Regulation Of Skeletal Growth

在哺乳动物中，长骨的生长发生在生长板上，生长板是一种软骨结构，包含三个主要层，即静止区、增殖区和肥大区。休息区的功能还不清楚。我们通过手术切除了兔远端尺骨生长板的增生区和肥大区，只留下静止区。在一周内，一个完整的增生和肥大区往往再生。接下来，我们通过外科手术操作生长板，将静止区软骨异位地放置在增殖柱旁边。异位静止区软骨诱导附近的增殖区软骨细胞的方向发生90度移位，并抑制其肥大分化。我们的研究结果表明，静止区软骨在生长板的软骨内骨形成中至少起三个重要作用：（1）它含有产生增殖软骨细胞克隆的干细胞样细胞，（2）它产生生长板定向因子，一种引导增殖克隆排列成平行于骨长轴的柱状的形态发生素，和（3）它产生抑制附近增殖区软骨细胞的终末分化的形态发生素，因此负责生长板组织成不同的增殖和肥大区。 随着年龄的增长，哺乳动物的生长板经历结构和功能的衰老变化，导致线性生长速率下降。在一些哺乳动物中，包括人类，生长板最终在性成熟时被骨骼取代。这个过程称为骨骺融合，关键取决于雌激素。雌激素引起骨骺闭合的机制尚不清楚。使用体内模型，我们发现雌激素加速生长板结构和功能的正常衰老衰退，包括胫骨生长速率、软骨细胞增殖速率、生长板高度、增殖软骨细胞数量、肥大软骨细胞数量、终末肥大软骨细胞大小和柱密度的下降。在衰老的生长板中，观察到骨骺融合是一个突然的事件，其中所有剩余的软骨细胞迅速被骨元素取代。当软骨细胞增殖率接近于零时发生融合。雌激素导致这种增殖性衰竭和融合更早发生。我们的数据表明：1）当生长板软骨细胞的增殖潜能耗尽时，骨骺融合被触发; 2）雌激素不直接诱导生长板骨化;相反，雌激素加速生长板的程序性衰老，从而导致更早的增殖耗尽，从而导致更早的融合。 我们也在进行一项阿仑膦酸钠治疗特发性青少年骨质疏松症的随机试验。在本研究的一个分支中，我们评估了可能纳入试验的患者诊断的准确性。我们发现，大多数被诊断为骨质疏松症的儿童实际上具有正常的骨密度。这种惊人的过度诊断率是由于双能X线吸收测定法（DEXA）扫描的解释错误。最常见的错误是使用T评分（与年轻人相比的标准差评分）来诊断尚未达到峰值骨量的儿童的骨质疏松症。这种过度诊断可能会导致不必要的额外检查，孩子和父母的焦虑，身体活动的限制和医疗干预。 我们还完成了一项随机双盲安慰剂对照试验，在非生长激素缺乏性极矮身材儿童中进行生长激素治疗。本研究的数据分析表明，与以前基于非随机试验的报告相反，生长激素治疗不会改变青春期的时间或速度。

In mammals, growth of long bones occurs at the growth plate, a cartilage structure that contains three principal layers, the resting, proliferative, and hypertrophic zones. The function of the resting zone is not well understood. We surgically removed the proliferative and hypertrophic zones from the rabbit distal ulnar growth plate, leaving only the resting zone. Within one week, a complete proliferative and hypertrophic zone often regenerated. Next, we surgically manipulated growth plates to place resting zone cartilage ectopically alongside the proliferative columns. Ectopic resting zone cartilage induced a 90-degree shift in the orientation of nearby proliferative zone chondrocytes and inhibited their hypertrophic differentiation. Our findings suggest that resting zone cartilage serves at least three important roles in endochondral bone formation at the growth plate: (1) it contains stem-like cells that give rise to clones of proliferative chondrocytes, (2) it produces a growth plate-orienting factor, a morphogen that directs the alignment of the proliferative clones into columns parallel to the long axis of the bone, and (3) it produces a morphogen that inhibits terminal differentiation of nearby proliferative zone chondrocytes and thus is responsible for the organization of the growth plate into distinct zones of proliferation and hypertrophy. With age, the mammalian growth plate undergoes structural and functional senescent changes which cause a decrease in linear growth rate. In some mammals, including humans, the growth plates are eventually replaced by bone at the time of sexual maturation. This process, termed epiphyseal fusion, depends critically on estrogen. The mechanism by which estrogen causes epiphyseal closure is not known. Using an in vivo model, we found that estrogen accelerated the normal senescent decline in growth plate structure and function, including the decline in tibial growth rate, rate of chondrocyte proliferation, growth plate height, number of proliferative chondrocytes, number of hypertrophic chondrocytes, size of terminal hypertrophic chondrocytes, and column density. In senescent growth plates, epiphyseal fusion was observed to be an abrupt event in which all remaining chondrocytes were rapidly replaced by bone elements. Fusion occurred when the rate of chondrocyte proliferation approached zero. Estrogen caused this proliferative exhaustion and fusion to occur earlier. Our data suggest that 1) Epiphyseal fusion is triggered when the proliferative potential of growth plate chondrocytes is exhausted; 2) Estrogen does not induce growth plate ossification directly; instead, estrogen accelerates the programmed senescence of the growth plate, thus causing earlier proliferative exhaustion and consequently earlier fusion. We are also conducting a randomized trial of alendronate for the treatment of idiopathic juvenile osteoporosis. In an offshoot of this study, we assessed the accuracy of diagnosis in patients referred for possible inclusion in the trial. We found that the majority of children referred with a diagnosis of osteoporosis, in fact, have a normal bone mineral density. This alarming rate of overdiagnosis was due to errors in the interpretation of the dual-energy x-ray absorptiometry (DEXA) scan. The most frequent error was the use of T-score (standard deviation score compared to young adults) to diagnose osteoporosis in children who had not yet achieved peak bone mass. This overdiagnosis can precipitate unnecessary additional testing, anxiety on the part of the child and parents, restriction of physical activity, and medical intervention. We are also completing a randomized double-blind placebo-controlled trial of growth hormone therapy in children with non-growth hormone-deficient extreme short stature. Analysis of data from this study indicate that, contrary to previous reports based on nonrandomized trials, growth hormone therapy does not alter the timing or pace of puberty.