Branched structures are found in many natural settings, and the molecular and cellular mechanisms underlying their formation in animal development have extensively studied in recent years. Despite their importance and the accumulated knowledge from studies on several organs of Drosophila and mammals, much remains unknown about branching mechanisms in other animal species. We chose to study the jellyfish species Cladonema pacificum. Unlike many other jellyfish, this species has branched medusa tentacles, and its basal phylogenetic position in animal evolution makes it an ideal organism for studying and understanding branching morphogenesis more broadly. Branched tentacles are unique compared to other well-studied branched structures in that they have two functionally distinct identities: one with adhesive organs for attaching to a substratum, and another with nematocyst clusters for capturing prey. We began our analyses on C. pacificum tentacles by observing their branching during growth. We found that tentacle branches form through repeated addition of new branches to the proximal region of the main tentacle while it is elongating. At the site of branch bud formation, we observed apical thickening of the epidermal epithelial layer, possibly caused by extension of the epithelial cells along the apico-basal axis. Interestingly, tentacle branch formation required receptor tyrosine kinase signaling, which is an essential factor for branching morphogenesis in Drosophila and mammals. We also found that new branches form adhesive organs first, and then are transformed into branches with nematocyst clusters as they develop. These results highlight unique features in branch generation in C. pacificum medusa tentacles and illuminate conserved and fundamental mechanisms by which branched structures are created across a variety of animal species.

中文翻译：

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水母 Cladonema pacificum (Hydrozoa, Cnidaria) 中水母触须的分枝模式和形态发生

在许多自然环境中都发现了分支结构，并且近年来对它们在动物发育中形成的分子和细胞机制进行了广泛的研究。尽管它们很重要，并且从对果蝇和哺乳动物的几个器官的研究中积累了知识，但对于其他动物物种的分支机制仍有很多未知之处。我们选择研究水母物种 Cladonema pacificum。与许多其他水母不同，该物种具有分枝的水母触须，其在动物进化中的基础系统发育位置使其成为更广泛地研究和理解分枝形态发生的理想生物。与其他经过充分研究的分支结构相比，分支触手是独一无二的，因为它们具有两种功能上不同的特性：一种具有用于附着在基质上的粘合器官，另一个带有用于捕获猎物的刺丝囊簇。我们通过观察它们在生长过程中的分枝来开始对 C. pacificum 触手的分析。我们发现，触手分支是通过在主触手伸长的近端区域反复添加新分支而形成的。在分支芽形成部位，我们观察到表皮上皮层的顶端增厚，这可能是由于上皮细胞沿顶基轴延伸所致。有趣的是，触手分支的形成需要受体酪氨酸激酶信号传导，这是果蝇和哺乳动物分支形态发生的重要因素。我们还发现，新的分支首先形成粘附器官，然后随着它们的发育转变为带有刺胞簇的分支。这些结果突出了 C 中分支生成的独特特征。