Dorsal Closure Research Articles

Bilateral axillary skin fold flaps used for dorsal thoracic skin wound closure in a dog : clinical communication

A 10-year-old greyhound-cross dog was presented with a large, chronic skin wound extending over the interscapular region. The substantial skin defect was closed by making use of bilateral axillary skin fold flaps. It was possible to elevate the 2 skin flaps sufficiently to allow them to meet at the dorsal midline and thus facilitate complete closure of a large and awkwardly positioned wound. Small dorsal areas of the skin flaps underwent necrosis, but the resulting defects were closed without difficulty in a subsequent procedure. To the author's knowledge, this is the 1st clinical report of the use of bilateral axillary skin fold flaps in this fashion and describes an additional use of a versatile skin flap procedure.

A Developmentally Regulated Two-Step Process Generates a Noncentrosomal Microtubule Network in Drosophila Tracheal Cells

Microtubules (MTs) are essential for many cell features, such as polarity, motility, shape, and vesicle trafficking. Therefore, in a multicellular organism, their organization differs between cell types and during development; however, the control of this process remains elusive. Here, we show that during Drosophila tracheal morphogenesis, MT reorganization is coupled to relocalization of the microtubule organizing centers (MTOC) components from the centrosome to the apical cell domain from where MTs then grow. We reveal that this process is controlled by the trachealess patterning gene in a two-step mechanism. MTOC components are first released from the centrosome by the activity of the MT-severing protein Spastin, and then anchored apically through the transmembrane protein Piopio. We further show that these changes are essential for tracheal development, thus stressing the functional relevance of MT reorganization for morphogenesis.

The PAR complex regulates pulsed actomyosin contractions during amnioserosa apical constriction in Drosophila

Apical constriction is a major mechanism underlying tissue internalization during development. This cell constriction typically requires actomyosin contractility. Thus, understanding apical constriction requires characterization of the mechanics and regulation of actomyosin assemblies. We have analyzed the relationship between myosin and the polarity regulators Par-6, aPKC and Bazooka (Par-3) (the PAR complex) during amnioserosa apical constriction at Drosophila dorsal closure. The PAR complex and myosin accumulate at the apical surface domain of amnioserosa cells at dorsal closure, the PAR complex forming a patch of puncta and myosin forming an associated network. Genetic interactions indicate that the PAR complex supports myosin activity during dorsal closure, as well as during other steps of embryogenesis. We find that actomyosin contractility in amnioserosa cells is based on the repeated assembly and disassembly of apical actomyosin networks, with each assembly event driving constriction of the apical domain. As the networks assemble they translocate across the apical patch of PAR proteins, which persist at the apical domain. Through loss- and gain-of-function studies, we find that different PAR complex components regulate distinct phases of the actomyosin assembly/disassembly cycle: Bazooka promotes the duration of actomyosin pulses and Par-6/aPKC promotes the lull time between pulses. These results identify the mechanics of actomyosin contractility that drive amnioserosa apical constriction and how specific steps of the contractile mechanism are regulated by the PAR complex.

Rho-family small GTPases are required for cell polarization and directional sensing in Drosophila wound healing

A wound induces cell polarization, in which myosin II is localized at the rear end of individual cells in a migrating epithelial sheet of the Drosophila larval epidermis. Here, we use myosin localization to demonstrate that Rac1, Cdc42, and Rho1 are each required for cell polarization and directional sensing of the wound. The three GTPases are also required for actin cable formation at the wound leading edge. Rac1, Cdc42, and Rho1 act upstream of c-Jun N-terminal kinase (JNK) to organize actin assembly. These results highlight the similarities between the molecular mechanism of Drosophila wound healing and those of Drosophila embryonic dorsal closure and the chemotactic response of Dictyostelium and leukocytes.

Tusk-bearing beaked whales from the Miocene of Peru: sexual dimorphism in fossil ziphiids?

New well-preserved fossils from Peru reveal details of the dentition and morphology of the mandible and rostrum in 2 late middle to early late Miocene beaked whales (Cetacea, Odontoceti, Ziphiidae). Apical mandibular tusks are present in both Nazcacetus urbinai and Messapicetus sp. In the former the tusks are associated with a strong reduction of the postapical dentition, whereas in Messapicetus sp. a complete series of functional upper and lower teeth is retained. The larger sample of Messapicetus sp. from a single locality and age reveals intraspecific variation in size and shape of the tusks and surrounding structures. In addition, the rostrum of Messapicetus displays thickened premaxillae, dorsally closing the mesorostral groove. By comparison with modern beaked whales, most of them highly sexually dimorphic at the level of the tusks and rostrum, we propose that the tusks of Messapicetus were used in intraspecific fights between adult males. Strengthening of the rostrum through the dorsal closure of its transverse section would have reduced the risk of fractures when facing impacts.

Postgastrular zen expression is required to develop distinct amniotic and serosal epithelia in the scuttle fly Megaselia

The amnioserosa is an extraembryonic epithelium that evolved in higher cyclorrhaphan flies from distinct serosal and amniotic epithelia. The underlying genetic mechanism of this evolutionary transition is unknown. Amnioserosa development of Drosophila correlates with novel expression characteristics of the homeobox gene zerknüllt ( zen), including a broad zen expression domain in the syncytial blastoderm and the complete absence of postgastrular zen expression. Here we examine the functional significance of these features by altering the activity profile of zen in Megaselia (a lower cyclorrhaphan fly with distinct serosal and amniotic epithelia) and Drosophila, and by examining in Megaselia the function of u-shaped group (ush-group) genes, which in Drosophila maintain the amnioserosa after gastrulation when zen is no longer expressed. In Megaselia, loss of postgastrular zen expression abrogates serosa development but allows amnion development. Ectopic expression of zen in early Megaselia embryos allows serosa formation but perturbs amnion development. Megaselia homologues of u-shaped group genes are not essential for serosa formation but mediate germband retraction and dorsal closure. Finally, ectopic postgastrular zen expression in Drosophila causes an enlargement of amnioserosa cells and interferes with the morphogenetic functions of the amnioserosa. Our results suggest that the origin of the amnioserosa involved the loss of postgastrular zen expression from extraembryonic tissue, that the early broad expression domain of Drosophila zen evolved afterwards, and that the ush-group genes ancestrally played a role in morphogenetic functions of the amnion.

Sec61α is required for dorsal closure duringDrosophilaembryogenesis through its regulation of Dpp signaling

During dorsal closure in Drosophila, signaling events in the dorsalmost row of epidermal cells (DME cells) direct the migration of lateral epidermal sheets towards the dorsal midline where they fuse to enclose the embryo. A Jun amino-terminal kinase (JNK) cascade in the DME cells induces the expression of Decapentaplegic (Dpp). Dpp signaling then regulates the cytoskeleton in the DME cells and amnioserosa to affect the cell shape changes necessary to complete dorsal closure. We identified a mutation in Sec61alpha that specifically perturbs dorsal closure. Sec61alpha encodes the main subunit of the translocon complex for co-translational import of proteins into the ER. JNK signaling is normal in Sec61alpha mutant embryos, but Dpp signaling is attenuated and the DME cells fail to maintain an actinomyosin cable as epithelial migration fails. Consistent with this model, dorsal closure is rescued in Sec61alpha mutant embryos by an activated form of the Dpp receptor Thick veins.

POSH misexpression induces caspase‐dependent cell death inDrosophila

POSH (Plenty of SH3 domains) is a scaffold for signaling proteins regulating cell survival. Specifically, POSH promotes assembly of a complex including Rac GTPase, mixed lineage kinase (MLK), MKK7, and Jun kinase (JNK). In Drosophila, genetic analysis implicated POSH in Tak1-dependent innate immune response, in part through regulation of JNK signaling. Homologs of the POSH signaling complex components, MLK and MKK7, are essential in Drosophila embryonic dorsal closure. Using a gain-of-function approach, we tested whether POSH plays a role in this process. Ectopic expression of POSH in the embryo causes dorsal closure defects due to apoptosis of the amnioserosa, but ectodermal JNK signaling is normal. Phenotypic consequences of POSH expression were found to be dependent on Drosophila Nc, the caspase-9 homolog, but only partially on Tak1 and not at all on Slpr and Hep. These results suggest that POSH may use different signaling complexes to promote cell death in distinct contexts.

Tissue-autonomous EcR functions are required for concurrent organ morphogenesis in the Drosophila embryo

The insect hormone 20-hydroxy-ecdysone (20E) peaks at different stages during the life cycle. The hormone signal is commonly transmitted by a nuclear receptor consisting of the ecdysone receptor (EcR) and Ultraspiracle (Usp, orthologous to vertebrate RXR). EcR:Usp then initiate the expression of a series of gene regulators that help mediate biological responses to the hormone. Here, we investigated the embryonic ecdysone-signalling mechanism. The rise in 20E levels that occurs at mid-embryogenesis is required for major tissue movements to complete organ morphogenesis, but the functions of EcR and Usp during embryogenesis have remained unclear. We find that both EcR and Usp are essential for head involution, dorsal closure and tracheal and midgut morphogenesis, processes that also depend on 20E, arguing that embryonic 20E signals via EcR:Usp. We also show that EcR mediates the effects on organ morphogenesis in a tissue-autonomous manner and thus, that embryonic EcR functions are not fully reflected by the commonly used EcR activity assays. Finally, we show that embryonic 20E via EcR instructs the temporal and tissue-specific expression of four transcription factors that are needed for late embryogenesis and are common to the metamorphic 20E response. The results suggest that mid-embryonic EcR-activation imparts a level of gene regulation during embryonic organogenesis that has been largely overlooked, and possibly facilitates synchronized development of individual organs.

Epithelial reorganization events during late extraembryonic development in a hemimetabolous insect

As extra-embryonic tissues, the amnion and serosa are not considered to contribute materially to the insect embryo, yet they must execute an array of morphogenetic movements before they are dispensable. In hemimetabolous insects, these movements have been known for over a century, but they have remained virtually unexamined. This study addresses late extraembryonic morphogenesis in the milkweed bug, Oncopeltus fasciatus. Cell shape changes and apoptosis profiles are used to characterize the membranes as they undergo a large repertoire of final reorganizational events that reposition the embryo (katatrepsis), and eliminate the membranes themselves in an ordered fashion (dorsal closure). A number of key features were identified. First, amnion–serosa “fusion” involves localized apoptosis in the amnion and the formation of a supracellular actin purse string at the amnion–serosa border. During katatrepsis, a ‘focus’ of serosal cells undergoes precocious columnarization and may serve as an anchor for contraction. Lastly, dorsal closure involves novel modifications of the amnion and embryonic flank that are without counterpart during the well-known process of dorsal closure in the fruit fly Drosophila melanogaster. These data also address the long-standing question of the final fate of the amnion: it undergoes apoptosis during dorsal closure and thus is likely to be solely extraembryonic.

Altricial development in subsocial cockroach ancestors: foundation for the evolution of phenotypic plasticity in termites

Basal termites possess two developmental features that eusocial Hymenoptera lack: the majority of colony members are juveniles whose somatic and reproductive development is temporarily or permanently suspended, and individual development is characterized by extreme phenotypic plasticity. An examination of the literature indicates that the basis for these unique ontogenetic characters is not the prolongation of a pronymphal stage into postembryonic development, as recently suggested. Like other hemimetabolous insects, termites have three embryonic cuticles, and the pronymphal (EC3) cuticle is shed during or shortly after hatch. Nonetheless, a different developmental landmark, dorsal closure, occurs later during embryogenesis in termites than it does in their cockroach relatives, clearly indicating ontogenetic repatterning from an ancestral state. An alternate hypothesis for the origin of isopteran phenotypic plasticity becomes apparent if we remain focused on the phylogenetic and social context of termite evolution. Altricial development occurs in both vertebrate and invertebrate taxa, evolves in response to the parental environment, and is displayed by two distantly related, biparental, wood-feeding cockroaches, including Cryptocercus, the sister-group to termites. It is therefore likely the condition was present in subsocial termite ancestors, and played a complex, multidimensional role in the transition to eusociality. Most relevant to current arguments is that a shift in responsibility for the care of altricial dependents, from parents to the first nutritionally independent nymphs in the family (alloparents), resulted in the developmental stasis of alloparents at a relatively young age. Because early instar cockroaches are not metamorphically competent, these young alloparents would have provided a novel developmental template on which selection could act.

Interaction betweenDrosophilabZIP proteins Atf3 and Jun prevents replacement of epithelial cells during metamorphosis

Epithelial sheet spreading and fusion underlie important developmental processes. Well-characterized examples of such epithelial morphogenetic events have been provided by studies in Drosophila, and include embryonic dorsal closure, formation of the adult thorax and wound healing. All of these processes require the basic region-leucine zipper (bZIP) transcription factors Jun and Fos. Much less is known about morphogenesis of the fly abdomen, which involves replacement of larval epidermal cells (LECs) with adult histoblasts that divide, migrate and finally fuse to form the adult epidermis during metamorphosis. Here, we implicate Drosophila Activating transcription factor 3 (Atf3), the single ortholog of human ATF3 and JDP2 bZIP proteins, in abdominal morphogenesis. During the process of the epithelial cell replacement, transcription of the atf3 gene declines. When this downregulation is experimentally prevented, the affected LECs accumulate cell-adhesion proteins and their extrusion and replacement with histoblasts are blocked. The abnormally adhering LECs consequently obstruct the closure of the adult abdominal epithelium. This closure defect can be either mimicked and further enhanced by knockdown of the small GTPase Rho1 or, conversely, alleviated by stimulating ecdysone steroid hormone signaling. Both Rho and ecdysone pathways have been previously identified as effectors of the LEC replacement. To elicit the gain-of-function effect, Atf3 specifically requires its binding partner Jun. Our data thus identify Atf3 as a new functional partner of Drosophila Jun during development.

Drosophilamorphogenesis: Tissue force laws and the modeling of dorsal closure

Dorsal closure, a stage of Drosophila development, is a model system for cell sheet morphogenesis and wound healing. During closure, two flanks of epidermal tissue progressively advance to reduce the area of the eye-shaped opening in the dorsal surface, which contains amnioserosa tissue. To simulate the time evolution of the overall shape of the dorsal opening, we developed a mathematical model, in which contractility and elasticity are manifest in model force-producing elements that satisfy force-velocity relationships similar to muscle. The action of the elements is consistent with the force-producing behavior of actin and myosin in cells. The parameters that characterize the simulated embryos were optimized by reference to experimental observations on wild-type embryos and, to a lesser extent, on embryos whose amnioserosa was removed by laser surgery and on myospheroid mutant embryos. Simulations failed to reproduce the amnioserosa-removal protocol in either the elastic or the contractile limit, indicating that both elastic and contractile dynamics are essential components of the biological force-producing elements. We found it was necessary to actively upregulate forces to recapitulate both the double and single-canthus nick protocols, which did not participate in the optimization of parameters, suggesting the existence of additional key feedback mechanisms.

Dorsal closure in Drosophila: cells cannot get out of the tight spot

Dorsal closure (DC), the closure of a hole in the dorsal epidermis of Drosophila embryos by the joining of opposing epithelial cell sheets, has been used as a model process to study the molecular and cellular mechanisms underlying epithelial spreading and wound healing. Recent studies have provided novel insights into how different tissues function cooperatively in this process. Specifically, they demonstrate a critical function of the epidermis surrounding the hole in modulating the behavior of the amnioserosa cells inside. These findings shed light not only on the mechanisms by which the behavior of different tissues is coordinated during DC, but also on the general mechanisms by which tissues interact to trigger global morphogenesis, an essential but yet poorly explored aspect of embryogenesis.

A critical role for sFRP proteins in maintaining caudal neural tube closure in mice via inhibition of BMP signaling

Both the BMP and Wnt pathways have been implicated in directing aspects of dorsal neural tube closure and cell fate specification. However, the mechanisms that control the diverse responses to these signals are poorly understood. In this study, we provide genetic and functional evidence that the secreted sFRP1 and sFRP2 proteins, which have been primarily implicated as negative regulators of Wnt signaling, can also antagonize BMP signaling in the caudal neural tube and that this function is critical to maintain proper neural tube closure and dorsal cell fate segregation. Our studies thus reveal a novel role for specific sFRP proteins in balancing the response of cells to two critical extracellular signaling pathways.

Cdc42- and IRSp53-dependent contractile filopodia tether presumptive lens and retina to coordinate epithelial invagination

The vertebrate lens provides an excellent model with which to study the mechanisms required for epithelial invagination. In the mouse, the lens forms from the head surface ectoderm. A domain of ectoderm first thickens to form the lens placode and then invaginates to form the lens pit. The epithelium of the lens placode remains in close apposition to the epithelium of the presumptive retina as these structures undergo a coordinated invagination. Here, we show that F-actin-rich basal filopodia that link adjacent presumptive lens and retinal epithelia function as physical tethers that coordinate invagination. The filopodia, most of which originate in the presumptive lens, form at E9.5 when presumptive lens and retinal epithelia first come into close contact, and have retracted by E11.5 when invagination is complete. At E10.5--the lens pit stage--there is approximately one filopodium per epithelial cell. Formation of filopodia is dependent on the Rho family GTPase Cdc42 and the Cdc42 effector IRSp53 (Baiap2). Loss of filopodia results in reduced lens pit invagination. Pharmacological manipulation of the actin-myosin contraction pathway showed that the filopodia can respond rapidly in length to change inter-epithelial distance. These data suggest that the lens-retina inter-epithelial filopodia are a fine-tuning mechanism to assist in lens pit invagination by transmitting the forces between presumptive lens and retina. Although invagination of the archenteron in sea urchins and dorsal closure in Drosophila are known to be partly dependent on filopodia, this mechanism of morphogenesis has not previously been identified in vertebrates.

Morphogenetic movement of cells in embryogenesis of Drosophila melanogaster: Mechanism and genetic control

Morphogenetic movement of cells is of significant importance in embryogenesis. It is necessary to identify the final position and configuration of the embryoTs tissues and share the positional information in differentiation. The healing of tissular injuries happens in the adult organism due to the cell layers' movement, and the directed macrophages migration to the nidus of infection assists the neutralization of inflammatory processes. Owing to the good level of knowledge, the fruit fly D. melanogaster is a perfect model object for the study of morphogenetic events in embryogenesis in connection with the cells movement. The description of Drosophila embryogenesis, mechanisms of ventral furrow formation, elongation of the germ band, and its contraction accompanied by dorsal closure was given. During these processes the movement of epithelial cells and the entire epithelial strata happens. The information about the genetic regulation of morphogenetic movement of D. melanogaster taking into consideration the scale of evolutionary conservatism of the cascades of the main signal, which control these processes in vertebrates, is presented.

Efficacité du capitonnage dans la prévention du sérome après lambeau de grand dorsal. La technique de « Chippendale »

Seroma is the most frequent minor complication after harvesting latissimus dorsi flap for breast reconstruction. It induces patient's discomfort and multiple consultations for punctions. The dead space resulting from the harvest has to be closed by the "quilting suture" in order to prevent the seroma. Our aim is to evaluate the efficiency and the tolerance of the quilting suture by comparing two groups of 100 patients who had a breast reconstruction by the same technic of extended latissimus dorsi flap, performed by the same surgeon, from 2004 to 2007. Half of patients had the classic way of dorsal closure, and the other half of patients had the dorsal quilting suture. In order to compare the two groups we have collected data concerning age, body mass index (BMI), tobacco use, postoperative complications, number and volume of punctions, draining time and postoperative pain. The efficiency of the quilting suture lies on a rigorous repartition of at least six sutures on the upper skin flap, 12 on the lower skin flap and under the skin suture line. The suture model is based on the one used for the Chippendale-designed sofa. We suture the skin flap while pushing down the shoulder, in order to split the skin tension and avoid traction on the final skin suture line. The procedure takes 15 minutes. The "Chippendale" technic allows to reduce draining time from 12 days to 6 days. The incidence of chronic seroma is reduced by 50%. The dorsal wound healing seems also better thanks to tension reduction resulting from the quilting suture. The "Chippendale" technic is a quick, cheap and easy learned procedure, efficient for preventing chronic seroma after the latissimus dorsi flap. The postoperative recovery is eased and the patients comforted.

Pulsed Forces Timed by a Ratchet-like Mechanism Drive Directed Tissue Movement during Dorsal Closure

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.

Closing in on Mechanisms of Tissue Morphogenesis

It remains largely unknown how large-scale tissue movements during development emerge from the interplay of different tensile forces associated with actomyosin networks. Solon et al. (2009) now report that a ratchet-like mechanism drives the movement of epithelial sheets during dorsal closure in embryos of the fruit fly Drosophila.