Anteroposterior patterning of the epidermis by inductive influences from the vegetal hemisphere cells in the ascidian embryo

Patterning along the anteroposterior axis is a critical step during animal embryogenesis, Although mechanisms of anteroposterior patterning in the neu ral tube have been studied in various chordates, little is known about thos e of the epidermis, To approach this issue, we investigated patterning mech anisms of the epidermis in the ascidian embryo. First we examined expressio n of homeobox genes (Hrdll-1, Hroth, HI Hox-l and Hrcad) in the epidermis, Hrdll-1 is expressed in the anterior tip of the epidermis that later forms the adhesive papillae, while Hroth is expressed in the anterior part of the trunk epidermis, HrHox-1 and Hrcad are expressed in middle and posterior p arts of the epidermis, respectively. These data suggested that the epidermi s of the ascidian embryo is patterned anteroposteriorly, In ascidian embryogenesis, the epidermis is exclusively derived from animal hemisphere cells. To investigate regulation of expression of the four home obox genes in the epidermis by vegetal hemisphere cells, we next performed hemisphere isolation and cell ablation experiments. We showed that removal of the vegetal cells before the late 16-cell stage results in loss of expre ssion of these homeobox genes in the animal hemisphere cells. Expression of Hrdll-1 and Hroth depends on contact with the anterior-vegetal (the A-line ) cells, while expression of HrHox-1 and Hrcad requires contact with the po sterior-vegetal (the B-line) cells, We also demonstrated that contact with the vegetal cells until the late 32-cell stage is sufficient for animal cel ls to express Hrdll-1, Hroth and Hrcad, while longer contact is necessary f or HrHox-1 expression. Contact with the A-line cells until the late 32-cell stage is also sufficient for formation of the adhesive papillae, Our data indicate that the epidermis of the ascidian embryo is patterned al ong the anteroposterior axis by multiple inductive influences from the vege tal hemisphere cells and provide the first insight into mechanisms of epide rmis patterning in the chordate embryos.