Transdifferentiation of pigmented multipotent epithelium during morphallactic development of budding tunicates.

In the budding tunicate, Polyandrocarpa misakiensis, the atrial epithelium is the major formative tissue giving rise to the pharynx, digestive tract, brain and endostyle of a bud. We show here that this multipotent epithelium carries several differentiation markers that are lost in the process of bud development. In both adult animals and growing buds, the atrial epithelium contained orange-pigmented granules in the cytoplasm. In developing buds, on the other hand, the cells committed to organ primordia have lost the granules, taken a cuboidal shape and have a large nucleus with a prominent nucleolus, like undifferentiated cells. The atrial epithelium was also characterized by ALP expressed on the apical surface of the cell. During budding the enzyme activity disappeared from the atrial epithelium and reappeared in the primordial digestive tract. Immunohistochemical studies suggested strongly that during gut formation, ALP antigens has been switched from the epithelial isoform to the intestinal isoform. These results have shown that in P. misakiensis budding involves transdifferentiation of multipotent, but differentiated epithelium, confirming our previous results (Fujiwara and Kawamura, Dev. Growth Differ. 34:463-472, 1992).