The shaping of the embryo during development is driven by large-scale movements of epithelial cell sheets as they bend, spread, and fuse with each other. One such major, well-defined morphogenetic event is dorsal closure, which occurs during the early stages of Drosophila embryogenesis. It involves the closing of an elliptical gap at the dorsal midline of the embryo by the convergence and fusion of epithelial cell sheets on either side of the embryo. The dorsal gap is underlined by the amnioserosal tissue that is made up of squamous epithelial cells and the lateral epithelial sheets contract and pull over the amnioserosal cells in a mechanism that closely resembles wound healing .
In live fly embryos, dorsal closure is observed to commence as soon as germ-band retraction starts. Germ-band retraction is the preceding event in Drosophila development in which, a germ-band that extended from the ventral and posterior side of the embryo to its dorsal side during the gastrulation stage, now retracts back to the posterior end of the embryo. This exposes the dorsal amnioserosa, which causes the anterior-most lateral epithelial leading edges to move in and seal regions of the dorsal gap. Following this, closure continues progressively from the anterior and posterior ends towards the center of the amnioserosa. The precise sequence of cellular events that take place during dorsal closure can be described in four stages, which are initiation, epithelial sweeping, zippering, and termination .
Initiation: Although the exact cellular events that occur during the onset of dorsal closure are not well known, a combination of signals from preceding morphogenetic events such as dorsoventral patterning and germ-band retraction, are known to trigger the initiating steps by activating the Jun N-terminal Kinase (JNK) signaling pathway. During the initial stages of dorsal closure, JNK activity is upregulated in the leading edge cells of the lateral epithelia while it is down regulated in the amnioserosal cells . At this stage, there is minimal internal tension in the lateral epithelial cells as they do not yet contain any actin-based contractile structures and the cells assume an irregular, scalloped shape. The leading edge is not clearly defined and it advances very slowly, with no net progression over the amnioserosa. However, the amnioserosal cells undergo active pulsatile contractions, which leads to a significant reduction in their apical surface area.
Epithelial sweeping: A short time after the initiation of dorsal closure, there is an accumulation of actin filaments at the leading edge of each cell of the epithelial cell sheet. The actin filaments organize into contractile units and these individual units are linked through intercellular junctions to form a continuous, supracellular acto-myosin cable that contracts and generates intrinsic tension . The leading edge cells stiffen under the influence of this tension and change their appearance from a scalloped shape to a tightly organized, regular row of cells. They also become polarized as they elongate along the dorsoventral axis and start advancing over the amnioserosa in sweeping movements. In conjunction with amnioserosal cell contractions, the sweeping movement of the lateral epithelia causes the dorsal gap to shrink in size significantly during this stage.
Zippering: Towards the end of the epithelial sweeping stage, the leading edge starts to put forth numerous filopodial protrusions, as well as a few lamellipodial protrusions. These actin-rich membranous protrusions facilitates establishment of contacts with the opposing leading edge, as soon as the leading edges from the two lateral epithelial sheets come close enough at the anterior and posterior ends of the gap. Once the protrusions touch one another, they seal together the remaining gap in a zippering mechanism to form a tight seam at the midline. In addition to the zippering function, the filopodia also play a key role as specialized sensors for pairing up the correct embryonic segments from both sides of the embryo. This ensures that the free lateral edges fuse in a coordinated manner and the general patterning within the epithelium is conserved. In mutant flies lacking Cdc42, a major activator of the assembly of actin-based protrusions, the opposing cells on the lateral edges failed to fuse properly and form a tight seam at the midline . Although zippering forces dominate at this stage, the contractility of the actin cable and the amnioserosal cells are still contributing to the final stages of dorsal closure .
Termination: Upon the establishment of contacts between the opposing leading edges via filopodial protrusions, certain ‘stop’ signals are activated that lead to the disassembly of actin-based structures and prevent further advancement of the leading edges. These signals also activate biochemical pathways that are essential for the reinforcement of the initial, temporary contacts between epithelial cells into permanent, tight adherens junctions. This mechanism recapitulates contact inhibition, and has been used as an ideal platform to gain insights into this fundamental biological process and in particular, its relevance in cancer progression . With the formation of tightly sealing adherens junctions, the process of dorsal gap closure is considered to be completed.
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