Organ shape in the Drosophila salivary gland is controlled by regulated, sequential internalization of the primordia

Single-cell analysis of the early Drosophila salivary gland reveals that morphogenetic control involves both the induction and exclusion of gene expression programs

How tissue shape and therefore function is encoded by the genome remains in many cases unresolved. The tubes of the salivary glands in the Drosophila embryo start from simple epithelial placodes, specified through the homeotic factors Scr/Hth/Exd. Previous work indicated that early morphogenetic changes are prepatterned by transcriptional changes, but an exhaustive transcriptional blueprint driving physical changes was lacking. We performed single-cell-RNAseq-analysis of FACS-isolated early placodal cells, making up less than 0.4% of cells within the embryo. Differential expression analysis in comparison to epidermal cells analyzed in parallel generated a repertoire of genes highly upregulated within placodal cells prior to morphogenetic changes. Furthermore, clustering and pseudotime analysis of single-cell-sequencing data identified dynamic expression changes along the morphogenetic timeline. Our dataset provides a comprehensive resource for future studies of a simple but highly conserved morphogenetic process of tube morphogenesis. Unexpectedly, we identified a subset of genes that, although initially expressed in the very early placode, then became selectively excluded from the placode but not the surrounding epidermis, including hth, grainyhead and tollo/toll-8. We show that maintaining tollo expression severely compromised the tube morphogenesis. We propose tollo is switched off to not interfere with key Tolls/LRRs that are expressed and function in the tube morphogenesis.

Coordination of cell cycle and morphogenesis during organ formation

Organ formation requires precise regulation of cell cycle and morphogenetic events. Using the Drosophila embryonic salivary gland (SG) as a model, we uncover the role of the SP1/KLF transcription factor Huckebein (Hkb) in coordinating cell cycle regulation and morphogenesis. The hkb mutant SG exhibits defects in invagination positioning and organ size due to the abnormal death of SG cells. Normal SG development involves distal-to-proximal progression of endoreplication (endocycle), whereas hkb mutant SG cells undergo abnormal cell division, leading to cell death. Hkb represses the expression of key cell cycle and pro-apoptotic genes in the SG. Knockdown of cyclin E or cyclin-dependent kinase 1, or overexpression of fizzy-related rescues most of the morphogenetic defects observed in the hkb mutant SG. These results indicate that Hkb plays a critical role in controlling endoreplication by regulating the transcription of key cell cycle effectors to ensure proper organ formation.

Organogenetic transcriptomes of the Drosophila embryo at single cell resolution

To gain insight into the transcription programs activated during the formation of Drosophila larval structures, we carried out single cell RNA sequencing during two periods of Drosophila embryogenesis: stages 10-12, when most organs are first specified and initiate morphological and physiological specialization; and stages 13-16, when organs achieve their final mature architectures and begin to function. Our data confirm previous findings with regards to functional specialization of some organs - the salivary gland and trachea - and clarify the embryonic functions of another - the plasmatocytes. We also identify two early developmental trajectories in germ cells and uncover a potential role for proteolysis during germline stem cell specialization. We identify the likely cell type of origin for key components of the Drosophila matrisome and several commonly used Drosophila embryonic cell culture lines. Finally, we compare our findings with other recent related studies and with other modalities for identifying tissue-specific gene expression patterns. These data provide a useful community resource for identifying many new players in tissue-specific morphogenesis and functional specialization of developing organs.

Multifunctional role of GPCR signaling in epithelial tube formation

Epithelial tube formation requires Rho1-dependent actomyosin contractility to generate the cellular forces that drive cell shape changes and rearrangement. Rho1 signaling is activated by G-protein-coupled receptor (GPCR) signaling at the cell surface. During Drosophila embryonic salivary gland (SG) invagination, the GPCR ligand Folded gastrulation (Fog) activates Rho1 signaling to drive apical constriction. The SG receptor that transduces the Fog signal into Rho1-dependent myosin activation has not been identified. Here, we reveal that the Smog GPCR transduces Fog signal to regulate Rho kinase accumulation and myosin activation in the medioapical region of cells to control apical constriction during SG invagination. We also report on unexpected Fog-independent roles for Smog in maintaining epithelial integrity and organizing cortical actin. Our data support a model wherein Smog regulates distinct myosin pools and actin cytoskeleton in a ligand-dependent manner during epithelial tube formation.

Ribbon boosts ribosomal protein gene expression to coordinate organ form and function

Cell growth is well defined for late (postembryonic) stages of development, but evidence for early (embryonic) cell growth during postmitotic morphogenesis is limited. Here, we report early cell growth as a key characteristic of tubulogenesis in the Drosophila embryonic salivary gland (SG) and trachea. A BTB/POZ domain nuclear factor, Ribbon (Rib), mediates this early cell growth. Rib binds the transcription start site of nearly every SG-expressed ribosomal protein gene (RPG) and is required for full expression of all RPGs tested. Rib binding to RPG promoters in vitro is weak and not sequence specific, suggesting that specificity is achieved through cofactor interactions. Accordingly, we demonstrate Rib’s ability to physically interact with each of the three known regulators of RPG transcription. Surprisingly, Rib-dependent early cell growth in another tubular organ, the embryonic trachea, is not mediated by direct RPG transcription. These findings support a model of early cell growth customized by transcriptional regulatory networks to coordinate organ form and function.

Forced into shape: Mechanical forces in Drosophila development and homeostasis

Mechanical forces play a central role in shaping tissues during development and maintaining epithelial integrity in homeostasis. In this review, we discuss the roles of mechanical forces in Drosophila development and homeostasis, starting from the interplay of mechanics with cell growth and division. We then discuss several examples of morphogenetic processes where complex 3D structures are shaped by mechanical forces, followed by a closer look at patterning processes. We also review the role of forces in homeostatic processes, including cell elimination and wound healing. Finally, we look at the interplay of mechanics and developmental robustness and discuss open questions in the field, as well as novel approaches that will help tackle them in the future.

Correct regionalization of a tissue primordium is essential for coordinated morphogenesis

During organ development, tubular organs often form from flat epithelial primordia. In the placodes of the forming tubes of the salivary glands in the Drosophila embryo, we previously identified spatially defined cell behaviors of cell wedging, tilting, and cell intercalation that are key to the initial stages of tube formation. Here, we address what the requirements are that ensure the continuous formation of a narrow symmetrical tube from an initially asymmetrical primordium whilst overall tissue geometry is constantly changing. We are using live-imaging and quantitative methods to compare wild-type placodes and mutants that either show disrupted cell behaviors or an initial symmetrical placode organization, with both resulting in severe impairment of the invagination. We find that early transcriptional patterning of key morphogenetic transcription factors drives the selective activation of downstream morphogenetic modules, such as GPCR signaling that activates apical-medial actomyosin activity to drive cell wedging at the future asymmetrically placed invagination point. Over time, transcription of key factors expands across the rest of the placode and cells switch their behavior from predominantly intercalating to predominantly apically constricting as their position approaches the invagination pit. Misplacement or enlargement of the initial invagination pit leads to early problems in cell behaviors that eventually result in a defective organ shape. Our work illustrates that the dynamic patterning of the expression of transcription factors and downstream morphogenetic effectors ensures positionally fixed areas of cell behavior with regards to the invagination point. This patterning in combination with the asymmetric geometrical setup ensures functional organ formation.

Composite morphogenesis during embryo development

Morphogenesis drives the formation of functional living shapes. Gene expression patterns and signaling pathways define the body plans of the animal and control the morphogenetic processes shaping the embryonic tissues. During embryogenesis, a tissue can undergo composite morphogenesis resulting from multiple concomitant shape changes. While previous studies have unraveled the mechanisms that drive simple morphogenetic processes, how a tissue can undergo multiple and simultaneous changes in shape is still not known and not much explored. In this chapter, we focus on the process of concomitant tissue folding and extension that is vital for the animal since it is key for embryo gastrulation and neurulation. Recent pioneering studies focus on this problem highlighting the roles of different spatially coordinated cell mechanisms or of the synergy between different patterns of gene expression to drive composite morphogenesis.

A three-dimensional vertex model forDrosophilasalivary gland invagination

During epithelial morphogenesis, force generation at the cellular level not only causes cell deformation, but may also produce coordinated cell movement and rearrangement on the tissue level. In this paper, we use a novel three-dimensional vertex model to explore the roles of cellular forces during the formation of the salivary gland in theDrosophilaembryo. Representing the placode as an epithelial sheet of initially columnar cells, we focus on the spatial and temporal patterning of contractile forces due to three actomyosin pools: the apicomedial actomyosin in the pit of the placode, junctional actomyosin arcs outside the pit, and a supracellular actomyosin cable along the circumference of the placode. In anin silico'wild type' model, these pools are activated at different times according to experimental data. To identify the role of each myosin pool, we have also simulated variousin silico'mutants' in which only one or two of the myosin pools are activated. We find that the apicomedial myosin initiates a small dimple in the pit, but this is not essential for the overall invagination of the placode. The myosin arcs are the main driver of invagination and are responsible for the internalization of the apical surface. The circumferential actomyosin cable acts to constrict the opening of the developing tube, and is responsible for forming a properly shaped lumen. Cell intercalation tends to facilitate the invagination, but the geometric constraints of our model only allow a small number of intercalations, and their effect is minor. The placode invagination predicted by the model is in general agreement with experimental observations. It confirms some features of the current 'belt-and-braces' model for the process, and provides new insights on the separate roles of the various myosin pools and their spatio-temporal coordination.

Regulation of apical constriction via microtubule- and Rab11-dependent apical transport during tissue invagination

The formation of an epithelial tube is a fundamental process for organogenesis. During Drosophila embryonic salivary gland (SG) invagination, Folded gastrulation (Fog)-dependent Rho-associated kinase (Rok) promotes contractile apical myosin formation to drive apical constriction. Microtubules (MTs) are also crucial for this process and are required for forming and maintaining apicomedial myosin. However, the underlying mechanism that coordinates actomyosin and MT networks still remains elusive. Here, we show that MT-dependent intracellular trafficking regulates apical constriction during SG invagination. Key components involved in protein trafficking, such as Rab11 and Nuclear fallout (Nuf), are apically enriched near the SG invagination pit in a MT-dependent manner. Disruption of the MT networks or knockdown of Rab11 impairs apicomedial myosin formation and apical constriction. We show that MTs and Rab11 are required for apical enrichment of the Fog ligand and the continuous distribution of the apical determinant protein Crumbs (Crb) and the key adherens junction protein E-Cadherin (E-Cad) along junctions. Targeted knockdown of crb or E-Cad in the SG disrupts apical myosin networks and results in apical constriction defects. Our data suggest a role of MT- and Rab11-dependent intracellular trafficking in regulating actomyosin networks and cell junctions to coordinate cell behaviors during tubular organ formation.

Secrets of secretion-How studies of the Drosophila salivary gland have informed our understanding of the cellular networks underlying secretory organ form and function

Secretory organs are critical for organismal survival. Yet, the transcriptional regulatory mechanisms governing their development and maintenance remain unclear for most model secretory organs. The Drosophila embryonic salivary gland (SG) remedies this deficiency as one of the few organs wherein direct connections from the expression of the early patterning genes to cell specification to organ architecture and functional specialization can be made. Few other models of secretion can be accorded this distinction. Studies from the past three decades have made enormous strides in parsing out the roles of distinct transcription factors (TFs) that direct major steps in furnishing this secretory organ. In the first step of specifying the salivary gland, the activity of the Hox factors Sex combs reduced, Extradenticle, and Homothorax activate expression of fork head (fkh), sage, and CrebA, which code for the major suite of TFs that carry forward the task of organ building and maintenance. Then, in the second key step of building the SG, the program for cell fate maintenance and morphogenesis is deployed. Fkh maintains the secretory cell fate by regulating its own expression and that of sage and CrebA. Fkh and Sage maintain secretory cell viability by actively blocking apoptotic cell death. Fkh, along with two other TFs, Hkb and Rib, also coordinates organ morphogenesis, transforming two plates of precursor cells on the embryo surface into elongated internalized epithelial tubes. Acquisition of functional specialization, the third key step, is mediated by CrebA and Fkh working in concert with Sage and yet another TF, Sens. CrebA directly upregulates expression of all of the components of the secretory machinery as well as other genes (e.g., Xbp1) necessary for managing the physiological stress that inexorably accompanies high secretory load. Secretory cargo specificity is controlled by Sage and Sens in collaboration with Fkh. Investigations have also uncovered roles for various signaling pathways, e.g., Dpp signaling, EGF signaling, GPCR signaling, and cytoskeletal signaling, and their interactions within the gene regulatory networks that specify, build, and specialize the SG. Collectively, studies of the SG have expanded our knowledge of secretory dynamics, cell polarity, and cytoskeletal mechanics in the context of organ development and function. Notably, the embryonic SG has made the singular contribution as a model system that revealed the core function of CrebA in scaling up secretory capacity, thus, serving as the pioneer system in which the conserved roles of the mammalian Creb3/3L-family orthologues were first discovered.

Role of tbc1 in Drosophila embryonic salivary glands

BACKGROUND

CG4552/tbc1 was identified as a downstream target of Fork head (Fkh), the single Drosophila member of the FoxA family of transcription factors and a major player in salivary gland formation and homeostasis. Tbc1 and its orthologues have been implicated in phagocytosis, the innate immune response, border cell migration, cancer and an autosomal recessive form of non-degenerative Pontocerebellar hypoplasia. Recently, the mammalian Tbc1 orthologue, Tbc1d23, has been shown to bind both the conserved N-terminal domains of two Golgins (Golgin-97 and Golgin-245) and the WASH complex on endosome vesicles. Through this activity, Tbc1d23 has been proposed to link endosomally-derived vesicles to their appropriate target membrane in the trans Golgi (TGN).

RESULTS

In this paper, we provide an initial characterization of Drosophila orthologue, we call tbc1. We show that, like its mammalian orthologue, Tbc1 localizes to the trans Golgi. We show that it also colocalizes with a subset of Rabs associated with both early and recycling endosomes. Animals completely missing tbc1 survive, but females have fertility defects. Consistent with the human disease, loss of tbc1 reduces optic lobe size and increases response time to mechanical perturbation. Loss and overexpression of tbc1 in the embryonic salivary glands leads to secretion defects and apical membrane irregularities.

CONCLUSIONS

These findings support a role for tbc1 in endocytic/membrane trafficking, consistent with its activities in other systems.

Radially patterned cell behaviours during tube budding from an epithelium

The budding of tubular organs from flat epithelial sheets is a vital morphogenetic process. Cell behaviours that drive such processes are only starting to be unraveled. Using live-imaging and novel morphometric methods, we show that in addition to apical constriction, radially oriented directional intercalation of cells plays a major contribution to early stages of invagination of the salivary gland tube in the Drosophila embryo. Extending analyses in 3D, we find that near the pit of invagination, isotropic apical constriction leads to strong cell-wedging. Further from the pit cells interleave circumferentially, suggesting apically driven behaviours. Supporting this, junctional myosin is enriched in, and neighbour exchanges are biased towards the circumferential orientation. In a mutant failing pit specification, neither are biased due to an inactive pit. Thus, tube budding involves radially patterned pools of apical myosin, medial as well as junctional, and radially patterned 3D-cell behaviours, with a close mechanical interplay between invagination and intercalation.

Uncoupling apical constriction from tissue invagination

Apical constriction is a widely utilized cell shape change linked to folding, bending and invagination of polarized epithelia. It remains unclear how apical constriction is regulated spatiotemporally during tissue invagination and how this cellular process contributes to tube formation in different developmental contexts. Using Drosophila salivary gland (SG) invagination as a model, we show that regulation of folded gastrulation expression by the Fork head transcription factor is required for apicomedial accumulation of Rho kinase and non-muscle myosin II, which coordinate apical constriction. We demonstrate that neither loss of spatially coordinated apical constriction nor its complete blockage prevent internalization and tube formation, although such manipulations affect the geometry of invagination. When apical constriction is disrupted, compressing force generated by a tissue-level myosin cable contributes to SG invagination. We demonstrate that fully elongated polarized SGs can form outside the embryo, suggesting that tube formation and elongation are intrinsic properties of the SG.

DOI:http://dx.doi.org/10.7554/eLife.22235.001

Many organs in the human body – like the kidneys, lungs, and salivary glands – are organized as a single layer of cells that surround a hollow tube. There are a number of ways that cells can achieve this particular arrangement. In one mechanism, a small group of cells bud out of a single cell layer to become the end of a new tube or a new branch of an existing tube. Since all the cells are still connected, the first cells bring their neighbouring cells along behind them, rearranging these cells to form the walls of a tube.

In addition to changing position, the cells must change their shape to form a tube. One crucial change in cell shape is called apical constriction, and involves the side of the cell facing the inside of the tube becoming smaller than the other sides. This creates cells with a wedge-like shape that can fit together to form the curved wall of the tube, similar to shaped bricks in an archway. Apical constriction has been widely studied and is controlled by proteins that act like motors moving along protein-based filaments; however the roles of apical constriction in tube formation have not been fully explained.

Using the developing salivary glands of the fruit fly Drosophila melanogaster, Chung et al. confirmed that the motor protein known as myosin II controls apical constriction during tissue invagination. Further examination showed that proteins (called Fork Head and Fog) activate and localize an enzyme (Rho kinase) to control the localized accumulation of myosin II and thereby control apical constriction. Chung et al. then showed that salivary glands could still form tubes if apical constriction was blocked, indicating that it is not an essential part of tissue invagination in this organ. However, blocking apical constriction led the tube to develop unusual shapes at intermediate stages.

More work is now needed to better understand the links between apical constriction, cell rearrangement and tissue invagination. These processes are fundamental for organs to form correctly in many organisms and understanding their control could have wide-ranging impacts. A better understanding of these processes may provide insight into how the tubes can form while keeping all the cells adequately supplied with oxygen and nutrients, and into diseases that result if there are defects in the invagination process.

DOI:http://dx.doi.org/10.7554/eLife.22235.002

Ribbon regulates morphogenesis of the Drosophila embryonic salivary gland through transcriptional activation and repression

Transcription factors affect spatiotemporal patterns of gene expression often regulating multiple aspects of tissue morphogenesis, including cell-type specification, cell proliferation, cell death, cell polarity, cell shape, cell arrangement and cell migration. In this work, we describe a distinct role for Ribbon (Rib) in controlling cell shape/volume increases during elongation of the Drosophila salivary gland (SG). Notably, the morphogenetic changes in rib mutants occurred without effects on general SG cell attributes such as specification, proliferation and apoptosis. Moreover, the changes in cell shape/volume in rib mutants occurred without compromising epithelial-specific morphological attributes such as apicobasal polarity and junctional integrity. To identify the genes regulated by Rib, we performed ChIP-seq analysis in embryos driving expression of GFP-tagged Rib specifically in the SGs. To learn if the Rib binding sites identified in the ChIP-seq analysis were linked to changes in gene expression, we performed microarray analysis comparing RNA samples from age-matched wild-type and rib null embryos. From the superposed ChIP-seq and microarray gene expression data, we identified 60 genomic sites bound by Rib likely to regulate SG-specific gene expression. We confirmed several of the identified Rib targets by qRT-pCR and/or in situ hybridization. Our results indicate that Rib regulates cell growth and tissue shape in the Drosophila salivary gland via a diverse array of targets through both transcriptional activation and repression. Furthermore, our results suggest that autoregulation of rib expression may be a key component of the SG morphogenetic gene network.

Cellular and molecular mechanisms of single and collective cell migrations in Drosophila: themes and variations

The process of cell migration is essential throughout life, driving embryonic morphogenesis and ensuring homeostasis in adults. Defects in cell migration are a major cause of human disease, with excessive migration causing autoimmune diseases and cancer metastasis, whereas reduced capacity for migration leads to birth defects and immunodeficiencies. Myriad studies in vitro have established a consensus view that cell migrations require cell polarization, Rho GTPase-mediated cytoskeletal rearrangements, and myosin-mediated contractility. However, in vivo studies later revealed a more complex picture, including the discovery that cells migrate not only as single units but also as clusters, strands, and sheets. In particular, the role of E-Cadherin in cell motility appears to be more complex than previously appreciated. Here, we discuss recent advances achieved by combining the plethora of genetic tools available to the Drosophila geneticist with live imaging and biophysical techniques. Finally, we discuss the emerging themes such studies have revealed and ponder the puzzles that remain to be solved.

The Garz Sec7 domain guanine nucleotide exchange factor for Arf regulates salivary gland development in Drosophila

Surface delivery of proteins involved in cell-cell and cell-matrix interactions in cultured mammalian cells requires the GBF1 guanine nucleotide exchange factor. However, the role of GBF1 in delivery of adhesion proteins during organogenesis in intact animals has not been characterized. Here, we report the function of the fly GBF1 homolog, Gartenzwerg (Garz) in the development of the salivary gland in Drosophila melanogaster. We used the GAL4/UAS system to selectively deplete Garz from salivary gland cells. We show that depletion of Garz disrupts the secretory pathway as evidenced by the collapse of Golgi-localized Lava lamp (Lva) and the TGN-localized γ subunit of the clathrin-adaptor protein complex (AP-1). Additionally, Garz depletion inhibits trafficking of cell-cell adhesion proteins cadherin (DE-cad) and Flamingo to the cell surface. Disregulation of trafficking correlates with mistargeting of the tumor suppressor protein Discs large involved in epithelial polarity determination. Garz-depleted salivary cells are smaller and lack well-defined plasma membrane domains. Garz depletion also inhibits normal elongation and positioning of epithelial cells, resulting in a disorganized salivary gland that lacks a well defined luminal duct. Our findings suggest that Garz is essential for establishment of epithelial structures and demonstrate an absolute requirement for Garz during Drosophila development.

A dynamic microtubule cytoskeleton directs medial actomyosin function during tube formation

The cytoskeleton is a major determinant of cell-shape changes that drive the formation of complex tissues during development. Important roles for actomyosin during tissue morphogenesis have been identified, but the role of the microtubule cytoskeleton is less clear. Here, we show that during tubulogenesis of the salivary glands in the fly embryo, the microtubule cytoskeleton undergoes major rearrangements, including a 90° change in alignment relative to the apicobasal axis, loss of centrosomal attachment, and apical stabilization. Disruption of the microtubule cytoskeleton leads to failure of apical constriction in placodal cells fated to invaginate. We show that this failure is due to loss of an apical medial actomyosin network whose pulsatile behavior in wild-type embryos drives the apical constriction of the cells. The medial actomyosin network interacts with the minus ends of acentrosomal microtubule bundles through the cytolinker protein Shot, and disruption of Shot also impairs apical constriction.

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Large-scale rearrangement of microtubules accompanies early tube formation

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Loss of microtubules leads to loss of apical constriction during tube formation

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During tubulogenesis, apical constriction is driven by pulsatile medial actomyosin

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Microtubules and the cytolinker Shot stabilize the medial actomyosin

Building and specializing epithelial tubular organs: the Drosophila salivary gland as a model system for revealing how epithelial organs are specified, form and specialize

The past two decades have witnessed incredible progress toward understanding the genetic and cellular mechanisms of organogenesis. Among the organs that have provided key insight into how patterning information is integrated to specify and build functional body parts is the Drosophila salivary gland, a relatively simple epithelial organ specialized for the synthesis and secretion of high levels of protein. Here, we discuss what the past couple of decades of research have revealed about organ specification, development, specialization, and death, and what general principles emerge from these studies.

Controlling cell shape changes during salivary gland tube formation in Drosophila

Any type of tubulogenesis is a process that is highly coordinated between large numbers of cells. Like other morphogenetic processes, it is driven to a great extent by complex cell shape changes and cell rearrangements. The formation of the salivary glands in the fly embryo provides an ideal model system to study these changes and rearrangements, because upon specification of the cells that are destined to form the tube, there is no further cell division or cell death. Thus, morphogenesis of the salivary gland tubes is entirely driven by cell shape changes and rearrangements. In this review, we will discuss and distill from the literature what is known about the control of cell shape during the early invagination process and whilst the tubes extend in the fly embryo at later stages.

Hox targets and cellular functions

Hox genes are a group of genes that specify structures along the anteroposterior axis in bilaterians. Although in many cases they do so by modifying a homologous structure with a different (or no) Hox input, there are also examples of Hox genes constructing new organs with no homology in other regions of the body. Hox genes determine structures though the regulation of targets implementing cellular functions and by coordinating cell behavior. The genetic organization to construct or modify a certain organ involves both a genetic cascade through intermediate transcription factors and a direct regulation of targets carrying out cellular functions. In this review I discuss new data from genome-wide techniques, as well as previous genetic and developmental information, to describe some examples of Hox regulation of different cell functions. I also discuss the organization of genetic cascades leading to the development of new organs, mainly using Drosophila melanogaster as the model to analyze Hox function.

Apical constriction initiates new bud formation during monopodial branching of the embryonic chicken lung

Branching morphogenesis sculpts the airway epithelium of the lung into a tree-like structure to conduct air and promote gas exchange after birth. In the avian lung, a series of buds emerges from the dorsal surface of the primary bronchus via monopodial branching to form the conducting airways; anatomically, these buds are similar to those formed by domain branching in the mammalian lung. Here, we show that monopodial branching is initiated by apical constriction of the airway epithelium, and not by differential cell proliferation, using computational modeling and quantitative imaging of embryonic chicken lung explants. Both filamentous actin and phosphorylated myosin light chain were enriched at the apical surface of the airway epithelium during monopodial branching. Consistently, inhibiting actomyosin contractility prevented apical constriction and blocked branch initiation. Although cell proliferation was enhanced along the dorsal and ventral aspects of the primary bronchus, especially before branch formation, inhibiting proliferation had no effect on the initiation of branches. To test whether the physical forces from apical constriction alone are sufficient to drive the formation of new buds, we constructed a nonlinear, three-dimensional finite element model of the airway epithelium and used it to simulate apical constriction and proliferation in the primary bronchus. Our results suggest that, consistent with the experimental results, apical constriction is sufficient to drive the early stages of monopodial branching whereas cell proliferation is dispensable. We propose that initial folding of the airway epithelium is driven primarily by apical constriction during monopodial branching of the avian lung.

Organ-specific gene expression: the bHLH protein Sage provides tissue specificity to Drosophila FoxA

FoxA transcription factors play major roles in organ-specific gene expression, regulating, for example, glucagon expression in the pancreas, GLUT2 expression in the liver, and tyrosine hydroxylase expression in dopaminergic neurons. Organ-specific gene regulation by FoxA proteins is achieved through cooperative regulation with a broad array of transcription factors with more limited expression domains. Fork head (Fkh), the sole Drosophila FoxA family member, is required for the development of multiple distinct organs, yet little is known regarding how Fkh regulates tissue-specific gene expression. Here, we characterize Sage, a bHLH transcription factor expressed exclusively in the Drosophila salivary gland (SG). We show that Sage is required for late SG survival and normal tube morphology. We find that many Sage targets, identified by microarray analysis, encode SG-specific secreted cargo, transmembrane proteins, and the enzymes that modify these proteins. We show that both Sage and Fkh are required for the expression of Sage target genes, and that co-expression of Sage and Fkh is sufficient to drive target gene expression in multiple cell types. Sage and Fkh drive expression of the bZip transcription factor Senseless (Sens), which boosts expression of Sage-Fkh targets, and Sage, Fkh and Sens colocalize on SG chromosomes. Importantly, expression of Sage-Fkh target genes appears to simply add to the tissue-specific gene expression programs already established in other cell types, and Sage and Fkh cannot alter the fate of most embryonic cell types even when expressed early and continuously.

Sculpting organs: mechanical regulation of tissue development

The ramified architectures of organs such as the mammary gland and lung are generated via branching morphogenesis, a developmental process through which individual cells bud and pinch off of pre-existing epithelial sheets. Although specified by signaling programs, organ development requires integration of all aspects of the microenvironment. We describe the essential role of endogenous cellular contractility in the formation of branching tubes. We also highlight the role of exogenous forces in normal and aberrant branching.

Drosophila as a model for epithelial tube formation

Epithelial tubular organs are essential for life in higher organisms and include the pancreas and other secretory organs that function as biological factories for the synthesis and delivery of secreted enzymes, hormones, and nutrients essential for tissue homeostasis and viability. The lungs, which are necessary for gas exchange, vocalization, and maintaining blood pH, are organized as highly branched tubular epithelia. Tubular organs include arteries, veins, and lymphatics, high-speed passageways for delivery and uptake of nutrients, liquids, gases, and immune cells. The kidneys and components of the reproductive system are also epithelial tubes. Both the heart and central nervous system of many vertebrates begin as epithelial tubes. Thus, it is not surprising that defects in tube formation and maintenance underlie many human diseases. Accordingly, a thorough understanding how tubes form and are maintained is essential to developing better diagnostics and therapeutics. Among the best-characterized tubular organs are the Drosophila salivary gland and trachea, organs whose relative simplicity have allowed for in depth analysis of gene function, yielding key mechanistic insight into tube initiation, remodeling and maintenance. Here, we review our current understanding of salivary gland and trachea formation - highlighting recent discoveries into how these organs attain their final form and function.

Hemocyte-secreted type IV collagen enhances BMP signaling to guide renal tubule morphogenesis in Drosophila

Details of the mechanisms that determine the shape and positioning of organs in the body cavity remain largely obscure. We show that stereotypic positioning of outgrowing Drosophila renal tubules depends on signaling in a subset of tubule cells and results from enhanced sensitivity to guidance signals by targeted matrix deposition. VEGF/PDGF ligands from the tubules attract hemocytes, which secrete components of the basement membrane to ensheath them. Collagen IV sensitizes tubule cells to localized BMP guidance cues. Signaling results in pathway activation in a subset of tubule cells that lead outgrowth through the body cavity. Failure of hemocyte migration, loss of collagen IV, or abrogation of BMP signaling results in tubule misrouting and defective organ shape and positioning. Such regulated interplay between cell-cell and cell-matrix interactions is likely to have wide relevance in organogenesis and congenital disease.

Reiterative use of signalling pathways controls multiple cellular events during Drosophila posterior spiracle organogenesis

Organogenesis proceeds in multiple steps and events that need to be coordinated in time and space. Yet the genetic and molecular control of such coordination remains poorly understood. In this study we have investigated the contribution of three signalling pathways, Wnt/Wingless (Wg), Hedgehog (Hh), and epidermal growth factor receptor (EGFR), to posterior spiracle morphogenesis, an organ that forms under Abdominal-B (AbdB) control in the eighth abdominal segment. Using targeted signalling inactivation, we show that these pathways are reiteratively used to control multiple cellular events during posterior spiracle organogenesis, including cell survival and maintenance of cell polarity and adhesion required for tissue integrity. We propose that the reiterative use of the Wg, Hh, and EGFR signalling pathways serves to coordinate in time and space the sequential deployment of events that collectively allow proper organogenesis.

Endocytosis is required for efficient apical constriction during Xenopus gastrulation

Coordinated apical constriction (AC) in epithelial sheets drives tissue invagination [1, 2] and is required for diverse morphogenetic movements such as gastrulation [3], neurulation [4, 5], and organogenesis [6]. We showed previously that actomyosin contractility drives AC in Xenopus laevis bottle cells [7]; however, it remained unclear whether it does so in concert with other processes. Here we report that endocytosis-driven membrane remodeling is required for efficient AC. We found endosomes exclusively in bottle cells in the early gastrula. Disrupting endocytosis with dominant-negative dynamin or rab5 perturbed AC, with a significant decrease in constriction rate late in the process, suggesting that endocytosis operates downstream of actomyosin contractility to remove excess membrane. Additionally, disrupting endocytosis during neurulation inhibits AC in hingepoint cells, resulting in neural tube closure defects. Thus, membrane remodeling during AC could be a general mechanism to achieve efficient invagination in embryos.

Morphogenesis of epithelial tubes: Insights into tube formation, elongation, and elaboration

Epithelial tubes are a fundamental tissue across the metazoan phyla and provide an essential functional component of many of the major organs. Recent work in flies and mammals has begun to elucidate the cellular mechanisms driving the formation, elongation, and branching morphogenesis of epithelial tubes during development. Both forward and reverse genetic techniques have begun to identify critical molecular regulators for these processes and have revealed the conserved role of key pathways in regulating the growth and elaboration of tubular networks. In this review, we discuss the developmental programs driving the formation of branched epithelial networks, with specific emphasis on the trachea and salivary gland of Drosophila melanogaster and the mammalian lung, mammary gland, kidney, and salivary gland. We both highlight similarities in the development of these organs and attempt to identify tissue and organism specific strategies. Finally, we briefly consider how our understanding of the regulation of proliferation, apicobasal polarity, and epithelial motility during branching morphogenesis can be applied to understand the pathologic dysregulation of these same processes during metastatic cancer progression.

Apical constriction: a cell shape change that can drive morphogenesis

Biologists have long recognized that dramatic bending of a cell sheet may be driven by even modest shrinking of the apical sides of cells. Cell shape changes and tissue movements like these are at the core of many of the morphogenetic movements that shape animal form during development, driving processes such as gastrulation, tube formation, and neurulation. The mechanisms of such cell shape changes must integrate developmental patterning information in order to spatially and temporally control force production-issues that touch on fundamental aspects of both cell and developmental biology and on birth defects research. How does developmental patterning regulate force-producing mechanisms, and what roles do such mechanisms play in development? Work on apical constriction from multiple systems including Drosophila, Caenorhabditis elegans, sea urchin, Xenopus, chick, and mouse has begun to illuminate these issues. Here, we review this effort to explore the diversity of mechanisms of apical constriction, the diversity of roles that apical constriction plays in development, and the common themes that emerge from comparing systems.

A targeted gain-of-function screen identifies genes affecting salivary gland morphogenesis/tubulogenesis in Drosophila

During development individual cells in tissues undergo complex cell-shape changes to drive the morphogenetic movements required to form tissues. Cell shape is determined by the cytoskeleton and cell-shape changes critically depend on a tight spatial and temporal control of cytoskeletal behavior. We have used the formation of the salivary glands in the Drosophila embryo, a process of tubulogenesis, as an assay for identifying factors that impinge on cell shape and the cytoskeleton. To this end we have performed a gain-of-function screen in the salivary glands, using a collection of fly lines carrying EP-element insertions that allow the overexpression of downstream-located genes using the UAS-Gal4 system. We used a salivary-gland-specific fork head-Gal4 line to restrict expression to the salivary glands, in combination with reporters of cell shape and the cytoskeleton. We identified a number of genes known to affect salivary gland formation, confirming the effectiveness of the screen. In addition, we found many genes not implicated previously in this process, some having known functions in other tissues. We report the initial characterization of a subset of genes, including chickadee, rhomboid1, egalitarian, bitesize, and capricious, through comparison of gain- and loss-of-function phenotypes.

Kinetic and mechanical analysis of live tube morphogenesis

Ribbon is a nuclear Broad Tramtrack Bric-a-brac (BTB) -domain protein required for morphogenesis of the salivary gland and trachea. We recently showed that ribbon mutants exhibit decreased Crumbs and Rab11-coincident apical vesicles and increased apical Moesin activity and microvillar structure during tube elongation. To learn how these molecular and morphological changes affect the dynamics of tubulogenesis, we optimized an advanced two-photon microscope to enable high-resolution live imaging of the salivary gland and trachea. Live imaging revealed that ribbon mutant tissues exhibit slowed and incomplete lumenal morphogenesis, consistent with previously described apical defects. Because Moesin activity correlates with cortical stiffness, we hypothesize that ribbon mutants suffer from increased apical stiffness during morphogenesis. We develop this hypothesis through mechanical analysis, using the advantages of live imaging to construct computational elastic and analytical viscoelastic models of tube elongation, which suggest that ribbon mutant tubes exhibit three- to fivefold increased apical stiffness and twofold increased effective apical viscosity.

Ribbon modulates apical membrane during tube elongation through Crumbs and Moesin

Although the formation and maintenance of epithelial tubes are essential for the viability of multicellular organisms, our understanding of the molecular and cellular events coordinating tubulogenesis is relatively limited. Here, we focus on the activities of Ribbon, a novel BTB-domain containing nuclear protein, in the elongation of two epithelial tubes: the Drosophila salivary gland and trachea. We show that Ribbon interacts with Lola Like, another BTB-domain containing protein required for robust nuclear localization of Ribbon, to upregulate crumbs expression and downregulate Moesin activity. Our ultrastructural analysis of ribbon null salivary glands by TEM reveals a diminished pool of subapical vesicles and an increase in microvillar structure, cellular changes consistent with the known role of Crumbs in apical membrane generation and of Moesin in the cross-linking of the apical membrane to the subapical cytoskeleton. Furthermore, the subapical localization of Rab11, a small GTPase associated with apical membrane delivery and rearrangement, is significantly diminished in ribbon mutant salivary glands and tracheae. These findings suggest that Ribbon and Lola Like function as a novel transcriptional cassette coordinating molecular changes at the apical membrane of epithelial cells to facilitate tube elongation.

Rho GTPase controls invagination and cohesive migration of the Drosophila salivary gland through Crumbs and Rho-kinase

Coordinated cell movements shape simple epithelia into functional tissues and organs during embryogenesis. Regulators and effectors of the small GTPase Rho have been shown to be essential for epithelial morphogenesis in cell culture; however, the mechanism by which Rho GTPase and its downstream effectors control coordinated movement of epithelia in a developing tissue or organ is largely unknown. Here, we show that Rho1 GTPase activity is required for the invagination of Drosophila embryonic salivary gland epithelia and for directed migration of the internalized gland. We demonstrate that the absence of zygotic function of Rho1 results in the selective loss of the apical proteins, Crumbs (Crb), Drosophila atypical PKC and Stardust during gland invagination and that this is partially due to reduced crb RNA levels and apical localization. In parallel to regulation of crb RNA and protein, Rho1 activity also signals through Rho-kinase (Rok) to induce apical constriction and cell shape change during invagination. After invagination, Rho-Rok signaling is required again for the coordinated contraction and dorsal migration of the proximal half of the gland. We also show that Rho1 activity is required for proper development of the circular visceral mesoderm upon which the gland migrates. Our genetic and live-imaging analyses provide novel evidence that the proximal gland cells play an essential and active role in salivary gland migration that propels the entire gland to turn and migrate posteriorly.

A wave of EGFR signaling determines cell alignment and intercalation in the Drosophila tracheal placode

Invagination of organ placodes converts flat epithelia into three-dimensional organs. Cell tracing in the Drosophila tracheal placode revealed that, in the 30-minute period before invagination, cells enter mitotic quiescence and form short rows that encircle the future invagination site. The cells in the rows align to form a smooth boundary (;boundary smoothing'), accompanied by a transient increase in myosin at the boundary and cell intercalation oriented in parallel with the cellular rows. Cells then undergo apical constriction and invaginate, followed by radially oriented mitosis in the placode. Prior to invagination, ERK MAP kinase is activated in an outward circular wave, with the wave front often correlating with the smoothing cell boundaries. EGFR signaling is required for myosin accumulation and cell boundary smoothing, suggesting its propagation polarizes the planar cell rearrangement in the tracheal placode, and coordinates the timing and position of intrinsic cell internalization activities.

mab-7 encodes a novel transmembrane protein that orchestrates sensory ray morphogenesis in C. elegans

The tapered sensory rays of the male Caenorhabditis elegans are important for successful male/hermaphrodite copulation. A group of ram (ray morphology abnormal) genes encoding modifying enzymes and transmembrane protein have been reported as key regulators controlling ray morphogenesis. Here we report the characterization of another component essential for this morphogenetic process encoded by mab-7. This gene is active in the hypodermis, structural cells, the body seam and several head neurons. It encodes a novel protein with a hydrophobic region at the N-terminus, an EGF-like motif, an ShKT motif and a long C-terminal tail. All these domains are shown to be critical to MAB-7 activity except the EGF-like domain, which appears to be regulatory and dispensable. MAB-7 is shown to be a type II membrane protein, tethered on the cell surface by the N-terminal transmembrane domain with the remainder of the protein exposed to the extracellular matrix. Since ectopic mab-7 expression in any ray cell or even in touch neurons of non-ray lineage can rescue the mutant phenotype, mab-7 is probably acting non-autonomously. It may facilitate intercellular communication among ray cells to augment normal ray morphogenesis.

18 wheeler regulates apical constriction of salivary gland cells via the Rho-GTPase-signaling pathway

Rho GTPase and its upstream activator, guanine nucleotide exchange factor 2 (RhoGEF2), have emerged as key regulators of actin rearrangements during epithelial folding and invagination (Nikolaidou, K.K., Barrett, K. (2004). A Rho-GTPase-signaling pathway is used reiteratively in epithelial folding and potentially selects the outcome of Rho activation. Curr. Biol. 14, 1822-1826). Here, we show that Drosophila 18 wheeler (18W), a Toll-like receptor protein, is a novel component of the Rho-signaling pathway involved in epithelial morphogenesis. 18w Mutant embryos have salivary gland invagination defects similar to embryos that lack components of the Rho pathway, and ubiquitous expression of 18W results in an upregulation of Rho signaling. Transheterozygous genetic interactions and double mutant analysis suggest that 18W affects the Rho-GTPase-signaling pathway not through Fog and RhoGEF2, but rather by inhibiting Rho GTPase activating proteins (RhoGAPs). We show that RhoGAP5A and RhoGAP88C/Crossveinless-c (CV-C) are required for proper salivary gland morphogenesis, implicating them as potential targets of 18W.

Gastrulation: Wnts signal constriction

Recent work shows that Wnt signaling directly regulates the apical constriction that drives gastrulation movements in Caenorhabditis elegans, and also promotes invagination in sea urchins, providing a novel and possibly conserved mode of developmental regulation.

From fate to function: the Drosophila trachea and salivary gland as models for tubulogenesis

Tube formation is a ubiquitous process required to sustain life in multicellular organisms. The tubular organs of adult mammals include the lungs, vasculature, digestive and excretory systems, as well as secretory organs such as the pancreas, salivary, prostate, and mammary glands. Other tissues, including the embryonic heart and neural tube, have requisite stages of tubular organization early in development. To learn the molecular and cellular basis of how epithelial cells are organized into tubular organs of various shapes and sizes, investigators have focused on the Drosophila trachea and salivary gland as model genetic systems for branched and unbranched tubes, respectively. Both organs begin as polarized epithelial placodes, which through coordinated cell shape changes, cell rearrangement, and cell migration form elongated tubes. Here, we discuss what has been discovered regarding the details of cell fate specification and tube formation in the two organs; these discoveries reveal significant conservation in the cellular and molecular events of tubulogenesis.

Rac function in epithelial tube morphogenesis

Epithelial cell migration and morphogenesis require dynamic remodeling of the actin cytoskeleton and cell-cell adhesion complexes. Numerous studies in cell culture and in model organisms have demonstrated the small GTPase Rac to be a critical regulator of these processes; however, little is known about Rac function in the morphogenic movements that drive epithelial tube formation. Here, we use the embryonic salivary glands of Drosophila to understand the role of Rac in epithelial tube morphogenesis. We show that inhibition of Rac function, either through loss of function mutations or dominant-negative mutations, disrupts salivary gland invagination and posterior migration. In contrast, constitutive activation of Rac induces motile behavior and subsequent cell death. We further show that Rac regulation of salivary gland morphogenesis occurs through modulation of cell-cell adhesion mediated by the E-cadherin/beta-catenin complex and that shibire, the Drosophila homolog of dynamin, functions downstream of Rac in regulating beta-catenin localization during gland morphogenesis. Our results demonstrate that regulation of cadherin-based adherens junctions by Rac is critical for salivary gland morphogenesis and that this regulation occurs through dynamin-mediated endocytosis.

NETRIN and SLIT guide salivary gland migration

Directed migration is pivotal for the proper placement and function of nearly all organs. The majority of known guidance molecules involved in directed migration have been identified from studies of migrating axons during nervous system development. Here, we show that at least two of these axon guidance molecules, NETRIN and SLIT, act through their canonical receptors, to guide Drosophila embryonic salivary glands. NETRIN serves as a chemo-attractant while SLIT functions antagonistically to NETRIN as a chemo-repellent during salivary gland migration. CNS midline expression of both NETRIN and SLIT directs the glands to move unswervingly parallel to the CNS. NETRIN expression is also required in the visceral mesoderm, along which the glands move during their migration. We propose that analogous to axon guidance, a balance between chemo-attractants and chemo-repellents is required for the proper migratory path of the developing salivary glands.

Organ positioning in Drosophila requires complex tissue-tissue interactions

Positioning an organ with respect to other tissues is a complex process necessary for proper anatomical development and organ function. The local environment surrounding an organ can serve both as a substrate for migration and as a source of guidance cues that direct migration. Little is known about the factors guiding Drosophila salivary gland movement or about the contacts the glands establish along their migratory path. Here, we provide a detailed description of the spatial and temporal interactions between the salivary glands and surrounding tissues during embryogenesis. The glands directly contact five other tissues: the visceral mesoderm, gastric caecae, somatic mesoderm, fat body, and central nervous system. Mutational analysis reveals that all of the tissues tested in this study are important for normal salivary gland positioning; proper differentiation of the visceral and somatic mesoderm is necessary for the glands to attain their final correct position. We also provide evidence that the segment-polarity gene, gooseberry (gsb), controls expression of signals from the developing fat body that direct posterior migration of the glands. These data further the understanding of how organ morphology and position are determined by three-dimensional constraints and guidance cues provided by neighboring tissues.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Tec29 controls actin remodeling and endoreplication during invagination of the Drosophila embryonic salivary glands

Epithelial invagination is necessary for formation of many tubular organs, one of which is the Drosophila embryonic salivary gland. We show that actin reorganization and control of endocycle entry are crucial for normal invagination of the salivary placodes. Embryos mutant for Tec29, the Drosophila Tec family tyrosine kinase, showed delayed invagination of the salivary placodes. This invagination delay was partly the result of an accumulation of G-actin in the salivary placodes, indicating that Tec29 is necessary for maintaining the equilibrium between G- and F-actin during invagination of the salivary placodes. Furthermore, normal invagination of the salivary placodes appears to require the proper timing of the endocycle in these cells; Tec29 must delay DNA endoreplication in the salivary placode cells until they have invaginated into the embryo. Taken together, these results show that Tec29 regulates both the actin cytoskeleton and the cell cycle to facilitate the morphogenesis of the embryonic salivary glands. We suggest that apical constriction of the actin cytoskeleton may provide a temporal cue ensuring that endoreplication does not begin until the cells have finished invagination.

CrebA regulates secretory activity in theDrosophilasalivary gland and epidermis

Understanding how organs acquire the capacity to perform their respective functions is important for both cell and developmental biology. Here, we have examined the role of early-expressed transcription factors in activating genes crucial for secretory function in the Drosophila salivary gland. We show that expression of genes encoding proteins required for ER targeting and translocation, and proteins that mediate transport between the ER and Golgi is very high in the early salivary gland. This high level expression requires two early salivary gland transcription factors; CrebA is required throughout embryogenesis and Fkh is required only during late embryonic stages. As Fkh is required to maintain late CrebA expression in the salivary gland, Fkh probably works through CrebA to affect secretory pathway gene expression. In support of these regulatory interactions, we show that CrebA is important for elevated secretion in the salivary gland. Additionally, CrebA is required for the expression of the secretory pathway genes in the embryonic epidermis, where CrebA had previously been shown to be essential for cuticle development. We show that zygotic mutations in several individual secretory pathway genes result in larval cuticle phenotypes nearly identical to those of CrebA mutants. Thus, CrebA activity is linked to secretory function in multiple tissues.

A Rho GTPase Signaling Pathway Is Used Reiteratively in Epithelial Folding and Potentially Selects the Outcome of Rho Activation

A single Rho GTPase family member is capable of initiating several different processes, including cell cycle regulation, cytokinesis, cell migration, and transcriptional regulation . It is not clear, however, how the Rho protein selects which of these processes to initiate. Guanine nucleotide exchange factors (GEFs), proteins that activate Rho GTPases, could be important in making this selection. We show here that in vivo, DRhoGEF2, a GEF that is ubiquitously expressed and specific for Rho1, is reiteratively required for epithelial folding and invagination, but not for other processes regulated by Rho. The limitation of DRhoGEF2 function supports the hypothesis that the GEF selects the outcome of Rho activation. DRhoGEF2 exerts its effects in gastrulation through the regulation of Myosin II to orchestrate coordinated apical cell constriction. Apical myosin localization is also regulated by Concertina (Cta), a Galpha(12/13) family member that is thought to activate DRhoGEF2 and is itself activated by a putative ligand, Folded gastrulation (Fog). Fog and Cta also play a role in the morphogenetic events requiring DRhoGEF2, suggesting the existence of a conserved signaling pathway in which Fog, Cta, and DRhoGEF2 locally activate Myosin for epithelial invagination and folding.

bullwinkle is required for epithelial morphogenesis during Drosophila oogenesis

Many organs, such as the liver, neural tube, and lung, form by the precise remodeling of flat epithelial sheets into tubes. Here we investigate epithelial tubulogenesis in Drosophila melanogaster by examining the development of the dorsal respiratory appendages of the eggshell. We employ a culture system that permits confocal analysis of stage 10-14 egg chambers. Time-lapse imaging of GFP-Moesin-expressing egg chambers reveals three phases of morphogenesis: tube formation, anterior extension, and paddle maturation. The dorsal-appendage-forming cells, previously thought to represent a single cell fate, consist of two subpopulations, those forming the tube roof and those forming the tube floor. These two cell types exhibit distinct morphological and molecular features. Roof-forming cells constrict apically and express high levels of Broad protein. Floor cells lack Broad, express the rhomboid-lacZ marker, and form the floor by directed cell elongation. We examine the morphogenetic phenotype of the bullwinkle (bwk) mutant and identify defects in both roof and floor formation. Dorsal appendage formation is an excellent system in which cell biological, molecular, and genetic tools facilitate the study of epithelial morphogenesis.

Beyond homeosis—HOX function in morphogenesis and organogenesis

Hox genes encode conserved transcription factors expressed along the antero-posterior axis of vertebrates and invertebrates. In both phyla, HOX proteins control the formation of specific structures in the segments where they are expressed. Because of the global effect they have on segment morphology, the Hox genes are said to control segment identity. Here we review the data available on how HOX proteins regulate their downstream targets and how they mediate the formation of segment-specific structures. Within the segment, the information provided by HOX proteins, tissue-specific transcription factors, and signaling pathway effectors becomes integrated at the enhancer of the target genes, resulting in their localized activation. In general, HOX proteins regulate the morphogenesis of specific organs indirectly by activating networks of transcription factors and signaling molecules, but they can also directly regulate the so-called realizator genes: genes that control the cell behaviors that induce morphogenesis. Here we review some of the Hox-activated networks, the most interesting realizator genes known to date, and summarize how organogenesis is affected in Hox mutants. These examples reveal that only a fraction of the transformations caused by Hox mutations are in fact homeotic (leading to the morphological transformation of a structure present in one segment into that present in another segment). In the cases where Hox gene mutants do not cause homeotic transformations, the wild-type function of the Hox gene is to activate specific cell behaviors (cell proliferation, survival, shape changes, and rearrangements) that lead to the morphogenesis of particular organs. This second non-homeotic function is common to vertebrates and invertebrates, and we argue that it may actually constitute the original HOX function.

Posterior migration of the salivary gland requires an intact visceral mesoderm and integrin function

The final overall shape of an organ and its position within the developing embryo arise as a consequence of both its intrinsic properties and its interactions with surrounding tissues. Here, we focus on the role of directed cell migration in shaping and positioning the Drosophila salivary gland. We demonstrate that the salivary gland turns and migrates along the visceral mesoderm to become properly oriented with respect to the overall embryo. We show that salivary gland posterior migration requires the activities of genes that position the visceral mesoderm precursors, such as heartless, thickveins, and tinman, but does not require a differentiated visceral mesoderm. We also demonstrate a role for integrin function in salivary gland migration. Although the mutations affecting salivary gland motility and directional migration cause defects in the final positioning of the salivary gland, most do not affect the length or diameter of the salivary gland tube. These findings suggest that salivary tube dimensions may be an intrinsic property of salivary gland cells.

Constructing an organ: the Drosophila salivary gland as a model for tube formation

Tubes are required in metazoans to transport the liquids and gases that sustain life. The conservation of molecules and mechanisms involved in tube formation suggests that what we learn by studying simple systems will apply to related processes in higher animals. Studies over the past 10 years have revealed the molecules that specify cell fate in Drosophila salivary gland and the cellular events that mediate tube morphogenesis. Here, we discuss how anterior-posterior and dorsal-ventral patterning information specifies both the position of salivary-gland primordia and how many cells they contain. We examine the transformation of a polarized epithelial sheet into an elongated, unbranched tube, and the intrinsic and extrinsic factors that influence the final position of the salivary gland.