Ciliated cell observation by SEM on the surface of human incudo-malleolar-joint articular cartilage: are they a new chondrocyte phenotype?

Background: Scanning electron microscopy (SEM) study of the human incus bone is scanty whilst, to our knowledge, no information regarding human incudo-malleolar joint articular-cartilage morphology has previously been provided. Aims/objectives: Our aim was to shed some light on this morphological issue and to propose some theoretical perspectives on its functional role. Material and methods: The human incudo-malleolar joint was documented with field emission SEM on samples recovered during ear surgery procedures after patients’ informed consent. Results: Normal articular cartilage chondrocytes, flattened cells with prominent nucleus and short microvilli were observed. Interestingly, cells provided with long cilia were identified. Type A cilia are arranged in a pyramidal formation with extra-long cilia stemming from the cluster, projecting upwards in an antenna-like formation ending with a dilated structure that as a whole, resembles the stereocilia with kinocilium. Types B, C and D cilia resemble those of the genital and respiratory tracts. Conclusions and significance: It is therefore possible to hypothesize that the observed ciliated cells may be a new chondrocyte phenotype with sensory function. Motile cilia confer the ability to distinguish variations in synovial fluid chemical composition and, in addition, they perhaps may also play some role in the mechanism of sound transmission.