56 • Debra L. Martin 



skeletal tissue makes when under physiological stress. Gross 

 pathologic changes on bone reveal insults to which an indi- 

 vidual may have been subjected such as infectious disease, 

 trauma, and degenerative changes such as arthritis. Tradi- 

 tionally, the health status of prehistoric individuals was based 

 on these gross indicators alone. The analysis of bone his- 

 tology provides information on a more subtle level — 

 information concerning chronic or episodic undernutrition, 

 periods of physiological disruption which leave no trace on 

 the outer skeletal surfaces, the long-term effects of multi- 

 parity and lactation on female calcium metabolism, and the 

 effects of immobilization on skeletal health. 



In summary, analysis of physiological aspects of skeletal 

 remodeling provides a clue to an individual's lifestyle, diet, 

 reproductive activity, and nutritional adequacy. These in- 

 dicators of health are rarely "clinically" significant; they re- 

 flect the day-to-day physiological responses which individu- 

 als must make. It is these responses at the histological level 

 on which current research can focus. 



Bone biodynamics 



A general familiarity of normal bone histology, as well as the 

 normal processes involved with growth, development, main- 

 tenance and repair, is necessary in order to highlight and 

 understand the range of possible responses bone can make 

 when experiencing physiological stress. Bone is a highly 

 specialized kind of connective tissue and it is distinguished 

 from other tissues such as skin and cartilage by its hard and 

 crystalline structure. Bone has a cellular matrix composed of 

 collagen and protein fibers embedded in a ground substance 

 high in mucopolysaccharides. The hardness of bone comes 

 from crystalline salts of calcium, phosphate, and carbonate 

 deposited within the organic matrix. Specialized cells medi- 

 ate the deposition and withdrawal of the mineral component 

 of bone to keep an even balance between the body fluids and 

 the skeletal reserves (Raisz and Kream 1983). Bone exists in 

 a dynamic equilibrium with blood, and the "bone-body con- 

 tinuum" is regulated by nutrients and hormones (McLean and 

 Urist 1968). 



Much of the outward appearance, and most of the histo- 

 logical aspects of bone reflects the biological responses to 

 physical and structural requirements. The diverse set of func- 

 tions which a skeletal system provides (for example, struc- 

 tural support, locomotion, storage and regulation of miner- 

 als, control of ionic concentrations in body fluids, and 

 production of red blood cells) necessitates a high priority for 

 maintenance. Thus, as a connective tissue, bone is an open 

 and living system which changes constantly throughout life 

 to meet the demands for growth, development, maintenance 

 and repair. 



Disturbance in normal patterns of growth, mineralization, 

 and remodeling form the pathogenesis of nutritional, hormo- 

 nal, disease and aging problems (Jowscy 1964). These pro- 

 cesses can be measured, and the values can be used to define 



boundaries between health and disease. Quantification of 

 histological properties of bone aids in making diagnoses and 

 indicates the probabilities that the properties of an individual 

 case are within healthy or diseased states (Byers 1977). 



Osteons are discrete units of bone which are the major 

 quantifiable histological features used in diagnostic analy- 

 ses. In the femur, osteons measure approximately 0.25 mm 

 in diameter and are easily viewed under low or high magni- 

 fication. Osteons take a variable length of time to form. In a 

 ten-year-old individual osteons take 46 days to attain com- 

 pleteness and full mineralization. In a 60-year-old, osteons 

 can take as long as 108 days to attain completeness (Lips et 

 al. 1978). 



Both complete osteons and partial osteon fragments re- 

 main visible for many years, since at any given time only 3- 

 5% of the skeleton of an individual undergoes active re- 

 modeling during adulthood (Frost 1964). Osteons, resorption 

 spaces, fragments, and layers of bone without osteons 

 (lamellae) play a central and critical role in maintaining the 

 quality and quantity of bone. 



Once bone has been shaped by growth, it can be altered by 

 the remodeling process, or "turnover" of bone (Frost 1973). 

 There is activity of some degree in every part of the skeleton 

 throughout life. The two basic components of this process are 

 formation of osteons and resorption of older osteons. Re- 

 modeling is the resorption of older units of bone and forma- 

 tion of and replacement with newly mineralized bone. Re- 

 modeling can be viewed as quantitative changes in osteonal 

 size, degree of mineralization, and placement within the cor- 

 tex of bone. 



Resorption and formation are not independent phenome- 

 na; they are coupled. After each resorption activity, there is 

 always a formation activity although the rate is variable. The 

 cycle of resorption and subsequent formation can take from 

 three months to one year (Frost 1969). The formation/ 

 resorption ratio is a relative indicator of how many cycles are 

 in progress, but does not indicate at what point the cycles are, 

 or if the cycles are proceeding normally. 



Although remodeling continues at predictably different 

 rates in each decade of life, the consequence is an ever- 

 increasing population of osteons per unit volume of cortical 

 bone. Remodeling occurs slowly enough that previously re- 

 modeled bone remains unchanged for long periods of time. 

 The actual rate of remodeling can be measured by comparing 

 the number of older bone units with that of newer bone (Frost 

 and Villanueva 1960; Jowsey 1964) or by determining the 

 amount of bone laid down over the course of the life span 

 (Frost and Wu 1967). 



A healthy individual weighing 72 kg has approximately 14 

 kg of skeletal tissue. Of that 14kgofbone, half is composed 

 of calcium (Posncr 1979). Calcium is extremely important to 

 the biochemical constitution of skeletal tissue. Approxi- 

 mately 99% of the calcium ingested from food goes directly 

 to the skeletal tissue; only 1% of the calcium remains cir- 

 culating in the blood (Raisz 1977). The small percentage of 



Zagreb Paleopalholdity Symp. I91IH 



