In vitro models of muscle spindles: From traditional methods to 3D bioprinting strategies

Abstract

Muscle spindles are key proprioceptive mechanoreceptors composed of intrafusal fibres that regulate kinaesthetic sensations and reflex actions. Traumatic injuries and neuromuscular diseases can severely impair the proprioceptive feedback, yet the regenerative potential and cell-matrix interactions of muscle spindles remain poorly understood. There is a pressing need for robust tissue-engineered models to study spindle development, function and regeneration. Traditional approaches, while insightful, often lack physiological relevance and scalability. Three-dimensional (3D) bioprinting offers a promising approach to fabricate biomimetic, scalable, and animal-free muscle spindle constructs with controlled cellular architecture. Various bioprinting techniques - including inkjet, extrusion, digital light projection and laser-assisted bioprinting - have been explored for skeletal muscle fabrication, but replicating intrafusal fibre complexity remains a challenge. A major challenge lies in bioink development, where biocompatibility, printability and mechanical strength must be balanced to support intrafusal fibre differentiation and proprioceptive function. Recent molecular insights into spindle anatomy, innervation and extracellular matrix composition are shaping biofabrication strategies. This review discusses the current state of muscle spindle modelling, the application of 3D bioprinting in intrafusal fibre engineering, key challenges and future directions.

Keywords: bioink; bioprinting; hydrogel; intrafusal fibre; muscle spindle; proprioception; skeletal muscle; three-dimensional (3D) bioprinting; tissue engineering.

Figure 1.

Illustration of four 3D bioprinting technologies. (a) Inkjet-based bioprinting: dispensing of tiny bioink droplets through the printhead using thermal or piezoelectric driving pulses. (b) Extrusion-based bioprinting: continuous deposition of cylindrical filaments using pneumatic or mechanical pressures. (c) Digital light processing bioprinting: stacking of cured layers using projected UV or visible light. (d) Laser-assisted bioprinting: use of focused UV or visible light beam to photopolymerize photosensitive bioinks. Illustration created by Medgy Design.

Figure 2.

Biological insights guiding 3D bioprinting design and optimisation. The diagram highlights the role of biological insights in determining: (a) Structural Organisation: the spatial arrangement of cells within the bioprinted construct to mimic native tissue architecture; (b) Cell Type: selection of appropriate cell types and ratios to ensure the desired functionality; (c) Signalling Pathways: incorporation of biochemical cues to regulate cell behaviour, differentiation and tissue development; (d) Regenerative Potential: how the construct’s design supports tissue regeneration, including factors that influence cellular proliferation and repair; (e) Vascularisation: strategies for promoting blood vessel formation within the construct to ensure nutrient and oxygen supply; (f) Functional Requirement: how the design meets the mechanical and biological needs of the target tissue, such as load-bearing capacity or electrical conductivity; and (g) ECM Composition: the integration of ECM components to support cellular attachment, migration, and differentiation.

Figure 3.

The anatomical structure of muscle spindle. Illustration created by Medgy Design.

Figure 4.

Three Key factors to consider for 3D bioprinting design. This figure illustrates the interconnection between three critical factors – hydrogel formulation, tissue of interest, and bioprinting technique. The overlaps between these factors emphasise the importance of carefully considering how each element contributes to the development of a biomimetic and functional bioprinted construct with tuneable mechanical properties. Achieving an optimal balance between these factors is essential for creating tissue-engineered models that closely mimic native tissue behaviour and function.