Localization of DLL1- and NICD-positive osteoblasts in cortical bone during postnatal growth in rats.

The long bone midshaft expands by forming primary osteons at the periosteal surface of cortical bone in humans and rodents. Osteoblastic bone formation in the vascular cavity in the center of primary osteons is delayed during cortical bone development. The mechanisms of the formation of primary osteons is not fully understood, however. Focusing on NOTCH1 signaling, an inhibitory signaling on osteoblastic bone formation, our immunohistochemical analysis revealed Delta like1 (DLL1), a ligand of NOTCH1, and the NOTCH1 intracellular domain (NICD, an activated form of NOTCH1) immunoreactivity, in the cuboidal osteoblasts lining the bone surface in the vascular cavity of primary osteons during postnatal growth in rats. Interestingly, five days after treatment of primary osteoblasts with ascorbic acid and ß glycerophosphate, protein levels of both DLL1 and NICD increased transiently, indicating that DLL1 activates NOTCH1 in primary cultured osteoblasts. Thus, the results imply that DLL1-NOTCH1 signaling in osteoblasts is associated with primary osteonal bone formation.

Biorhythms, deciduous enamel thickness, and primary bone growth: a test of the Havers-Halberg Oscillation hypothesis.

Across mammalian species, the periodicity with which enamel layers form (Retzius periodicity) in permanent teeth corresponds with average body mass and the pace of life history. According to the Havers-Halberg Oscillation hypothesis (HHO), Retzius periodicity (RP) is a manifestation of a biorhythm that is also expressed in lamellar bone. Potentially, these links provide a basis for investigating aspects of a species' biology from fossilized teeth. Here, we tested intra-specific predictions of this hypothesis on skeletal samples of human juveniles. We measured daily enamel growth increments to calculate RP in deciduous molars (n = 25). Correlations were sought between RP, molar average and relative enamel thickness (AET, RET), and the average amount of primary bone growth (n = 7) in humeri of age-matched juveniles. Results show a previously undescribed relationship between RP and enamel thickness. Reduced major axis regression reveals RP is significantly and positively correlated with AET and RET, and scales isometrically. The direction of the correlation was opposite to HHO predictions as currently understood for human adults. Juveniles with higher RPs and thicker enamel had increased primary bone formation, which suggests a coordinating biorhythm. However, the direction of the correspondence was, again, opposite to predictions. Next, we compared RP from deciduous molars with new data for permanent molars, and with previously published values. The lowermost RP of 4 and 5 days in deciduous enamel extends below the lowermost RP of 6 days in permanent enamel. A lowered range of RP values in deciduous enamel implies that the underlying biorhythm might change with age. Our results develop the intra-specific HHO hypothesis.