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据可查的趋势的假设机制：近代人类比早期现代人更不健壮，来自温暖气候的人类比来自寒冷气候的人类有更长，更不健壮的四肢。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的长度和厚度都受到雌激素的影响。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，这会影响骨骼对机械负荷（例如运动）的反应，以及2）生长板中软骨细胞的活性，这会影响骨骼的长度。 如果骨骼对机械负荷的反应程度在个体或群体之间存在差异，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。区分这些替代品对于改善我们对人类适应和行为的解释至关重要。45天的实验将使用24只绵羊（Ovis aries），分为低，正常和高雌激素治疗组。每组中有一半人将久坐不动，另一半人将每天锻炼。荧光染料将标记骨生长。实时逆转录聚合酶链反应（rtPCR）将定量测量雌激素受体-a的表达。增殖细胞核抗原（PCNA）免疫反应性将测量软骨细胞增殖。初步的实验结果支持这一假设，即雌激素水平的变化影响骨骺生长和骨干负荷的反应，从而可能会影响人类骨骼健壮性的模式。在一个小样本的少年羊，雌激素水平和运动的差异有显着和戏剧性的影响股骨和胫骨骨膜生长速度。例如，运动动物股骨中的平均每日沉积率在高雌激素动物中比在低雌激素动物中高44%（p.05），并且比久坐的高雌激素动物高26%（p.05）。骨骺高度也随雌激素水平和运动而变化：在高雌激素运动的动物中，跖骨生长板的高度比久坐的动物高41%（p.025），增殖性软骨细胞多29%（p.05），在低雌激素运动的动物中，胫骨远端生长板的高度比久坐的动物高25%（p.025）。 该项目将促进体质人类学与实验生理学和骨骼内分泌学的研究生培训和研究的整合。这些实验将解决一些基本的生物学问题，即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。激素水平和环境刺激之间的相互作用对人类骨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。出于这个原因，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.