Ultrastructural, tomographic and confocal imaging of the chondrocyte primary cilium in situ

TLDR

Hyaline cartilage chondrocytes possess a single primary cilium per cell, yet its function remains unknown. The study examined the ultrastructure of chick embryo sternal chondrocyte cilia and their interaction with extracellular matrix molecules using TEM and double‑tilt electron tomography. The authors employed TEM, double‑tilt electron tomography, and confocal immunohistochemistry to analyze ciliary structure and bending. Tomography and TEM showed the ciliary axoneme interdigitating with collagen fibers and proteoglycans, containing electron‑opaque particles that may be intraciliary transport, and exhibiting diverse bending patterns that align with shear stress and direct deformation by matrix components, indicating the primary cilium acts as a mechanosensor in skeletal patterning and growth.

Abstract

Hyaline cartilage chondrocytes express one primary cilium per cell, but its function remains unknown. We examined the ultrastructure of chick embryo sternal chondrocyte cilia and their interaction with extracellular matrix molecules by transmission electron microscopy (TEM) and, for the first time, double-tilt electron tomography. Ciliary bending was also examined by confocal immunohistochemistry. Tomography and TEM showed the ciliary axoneme to interdigitate amongst collagen fibres and condensed proteoglycans. TEM also revealed the presence of electron-opaque particles in the proximal axoneme which may represent intraciliary-transport (ICT) particles. We observed a wide range of ciliary bending patterns. Some conformed to a heavy elastica model associated with shear stress. Others were acutely deformed, suggesting ciliary deflection by collagen fibres and proteoglycans with which the cilia make contact. We conclude that mechanical forces transmitted through these matrix macromolecules bend the primary cilium, identifying it as a potential mechanosensor involved in skeletal patterning and growth.