Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aobayama Aoba-ku, Sendai 980-8578, Japan.
Development. 2012 Aug;139(16):2916-25. doi: 10.1242/dev.075572. Epub 2012 Jul 12.
Fins and limbs, which are considered to be homologous paired vertebrate appendages, have obvious morphological differences that arise during development. One major difference in their development is that the AER (apical ectodermal ridge), which organizes fin/limb development, transitions into a different, elongated organizing structure in the fin bud, the AF (apical fold). Although the role of AER in limb development has been clarified in many studies, little is known about the role of AF in fin development. Here, we investigated AF-driven morphogenesis in the pectoral fin of zebrafish. After the AER-AF transition at ∼36 hours post-fertilization, the AF was identifiable distal to the circumferential blood vessel of the fin bud. Moreover, the AF was divisible into two regions: the proximal AF (pAF) and the distal AF (dAF). Removing the AF caused the AER and a new AF to re-form. Interestingly, repeatedly removing the AF led to excessive elongation of the fin mesenchyme, suggesting that prolonged exposure to AER signals results in elongation of mesenchyme region for endoskeleton. Removal of the dAF affected outgrowth of the pAF region, suggesting that dAF signals act on the pAF. We also found that the elongation of the AF was caused by morphological changes in ectodermal cells. Our results suggest that the timing of the AER-AF transition mediates the differences between fins and limbs, and that the acquisition of a mechanism to maintain the AER was a crucial evolutionary step in the development of tetrapod limbs.
鳍和肢被认为是同源的成对脊椎动物附肢，它们在发育过程中具有明显的形态差异。它们在发育过程中的一个主要区别是，在鳍芽中，组织鳍/肢发育的顶端外胚层嵴（AER）转变为不同的、伸长的组织结构，即顶端褶皱（AF）。尽管 AER 在肢体发育中的作用在许多研究中已经得到阐明，但关于 AF 在鳍发育中的作用知之甚少。在这里，我们研究了斑马鱼胸鳍的 AF 驱动的形态发生。在受精后约 36 小时的 AER-AF 转换后，AF 可在鳍芽的周向血管远端识别。此外，AF 可分为两个区域：近端 AF（pAF）和远端 AF（dAF）。去除 AF 会导致 AER 和新的 AF 重新形成。有趣的是，反复去除 AF 会导致鳍间充质过度伸长，这表明长时间暴露于 AER 信号会导致中胚层区域的延伸，以形成内骨骼。去除 dAF 会影响 pAF 区域的生长，这表明 dAF 信号作用于 pAF。我们还发现，AF 的伸长是由外胚层细胞形态变化引起的。我们的结果表明，AER-AF 转换的时间调节了鳍和肢之间的差异，并且获得维持 AER 的机制是四足动物肢发育的一个关键进化步骤。
