Signaling Molecules and Long Bone Formation

长骨是形态复杂的结构。当它们在胚胎中开始形成时，它们完全是软骨。软骨细胞随后经历成熟，包括增生、前肥大和增厚阶段。增生软骨一旦形成，就会被软骨周围细胞侵入，并被软骨内骨所取代。软骨周细胞也在肥大前软骨和肥大软骨周围产生膜内骨颈，它决定骨轴的大小和形状。因此，长骨的形成包括一个多步骤的软骨细胞成熟过程、膜内和软骨内成骨以及软骨和软骨周围组织的地形相关事件。目前还不清楚所有这些过程和事件是如何被调控的。印度刺猬（IHH）和类维生素a是强有力的信号分子。本小组和其他小组的研究表明，IHH是前增生性软骨细胞的产物，影响软骨细胞增殖和膜内骨颈形成，并抑制肥大。另一方面，类维生素a存在于肥大软骨和周围的软骨膜组织中，刺激肥大和软骨内骨形成。这些和许多其他数据表明，IHH和类维甲酸是调节软骨细胞成熟和协调软骨和软膜组织事件的信号分子。我们的主要假设是：(A) IHH是软骨细胞增殖和膜内项圈形成的直接诱导剂，是软骨细胞肥大的抑制剂；(B)类维生素a关闭IHH表达，刺激软骨细胞肥大，并允许软骨内成骨。该项目旨在确定IHH和类维生素a如何发挥这些不同但相互关联的作用，从而使长骨发育正常进行。我们将使用鸟类和小鼠动物、显微外科肢体操作和细胞培养来分析：(a) IHH和类视黄醇作用的受体和核介质的表达和功能；(b)类维生素a的合成、降解和递送途径；(c)异位蛋白表达的影响。我们还将确定上述机制的异常是否导致了IHH-缺失小鼠的长骨缺陷。研究结果将为长骨形成的调控提供急需的信息，并将与骨骼生长、骨折修复和软骨和骨的先天性疾病有关。

Long bones are morphologically complex structures. When they begin to form in the embryo, they are entirely cartilaginous. The chondrocytes then undergo maturation, involving a proliferative, prehypertrophic and hypertrophic phase. Once formed, hypertrophic cartilage is invaded by perichondrial cells and replaced by endochondral bone. Perichondrial cells produce also an intramembranous bone collar around prehypertrophic and hypertrophic cartilage that determines size and shape of the shaft. Thus, long bone formation involves a multi-step chondrocyte maturation process, intramembranous and endochondral ossification, and topographically related events in cartilage and perichondrial tissues. It is far from clear how all these processes and events are regulated. Indian hedgehog (IHH) and retinoids are powerful signaling molecules. Studies from this and other groups have suggested that IHH, a product of prehypertrophic chondrocytes, influences chondrocyte proliferation and intramembranous bone collar formation and inhibits hypertrophy. On the other hand, retinoids which are present in hypertrophic cartilage and surrounding perichondrial tissue, stimulate hypertrophy and endochondral bone formation. These and many additional data indicate that IHH and retinoids are signaling molecules regulating steps in chondrocyte maturation and coordinating events in cartilage and perichondrial tissues. Our central hypotheses are: (A) IHH is a direct inducer of chondrocyte proliferation and intramembranous collar formation, and an inhibitor of chondrocyte hypertrophy; and (B) Retinoids shut off IHH expression, stimulate chondrocyte hypertrophy, and permit endochondral ossification. This project aims to determine how IHH and retinoids exert these distinct but interrelated roles and thus allow normal progression of long bone development. We will use avian and murine animals, microsurgical limb manipulations, and cell cultures to analyze: (a) expression and function of receptors and nuclear mediators of IHH and retinoid action; (b) pathways of synthesis, degradation and delivery of retinoids; and (c) effects of ectopic protein expression. We will also determine whether abnormalities in above mechanisms underlie the defects in long bones seen in IHH- null mice. The results will provide much needed information on the regulation of long bone formation, and will have relevance to questions of skeletal growth, fracture repair and congenital conditions of cartilage and bone.