摘要：

It was previously discovered that tail fin rays of larval amphioxus are long ciliary rootlets in posterior epidermal cells. This work describes the heretofore unknown origin and fate of these organelles in the Florida amphioxus (Branchiostoma floridae). In late embryos, epidermal cells at the posterior end of the body increase in height, thus producing a tail fin. One ciliary rootlet in each cell elongates and also rotates through about 90°, soon becoming oriented parallel to the long axis of the cell and running continuously from the apical to the basal plasma membrane. During the subsequent growth of the larval tail, the rootlets and epidermal cells housing them reach lengths up to 120 μm. At metamorphosis, the rootlets become vacuolated and rapidly decrease in length along with the height of the tail epidermis. Contemporaneously, abundant extracellular dermal matrix accumulates in the sagittal plane of the body to produce a predominantly dermal tail fin. Throughout postmetamorphic life, the posterior epidermal cells, now without ciliary rootlets, thinly cover a largely dermal tail flange. Thus, the specialized morphology of the amphioxus tail fin is generated by two different cellular mechanisms, involving different cell populations (ectodermal and mesodermal), at different life-history stages.

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