The biorhythm of human juvenile skeletal growth

Biorhythms are cyclic changes in an organism's growth or functioning that can be driven by an internal biological clock and synchronised through environmental cues. Evidence of two biorhythms is retained in human tooth enamel in the form of incremental growth lines. The first biorhythm corresponds with a daily, circadian, rhythm that has been linked to the secretory activity of ameloblasts. The second biorhythm is a longer period infradian biorhythm that is marked by Retzius lines and can be quantified in days as Retzius periodicity (RP). There is some evidence to suggest that body size, basal metabolic rate, life history traits, and RP might be linked by a centrally coordinated autonomic biorhythm that is related to growth and development. Earlier studies have demonstrated links in adult humans between RP and body size, and between RP and aspects of cortical bone microstructure. If the biorhythm is related to the growth and development of the body, there should also be indications of this biorhythm retained in the juvenile skeleton. This thesis examines these links, for the first time, in skeletal samples of human children from one population. The aim of this thesis is to use 2D static histomorphometry to explore how, or if, evidence of an infradian biorhythm retained in tooth enamel as Retzius line periodicity is linked to the growth of human bone growth during ontogeny. In addition to the potential effect of a biorhythm on skeletal growth, many other factors may potentially affect bone microstructure. This thesis examines some of these influences prior to investigating the biorhythm. Variation in histomorphometric measures of cortical bone microstructure is sought between children of different regions in England (Canterbury, York, Newcastle), and between high- and low-status children from Canterbury. Additionally, bone microstructure varies between the bones in a single skeleton, so the intra-skeletal microstructural variation between eight bones from ten young adults was explored and compared to their RP. Finally, the potential for a relationship between RP and rib cortical bone microstructure is investigated. This thesis begins with a background review of the underlying biology of bone growth, enamel secretion, and biorhythms. The introductory sections are followed by a detailed materials and methods chapter. The investigative parts of the thesis are presented as three data chapters that have been designed as manuscripts that are suitable for publication. Each has a focus on a specific aim and has individual introduction, aims, methods, results, and discussion sections. Together, the three data chapters contain histomorphometric variables that were measured from the skeletal remains of 188 juvenile and young adults from four archaeological sites from medieval England. The RP of a permanent tooth from each skeleton was calculated and combined with measures of bone modelling - indicated by bone size variables - and remodelling - indicated by osteon population density, osteocyte lacunae density, and osteon size and shape. This thesis ends with a discussion of the main findings of each data chapter and how they contribute to the overall aim of the thesis, which is to assess evidence of the biorhythm in tooth enamel against aspects of growth in the juvenile skeleton. The results support the idea that an infradian biorhythm coordinates aspects of human hard tissue growth, particularly relative amounts of lamellar formation during modelling and remodelling. The biorhythm was related to the proportion of interstitial and osteonal lamellar bone, and to body size. These results imply that a child with a fast biorhythm tends to have a high proportion of lamellar bone and small Haversian canals in their osteons and a high proportion of bone in their ribs compared to the medullary cavities. Whereas, a child with a slow biorhythm tends to have a low proportion of lamellar bone and large Haversian canals in their osteons and a low proportion of bone in their ribs compared to the medullary cavities. The biorhythm did not relate to any measure of bone turnover, including osteon population density, intact osteon density, fragmentary osteon density, or osteocyte density. This biorhythm could be one of the many factors that affect macroscopic bone growth and microscopic bone modelling and remodelling during childhood growth in medieval Canterbury.