2 DISTINCT CELL-TYPES IDENTIFIED IN THE ASCIDIAN NOTOCHORD

We established a monoclonal antibody, designated CiNot-1,that recogniz es ascidian Ciona notochordal cells in the early tailbud embryo. It di stinguished two types of cells in the notochord, according to whether or not they expressed CiNot-1 antigen. Only A-line notochord cells exp ressed CiNot-1 antigen in cleavage-arrested embryos. Since A- and the B-line notochord precursor cells respond differently to inductive sign als, CiNot-1 distinguished two specification mechanisms in the notocho rd. After the middle tailbud stage, CiNot-1 recognized the nerve cord and endodermal strand in addition to the notochord. In these tissues, several cells in the distal tip of the tail did not express CiNot-1 an tigen. This revealed the distinct feature of unity in the distal tip o f the larval tail and suggests the specification mechanism of this reg ion.