dynamics and disease mechanisms not recapitulated in animal models or in conventional monolayer cell cultures. There has been impressive progress towards creating such models by using tissue engineering approaches to recapitulate a range of physical and biochemical components of native human skeletal muscle tissue. In this review, we discuss recent studies focussed on developing complex in vitro models of human skeletal muscle beyond monolayer cell cultures, involving skeletal myogenic differentiation from human primary myoblasts or pluripotent stem cells, often in the presence of structural scaffolding support. We conclude with our outlook on the future of advanced skeletal muscle three-dimensional cultures (e.g. organoids and biofabrication) to produce physiologically and clinically relevant platforms for disease modelling and therapy development in musculoskeletal and neuromuscular disorders.

Introduction

The skeletal muscle, an architecturally complex tissue that accounts for the largest tissue mass in the human body, is responsible for supporting posture, voluntary movement, guarding soft tissues and body openings, as well as regulating several metabolic and homoeostatic functions. Functional skeletal muscle not only contains myofibres and their progenitor cells but also requires their constant interaction with other cell types and tissues including, but not limited to, connective tissue, vasculature and motor neurons [1]. The hierarchical organisation of skeletal muscle (Figure 1a) consists of organised bundles of fascicles which in turn are composed of bundles of myofibres embedded within three layers of extracellular matrix (the endomysium, perimysium and epimysium) [2]. The importance of the interplay between different compartments of the skeletal muscle niche (Figure 1b) is exemplified on acute injury, when multiple mechanisms are initiated within the different compartments that eventually converge to activate tissue-resident muscle stem cells (MuSCs, also known as satellite cells). For instance, damaged blood vessels can release cytokines [3] or inflammatory cells [4] to support regeneration at an injury site.

Normal tissue function and repair/regeneration can be overcome in large acute muscle injuries as well as in chronic severe musculoskeletal disorders such as muscular dystrophy [5], where different components of the skeletal muscle tissue functional units and niche are compromised. Given ethical considerations and limited tissue availability, it is often difficult to study skeletal muscle developmental dynamics, regeneration and disease pathogenesis in human subjects or their biopsies. Although traditional cell culture and animal models have been used to elucidate some molecular aspects behind these processes, limitations in using different species [6] and systems lacking physiologically relevant extracellular cues [7] make it difficult to translate such findings to the human context. Bioengineering human models with higher fidelity to native skeletal muscle tissues can overcome these limitations and enable researchers to advance our fundamental understanding of the mechanistic processes behind muscle development and regeneration. Such insights can be further applied to disease modelling, biomarker detection, drug screening and regenerative medicine.

In this review, we will start with a brief overview of skeletal myogenic cell generation and differentiation followed by a discussion on recently developed three-dimensional (3D) platforms, developed with human biopsy-derived myoblasts (primary or immortalised) or pluripotent stem cells. We then conclude with our perspectives on the future of artificial skeletal muscle models by discussing methods to develop physiologically complex models able to deliver clinically relevant phenotypic readouts that can be used as outcome measures for therapy development. We will not highlight studies based on platforms using rodent myogenic cells, nor those involving top-down approaches such as tissue decellularization, for which we redirect the reader to recent comprehensive reviews [8,9].