Dipartimento di Biologia, Università degli Studi di Padova, I-35121 Padova, Italy.
J Comp Neurol. 2010 Feb 15;518(4):547-66. doi: 10.1002/cne.22222.
During the metamorphosis of tunicate ascidians, the swimming larva uses its three anterior papillae to detect the substrate for settlement, reabsorbs its chordate-like tail, and becomes a sessile oozooid. In view of the crucial role played by the anterior structures and their nerve relations, we applied electron microscopy and immunocytochemistry to study the larva of the colonial ascidian Botryllus schlosseri, following differentiation of the anterior epidermis during late embryogenesis, the larval stage, and the onset of metamorphosis. Rudiments of the papillae appear in the early tail-bud stage as ectodermic protrusions, the apexes of which differentiate into central and peripheral bipolar neurons. Axons fasciculate into two nerves direct to the brain. Distally, the long, rod-like dendritic terminations extend during the larval stage, becoming exposed to sea water. After the larva selects and adheres to the substrate, these neurons retract and regress. Adjacent to the papillae, other scattered neurons insinuate dendrites into the tunic and form the net of rostral trunk epidermal neurons (RTENs) which fasciculate together with the papillary neurons. Our data indicate that the papillae are simple and coniform, the papillary neurons are mechanoreceptors, and the RTENs are chemoreceptors. The interpapillary epidermal area, by means of an apocrine secretion, provides sticky material for temporary adhesion of the larva to the substrate.
在被囊动物海鞘的变态过程中，游泳幼虫利用其三个前乳突来探测附着基质，然后再吸收其脊索动物样的尾巴，变成固着的幼体。鉴于前部结构及其神经关系的重要作用，我们应用电子显微镜和免疫细胞化学方法，对群体海鞘 Botryllus schlosseri 的幼虫进行了研究，该幼虫经历了胚胎后期、幼虫期和变态起始期的前表皮分化。乳突原基在早期的尾芽期作为外胚层突起出现，其尖端分化为中央和外周双极神经元。轴突聚集形成两条神经直接通向大脑。在幼虫期，长而杆状的树突末端延伸，暴露于海水中。幼虫选择并附着于基质后，这些神经元会缩回和退化。在乳突旁边，其他分散的神经元将树突突入被囊中，并形成头胸部表皮神经元网络（RTEN），与乳突神经元一起聚集。我们的数据表明，乳突是简单的圆锥形，乳突神经元是机械感受器，而 RTEN 是化学感受器。通过顶浆分泌，乳突之间的表皮区域为幼虫暂时附着于基质提供粘性物质。
