Zipping Dynamics and Turgor Pressure during Dorsal Closure in Early Drosophila Development

Abstract

Biomechanical processes engaged in morphogenesis require forces to shape multiple tissues in a three-dimensional pattern during metazoan development. Dorsal closure, an essential stage of Drosophila embryogenesis, serves as an in-vivo model system for cell sheet movements during development and wound healing. During closure two flanks of lateral epidermis approach to close an eye-shaped gap that is initially occupied by a transient amnioserosa tissue in the dorsal opening. Based on a two-dimensional approximation, the time-dependent geometry of the dorsal opening previously has been quantified by four biomechanical processes collectively including apical constriction of amnioserosa cells, tension due to an actomyosin-rich purse string within each leading edge, adhesive zipping at each corner (canthus) of the eye-shaped opening, and resistance due to the lateral epidermis1. To more fully understand dorsal closure, we have moved beyond the two-dimensional approximation and report here our three-dimensional investigation. We investigated embryos with GFP/RFP labeled DE-cadherin, myosin, and/or moesin (actin) using time-lapsed confocal microscopy. We observed zipping to be an unexpectedly and remarkably three-dimensional process. The amnioserosa was pushed below the two leading edges of lateral epidermis as they zipped at each canthus. Just prior to zipping, the leading edges slid over the amnioserosa towards the anteroposterior axis. In addition, during early-to-mid stages of closure we observed the amnioserosa in the geometry of a dome. Segmenting this asymmetric dome and fitting with Laplace's formula quantified the turgor pressure. Furthermore, the purse strings that define the dorsal opening were curved in three dimensions with significant bends towards the embryo interior near each canthus. This research has been supported by the NIH, grant No. 33830.1. Science 300:145-149 (2003).