Continued Growth and Cell Proliferation Into Adulthood in the Notochord of the AppendicularianOikopleura dioica

The appendicularian urochordate Oikopleura dioica retains a free-swimming chordate body plan throughout life, in contrast to ascidian urochordates, whose metamorphosis to a sessile adult form involves the loss of chordate structures such as the notochord and dorsal nerve cord. Development to adult stages in Oikopleura involves a lengthening of the tail and notochord and an elaboration of the repertoire of tail movements. To investigate the cellular basis for this lengthening, we have used confocal microscopy and BrdU labeling to examine the development of the Oikopleura notochord from hatching through adult stages. We show that as the notochord undergoes the typical urochordate transition from a stacked row of cells to a tubular structure, cell number begins to increase. Addition of new notochord cells continues into adulthood, multiplying the larval complement of 20 cells by about 8-fold by the third day of life. In parallel, the notochord lengthens by about 4-fold. BrdU incorporation and a cell-cycle marker confirm that notochord cells continue to proliferate well into adulthood. The extensive postlarval proliferation of notochord cells, together with their arrangement in four circumferentially distributed longitudinal rows, presumably provides the Oikopleura tail with the necessary mechanical support for the complex movements exhibited at adult stages.