Koyama E, Leatherman J L, Noji S, Pacifici M
Department of Anatomy and Histology, School of Dental Medicine, University of Pennsylvania, Philadelphia 19104-6003, USA.
Dev Dyn. 1996 Nov;207(3):344-54. doi: 10.1002/(SICI)1097-0177(199611)207:3<344::AID-AJA11>3.0.CO;2-4.
Skeletal patterning and morphogenesis in the developing limb are thought to be regulated by instructive factors and cues from the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the dorsal ectoderm. However, the activities of the ZPA and AER dwindle early in embryogenesis and soon after ceases, when in fact the proximal skeletal elements are still rudimentary in structure and the more distal ones are yet to become recognizable. Thus, we asked whether the chondrocytes emerging within each mesenchymal condensation may themselves start expressing properties similar to those of ZPA and/or AER and, in so doing, may bring skeletal development to completion. Indeed, we found that the cartilaginous, but not precartilaginous, tissues in early chick limbs possess ZPA-like properties. They expressed an endogenous factor related to Sonic hedgehog (Shh), most likely Indian hedgehog (Ihh), and when fragments were grafted to the anterior margin of host stage 16-20 chick wing buds, they induced supernumerary skeletal elements (polarizing activity). The acquisition of polarizing activity by the cartilaginous structures followed clear proximo-to-distal and posterior-to-anterior routes. Thus, (1) stage 25 cartilaginous humerus had polarizing activity while stage 25 prospective radius did not, (2) posteriorly-located stage 29 ulna had stronger activity than anteriorly-located stage 29 radius, and (3) ulna's diaphysis had stronger activity at stage 29 than 31 while radius's diaphysis was stronger at stage 31 than 29. Prior to inducing extra digit formation, the cartilaginous grafts induced Hoxd-12 and Hoxd-13 gene expression in adjacent competent mesenchymal tissue. Strikingly, the cartilaginous grafts activity also expression of Shh and polarizing activity in adjacent mesenchyme, which ZPA grafts cannot do; thus, the cartilaginous structures displayed activities "upstream" of those of the ZPA. The results support our hypothesis that chondrocytes may themselves direct skeletal morphogenesis. In so doing and as a result of their inductive activities, the cells may also have an important role in the completion of limb patterning and morphogenesis.
发育中的肢体的骨骼模式形成和形态发生被认为受来自极化活性区（ZPA）、顶端外胚层嵴（AER）和背侧外胚层的诱导因子和信号调控。然而，ZPA和AER的活性在胚胎发育早期就开始减弱并很快停止，而此时近端骨骼元件在结构上仍处于雏形，更远端的元件甚至尚未可辨。因此，我们提出疑问，每个间充质凝聚中出现的软骨细胞自身是否可能开始表达与ZPA和/或AER相似的特性，并以此使骨骼发育完成。事实上，我们发现早期鸡胚肢体中的软骨组织而非前软骨组织具有类似ZPA的特性。它们表达一种与音猬因子（Shh）相关的内源性因子，很可能是印度刺猬因子（Ihh），当将其片段移植到宿主16 - 20期鸡胚翼芽的前缘时，会诱导额外的骨骼元件形成（极化活性）。软骨结构获得极化活性遵循明确的从近端到远端以及从后部到前部的路径。因此，（1）25期软骨肱骨具有极化活性，而25期预期的桡骨则没有；（2）29期位于后部的尺骨比位于前部的29期桡骨具有更强的活性；（3）尺骨干在29期比31期具有更强的活性，而桡骨干在31期比29期更强。在诱导额外指形成之前，软骨移植物在相邻的感受态间充质组织中诱导Hoxd - 12和Hoxd - 13基因表达。令人惊讶的是，软骨移植物的活性还能在相邻间充质中诱导Shh表达和极化活性，而ZPA移植物则不能；因此，软骨结构展现出在ZPA的“上游”活性。这些结果支持了我们的假设，即软骨细胞自身可能直接指导骨骼形态发生。在此过程中，由于它们的诱导活性，这些细胞在肢体模式形成和形态发生的完成中可能也起着重要作用。
