Doctoral Dissertation Improvement Grant: The Effect of Interactions between Estradiol and Mechanical Loading on Human Longitudinal and Periosteal Bone Growth

这项研究测试了人类骨骼进化中两个有充分证据的趋势的假设机制：最近的人类不如早期的现代人类强壮，来自温暖气候的人类的四肢比来自寒冷气候的人类更长，更不强壮。一般的假设是肢体比例和骨骼健壮性之间存在关系，因为骨骼生长过程中骨骼长度和厚度的生长过程都受到骨骼生长过程中雌激素的影响。具体的预测是，个体之间雌激素水平的变化会改变1)雌激素受体的表达，从而影响骨骼对机械负荷（如运动）的反应；2)生长板中软骨细胞的活性，从而影响骨骼的生长长度。如果骨骼对机械负荷的反应程度在个体或人群中有所不同，那么人类骨骼健壮性的模式可能反映了负荷历史和雌激素等激素水平的变化。同样，如果雌激素水平和机械负荷之间的相互作用影响骨骼长度，那么肢体长度的模式可能反映了习惯性活动以及对气候的适应。区分这些选择对于改善我们对人类适应和行为的解释至关重要。试验期45天，选取24只羊（Ovis aries），分为低、正常和高雌激素处理组。每组一半人久坐不动，另一半人每天锻炼。荧光染料将标记骨骼生长。实时逆转录聚合酶链反应（rt - pcr）将定量测量雌激素受体-a的表达。增殖细胞核抗原（PCNA）免疫反应性是软骨细胞增殖的指标。初步实验结果支持了雌激素水平变化影响骨骺生长和骨干对负荷反应的假设，从而可能影响人类骨骼健壮性的模式。在一个小样本的幼年羊中，雌激素水平和运动的差异对股骨和胫骨的骨膜生长速度有显著和戏剧性的影响。例如，高雌激素运动动物的股骨平均每日附着率比低雌激素运动动物高44% (p.05)，比久坐、高雌激素运动动物高26% （p.05）。骨骺高度也随雌激素水平和运动而变化：高雌激素运动的动物跖骨生长板高度比久坐的动物高41% (p.025)，增殖软骨细胞多29% (p.05)，低雌激素运动的动物胫骨远端生长板高度比久坐的动物高25% （p.025）。本计划将促进体质人类学的研究生训练和研究与实验生理学和骨骼内分泌学的整合。该实验将解决一些关于雌激素和应变之间的相互作用如何影响纵向和骨膜骨生长的基本生物学问题。激素水平和环境刺激之间的相互作用对人类骨骼生长的具体影响从未被研究过，但与许多临床问题非常相关，包括达到峰值骨密度和预防骨质疏松症。因此，这项研究的结果将在生物医学和人类学期刊上传播。

This study tests a hypothesized mechanism for two well-documented trends in human skeletal evolution: that recent humans are less robust than earlier modern humans, and that humans from warm climates have longer, less robust limbs than humans from cold climates. The general hypothesis is that there is a relationship between limb proportions and skeletal robusticity, because the processes by which bones grow in length and in thickness are both influenced by the hormone estrogen during skeletal growth. The specific prediction is that variation in estrogen levels among individuals alters 1) the expression of the estrogen receptor, which affects how bones respond to mechanical loading (e.g. from exercise), and 2) the activity of cartilage cells in the growth plate, which affects how bones grow in length. If the extent to which bones respond to mechanical loading varies among individuals or populations, then patterns of human skeletal robusticity may reflect both loading history and changes in the levels of hormones such as estrogen. Similarly, if interactions between estrogen levels and mechanical loading affect bone length, then patterns of limb length may reflect habitual activity as well as adaptations to climate. Distinguishing among these alternatives is crucial for improving our interpretations of human adaptation and behavior.The 45-day experiment will use 24 sheep (Ovis aries), divided into low, normal and high estrogen treatment groups. Half of each group will be sedentary, and half will exercise daily. Fluorochrome dyes will label bone growth. Real-time reverse transcriptase polymerase chain reaction (rtPCR) will quantitatively measure estrogen receptor-a expression. Proliferating cell nuclear antigen (PCNA) immunoreactivity will measure cartilage cell proliferation. The preliminary experimental results support the hypothesis that variation in estrogen level influences epiphyseal growth and diaphyseal responses to loading, and thus may affect patterns of skeletal robusticity in humans. In a small sample of juvenile sheep, differences in estrogen level and exercise had significant and dramatic effects on periosteal growth rate in the femur and tibia. For example, mean daily apposition rate in the femora of exercised animals is 44% higher in high-estrogen than in low-estrogen animals (p.05), and 26% higher than in sedentary, high-estrogen animals (p.05). Epiphyseal height also varies with estrogen level and exercise: the height of the metatarsal growth plate is 41% greater (p.025), and there are 29% more proliferative chondrocytes (p.05), in high-estrogen exercised vs. sedentary animals, and the height of the distal tibia growth plate is 25% greater in low-estrogen exercised vs. sedentary animals (p.025). This project will promote the integration of graduate training and research in physical anthropology with experimental physiology and skeletal endocrinology. The experiments will address a number of basic biological questions about how interactions between estrogen and strain affect longitudinal and periosteal bone growth. The specific effects of interactions between hormone levels and environmental stimuli on human bone growth have never been studied, but are quite relevant to a number of clinical issues, including the attainment of peak bone density and prevention of osteoporosis. For this reason, the results of this study will be disseminated in biomedical as well as anthropological journals.