CYTOPLASMIC FILAMENTS AND MORPHOGENESIS - ROLE OF NOTOCHORD IN ASCIDIAN METAMORPHOSIS

The notochord of Boltenia villosa is a tubular structure composed of squamous epithelioid cells and matrix material surrounded by a filamentous sheath. It acts asa skeleton in the larval tail. During resorption of the tail the notochordal cells shorten, dissociate and move along with the matrix through a newly formed breach in the anterior end of the notochordal sheath into the body cavity of the trunk. As the notochordal cells shorten they develop folds at their basal surfaces. These folds protrude into complementary folds in the notochordal sheath. The development of the folds is associated with the appearance of a thick layer of cytoplasmic filaments just beneath the plasmalemma of the basal surface. Most of these filaments are oriented in the axis of shortening. A new hypothesis is proposed to explain the resorption of the tail in this species: The notochordal cells are contractile and actively compress the notochordal sheath. The muscle cells, nerve cord, endodermal strand and epidermis collapse with the loss of the skeleton and are compressed by the notochord. The function of the notochord during resorption of the tail in Boltenia villosa is regarded as analogous to the function of the contractile epidermis in the compound ascidians Amaroucium constellatum, Distaplia occidentalis and Diplosoma macdonaldi. The oriented filaments and apical protrusions characteristic of the contractile epidermal cells of these ascidians were not found in the epidermis of Boltenia villosa. © 1969 Springer-Verlag.