Dorsal closure is a tissue-modeling process in the developing Drosophila
embryo during which an epidermal opening is closed. It begins with the appearance of a supracellular
actin cable that surrounds the opening and provides a contractile force. Amnioserosa cells that fill the opening produce an additional critical force pulling on the surrounding epidermal tissue. We show that this force is not gradual but pulsed and occurs long before
dorsal closure starts. Quantitative analysis, combined with
laser cutting experiments and simulations, reveals that tension-based dynamics and cell coupling control the force
pulses. These constitutively pull the surrounding epidermal tissue dorsally, but the displacement is initially transient. It is translated into dorsal-ward movement only with the help of the
actin cable, which acts like a ratchet, counteracting ventral-ward epidermis relaxation after force
pulses. Our work uncovers a sophisticated mechanism of cooperative force generation between two major forces
driving morphogenesis.
