Alternation in the gap-junctional intercellular communication capacity during the maturation of osteocytes in the embryonic chick calvaria

Introduction The intercellular network of cell-cell communication among osteocytes is mediated by gap junctions. Gap junctional intercellular communication (GJIC) is thought to play an important role in the integration and synchronization of bone remodeling. To further understand the mechanism of bone development it is important to quantify the difference in the GJIC capacity of young and developmentally mature osteocytes. Materials and methods We first established an embryonic chick calvaria growth model to show the growth of the calvaria in embryos at 13 to 21 days of age. We then applied a fluorescence recovery after photobleaching (FRAP) technique to compare the difference in the GJIC capacity of young osteocytes with that of developmentally mature osteocytes. Finally, we quantified the dye (Calcein) diffusion from the FRAP data using a mathematic model of simple diffusion which was also used to identify simple diffusion GJIC pattern cells (fitted model) and accelerated diffusion GJIC pattern cells (non-fitted model). Results The relationship between the longest medial-lateral length of the calvaria (frontal bone) and the embryonic age fit a logarithmic growth model: length = 5.144 × ln(day) − 11.340. The morphometric data during osteocyte differentiation showed that the cellular body becomes more spindle-shaped and that the cell body volume decreased by approximately 22% with an increase in the length of the processes between the cells. However, there were no significant differences in the cellular body surface area or in the distance between the mass centres of the cells. The dye-displacement rate in young osteocytes was significantly higher than that in developmentally mature osteocytes: dye displacement only occurred in 26.88% of the developmentally mature osteocytes, while it occurred in 64.38% of the young osteocytes. Additionally, in all recovered osteocytes, 36% of the developmentally mature osteocytes comprised non-fitted model cells while 53.19% of the young osteocytes were the non-fitted model, which indicates the active transduction of dye molecules. However, there were no statistically significant differences between the young and developmentally mature osteocytes with regard to the diffusion coefficient, permeability coefficient, or permeance of the osteocyte processes, which were 3.93 ± 3.77 (× 10^− 8 cm²/s), 5.12 ± 4.56 (× 10^− 5 cm²/s) and 2.99 ± 2.47 (× 10^− 13 cm²/s) (mean ± SD), respectively. Conclusions These experiments comprehensively quantified the GJIC capacity in the embryonic chick calvaria and indicated that the cell-cell communication capacity of the osteocytes in the embryonic chick calvaria was related to their development.