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Purpose: To investigate the presence and change of nerve fibers and neuropeptide during early development of articular cartilage in neonatal rats. Methods: Articular cartilage in distal-femoral epiphyses was collected from neonatal Sprague Dawley rats, which were 1-day, 5-day, and 10-day postnatal (P1, P5 and P10). Microscopy, immunofluorescence, transmission and scanning electron microscopy (TEM and SEM) were performed for detection of nerve fibers. Quantitative analysis for substance P (SP) and neuropeptide Y (NPY) was conducted using immunofluorescence and enzyme-linked immunosorbent assay (ELISA). Results: TEM showed the existence of myelinated nerve fibers in the extracellular matrix of articular cartilage in both P1, P5 and P10 rats, and they formed synaptic contacts with chondrocytes. During this time, chondrocytes proceeded with their development, and the nerve fibers gradually degraded. The ELISA results showed significant increase of the sensory neuropeptide SP and the sympathetic neuropeptide NPY in the cartilage tissue. Immunofluorescence results showed the distribution of SP and NPY in the perichondrium, the cartilage canals, the plasma of chondrocytes, and extracellular matrix in the cartilage tissue. Conclusions: Nerve fibers exist in the matrix of articular cartilage during early development of knee joints in neonatal rats. Nerve fibers form synaptic contacts with chondrocytes at the early stage and then degrade gradually in the course of chondrocyte development. SP and NPY significantly increase in articular cartilage during this very period. These results indicate that the nerve fibers and the neuropeptide they secrete may exert important effect on the development of articular cartilage. [ABSTRACT FROM AUTHOR]

Background: Mechanical loading has been widely considered to be essential for growth plate to maintain metabolism and development. Cyclic mechanical strain has been demonstrated to induce autophagy, whereas the relationship between cyclic tensile strain (CTS) and autophagy in growth plate chondrocytes (GPCs) is not clear. The objective of this study was to investigate whether CTS can regulate autophagy in GPCs in vitro and explore the potential mechanisms of this regulation. Methods: The 2-week-old Sprague–Dawley rat GPCs were subjected to CTS of varying magnitude and duration at a frequency of 2.0 Hz. The mRNA levels of autophagy-related genes were measured by RT-qPCR. The autophagy in GPCs was verified by transmission electron microscopy (TME), immunofluorescence and Western blotting. The fluorescence-activated cell sorting (FACS) was employed to detect the percentage of apoptotic and necrotic cells. Results: In GPCs, CTS significantly increased the mRNA and protein levels of autophagy-related genes, such as LC3, ULK1, ATG5 and BECN1 in a magnitude- and time-dependent manner. There was no significant difference in the proportion of apoptotic and necrotic cells between control group and CTS group. The autophagy inhibitors, 3-methyladenine (3MA) and chloroquine (CQ) reversed the CTS-induced autophagy via promoting the formation of autophagosomes. Cytochalasin D (cytoD), an inhibitor of G-actin polymerization into F-actin, could effectively block the CTS-induced autophagy in GPCs. Conclusion: Cyclic mechanical strain with high-tensile triggers autophagy in GPCs, which can be suppressed by 3MA and CQ, and cytoskeletal F-actin microfilaments organization plays a key role in chondrocytes' response to mechanical loading. [ABSTRACT FROM AUTHOR]