Expression of matrix Gla protein and osteocalcin in the developing tibial epiphysis of mice

This study aimed to investigate the expression of matrix Gla protein (MGP) and osteocalcin (OCN) in the tibial epiphysis of developing mice. At 1, 2, 3, and 4 weeks after birth, tibiae were removed and processed for histochemical observations and western blot analyses under anesthesia. To evaluate bone volume, the specimens were scanned with Micro CT Scanner from the articular cartilage through the growth plate, along the long axis of tibia. At 1 week after birth, OCN reactivity was faint in the region of vascular invasion, while hardly any MGP reactivity was discernible. Subsequently, MGP reactivity was seen on the cartilaginous lacunar walls of hypertrophic chondrocytes, while OCN reactivity was evenly found not only in the bone matrix, but also in the cartilaginous lacunar walls and on the bone surfaces. Furthermore, double-immunostaining clearly showed that MGP reactivity appeared closer to the cartilage matrix than OCN reactivity until postnatal week 3. Interestingly, the immunoreactivities for MGP and OCN both showed tidemarks in the articular cartilage at postnatal week 4, and MGP reactivity was more intense than OCN reactivity. Statistical analyses showed an overall upward trend in MGP and OCN expression levels during tibial epiphysis development, even though OCN was more abundant than MGP at every time-point. Taken together, our findings suggest that the expression of MGP and OCN increased gradually in the murine developing tibial epiphysis, and the two mineral-associated proteins may occur at the same location during a particular period, but at different levels.