Chondrocytes interconnecting tracks and cytoplasmic projections observed within the superficial zone of normal human articular cartilage--a transmission electron microscopy, atomic force microscopy, and two-photon excitation microscopy studies.

The morphology of the normal human and rat articular cartilage was assessed using transmission electron microscopy (TEM), atomic force microscopy (AFM), and two-photon excitation (2PE) microscopy. Spurr-embedded sections from fixed human cartilage were simultaneously evaluated using TEM and AFM. The presences of tracks among the chondrocytes from the superficial zone of the cartilage were observed. In order to ratify the presence of interconnecting tracks among superficial zone chondrocytes, whole fixed human and rat cartilage, as well as fresh whole rat cartilage, were examined under the 2PE. In all cases, these tracks were observed. In addition, porous matrix, well-defined lacunae, and cytoplasmic projections anchored to the extracellular matrix (ECM) were also detected. We conclude that normal human and rat flattened superficial chondrocytes might be interconnected by tracks running through the ECM. In addition, cytoplasmic projections were observed anchored to the ECM. All these structures may possibly be related to cell/cell and ECM/chondrocytes signaling. Our findings provide new information that possibly will be of relevant importance for a more profound study of normal cartilage physiology and eventually, the pathogenesis of osteoarthritis.