Sato Y, Terakado K, Morisawa M
Misaki Marine-Biological Station, Graduate School of Science, University of Tokyo, Kanagawa, Japan.
Dev Growth Differ. 1997 Feb;39(1):117-26. doi: 10.1046/j.1440-169x.1997.00013.x.
Morphological changes in the tunic layers and migration of the test cells during swimming period in the larva of the ascidian, Ciona intestinalis, were observed by light and electron microscopy. The swimming period was divided into three stages. In stage 1, further formation of juvenile tunic layer started only in the larval trunk and neck region. In stage 2, the layer became swollen in the ventral and dorsal sides of the neck region and in stage 3, the swelling expanded backward. Concomitantly with these changes, the outermost larval tunic layer (outer cuticular layer), which had been formed before hatching, also swelled in the neck region in stage 2 and formed two humps in stage 3, although the layer did not change in the tail region during the swimming period. Test cells that were present over the entire larval tunic layer in stage 1 began to move from the surface of the fin toward that of the side of the body in stage 2, and finally gathered to form six bands running radially from the anterior end to the posterior end of the trunk region and aligned along the lateral sides of body in the tail region in stage 3. In electron microscopic observations, pseudopodia protruding from the test cells invaded the larval tunic, following which they extended proximate to the juvenile tunic in the trunk region. In the tail region, which had no juvenile tunic layer as that described, the pseudopodia invaded and remained adjacent to the surface of the epidermis or the sensory cilia protruded from the epidermis. Metamorphosis of the larvae, further tunic formation, degradation of adhesive papilla, attachment of larva to the substratum and tail resorption commenced after these morphological changes occurred. The possible role of the test cells in metamorphosis is discussed.
通过光学显微镜和电子显微镜观察了海鞘（Ciona intestinalis）幼虫游泳期外套层的形态变化以及测试细胞的迁移情况。游泳期分为三个阶段。在第一阶段，幼年外套层仅在幼虫躯干和颈部区域进一步形成。在第二阶段，该层在颈部区域的腹侧和背侧肿胀，在第三阶段，肿胀向后扩展。伴随着这些变化，孵化前形成的最外层幼虫外套层（外角质层）在第二阶段也在颈部区域肿胀，并在第三阶段形成两个隆起，尽管在游泳期该层在尾部区域没有变化。在第一阶段覆盖整个幼虫外套层的测试细胞在第二阶段开始从鳍的表面向身体侧面移动，最终在第三阶段聚集形成六条从躯干区域前端到后端径向延伸并沿尾部区域身体两侧排列的带。在电子显微镜观察中，从测试细胞伸出的伪足侵入幼虫外套，随后它们在躯干区域靠近幼年外套延伸。在没有上述幼年外套层的尾部区域，伪足侵入并与表皮表面相邻或从表皮伸出的感觉纤毛相邻。在这些形态变化发生后，幼虫开始变态、进一步形成外套、粘附乳头退化、幼虫附着在基质上以及尾部吸收。讨论了测试细胞在变态中的可能作用。
