Compartment Borders Research Articles

Widespread contribution of transposable elements to the rewiring of mammalian 3D genomes

Transposable elements (TEs) are major contributors of genetic material in mammalian genomes. These often include binding sites for architectural proteins, including the multifarious master protein, CTCF, which shapes the 3D genome by creating loops, domains, compartment borders, and RNA-DNA interactions. These play a role in the compact packaging of DNA and have the potential to facilitate regulatory function. In this study, we explore the widespread contribution of TEs to mammalian 3D genomes by quantifying the extent to which they give rise to loops and domain border differences across various cell types and species using several 3D genome mapping technologies. We show that specific families and subfamilies of TEs have contributed to lineage-specific 3D chromatin structures across mammalian species. In many cases, these loops may facilitate sustained interaction between distant cis-regulatory elements and target genes, and domains may segregate chromatin state to impact gene expression in a lineage-specific manner. An experimental validation of our analytical findings using CRISPR-Cas9 to delete a candidate TE resulted in disruption of species-specific 3D chromatin structure. Taken together, we comprehensively quantify and selectively validate our finding that TEs contribute to shaping 3D genome organization and may, in some cases, impact gene regulation during the course of mammalian evolution.

β-Catenin is required for radial cell patterning and identity in the developing mouse cochlea

Development of multicellular organs requires the coordination of cell differentiation and patterning. Critical for sound detection, the mammalian organ of Corti contains functional units arranged tonotopically along the cochlear turns. Each unit consists of sensory hair cells intercalated by nonsensory supporting cells, both specified and radially patterned with exquisite precision during embryonic development. However, how cell identity and radial patterning are jointly controlled is poorly understood. Here we show that β-catenin is required for specification of hair cell and supporting cell subtypes and radial patterning of the cochlea in vivo. In 2 mouse models of conditional β-catenin deletion, early specification of Myosin7-expressing hair cells and Prox1-positive supporting cells was preserved. While β-catenin-deficient cochleae expressed FGF8 and FGFR3, both of which are essential for pillar cell specification, the radial patterning of organ of Corti was disrupted, revealed by aberrant expression of cadherins and the pillar cell markers P75 and Lgr6. Moreover, β-catenin ablation caused duplication of FGF8-positive inner hair cells and reduction of outer hair cells without affecting the overall hair cell density. In contrast, in another transgenic model with suppressed transcriptional activity of β-catenin but preserved cell adhesion function, both specification and radial patterning of the organ of Corti were intact. Our study reveals specific functions of β-catenin in governing cell identity and patterning mediated through cell adhesion in the developing cochlea.

Total mesorectal excision: fascial surgery or compartment surgery

Total mesorectal excision (TME) improves the prognosis of patients with rectal cancer (RC) and now is being established as the gold standard for RC surgery. Local spread of tumor is thought to be contained within a defined intact visceral mesorectal fascial envelope. Therefore, surgical dissection along correct plane with complete mesorectum excision is the essence of TME. Recent literatures stated that TME with the guidance of membrane anatomy could improve tumor radicality and reduce genitourinary dysfunction. But TME remains the object of ongoing controversy. In fact, the conventional function anatomy can′t provide theoretical support for TME. Observations of the anatomical studies found that Holy Plane was unlikely to become an almost impenetrable barrier to the spread of carcinoma and it didn′t ensure complete excision of mesorectum along the correct surgical plane. Compartment theory based on the ontogenetic anatomy suggested that tumors were always locally confined to a compartment derived from a common embryologic origin (primordium) for a relatively long phase. The potential reasons lie in that tumor propagation is primarily suppressed at the compartment borders. The compartment model of tumor spread provides explanations for TME which excise the complete rectum compartment including the rectum and its surrounding vascular and ligamentous mesenteries. The compartment theory may set up the new principles of tumor radicality. Key words: Rectal neoplasms; Total mesorectal excision; Compartment; Fascia; Anatomy

Striatal Cholinergic Interneurons Modulate Spike-Timing in Striosomes and Matrix by an Amphetamine-Sensitive Mechanism.

The striatum is key for action-selection and the motivation to move. Dopamine and acetylcholine release sites are enriched in the striatum and are cross-regulated, possibly to achieve optimal behavior. Drugs of abuse, which promote abnormally high dopamine release, disrupt normal action-selection and drive restricted, repetitive behaviors (stereotypies). Stereotypies occur in a variety of disorders including obsessive-compulsive disorder, autism, schizophrenia and Huntington's disease, as well as in addictive states. The severity of drug-induced stereotypy is correlated with induction of c-Fos expression in striosomes, a striatal compartment that is related to the limbic system and that directly projects to dopamine-producing neurons of the substantia nigra. These characteristics of striosomes contrast with the properties of the extra-striosomal matrix, which has strong sensorimotor and associative circuit inputs and outputs. Disruption of acetylcholine signaling in the striatum blocks the striosome-predominant c-Fos expression pattern induced by drugs of abuse and alters drug-induced stereotypy. The activity of striatal cholinergic interneurons is associated with behaviors related to sensory cues, and cortical inputs to striosomes can bias action-selection in the face of conflicting cues. The neurons and neuropil of striosomes and matrix neurons have observably separate distributions, both at the input level in the striatum and at the output level in the substantia nigra. Notably, cholinergic axons readily cross compartment borders, providing a potential route for local cross-compartment communication to maintain a balance between striosomal and matrix activity. We show here, by slice electrophysiology in transgenic mice, that repetitive evoked firing patterns in striosomal and matrix striatal projection neurons (SPNs) are interrupted by optogenetic activation of cholinergic interneurons either by the addition or the deletion of spikes. We demonstrate that this cholinergic modulation of projection neurons is blocked in brain slices taken from mice exposed to amphetamine and engaged in amphetamine-induced stereotypy, and lacking responsiveness to salient cues. Our findings support a model whereby activity in striosomes is normally under strong regulation by cholinergic interneurons, favoring behavioral flexibility, but that in animals with drug-induced stereotypy, this cholinergic signaling breaks down, resulting in differential modulation of striosomal activity and an inability to bias action-selection according to relevant sensory cues.

Robotically assisted peritoneal mesometrial resection (PMMR) in endometrial cancer supported by ICG labeling of the compartmental lymphatic system

•Peritoneal mesometrial resection is a compartment based radical hysterectomy in endometrial cancer•ICG staining of the lymph-vessel system facilitates identification of compartment borders•Fluorescence based HD-video documentation supports education in surgery of endometrial cancer.

Embryologically based radical hysterectomy as peritoneal mesometrial resection (PMMR) with pelvic and para-aortic lymphadenectomy for loco-regional tumor control in endometrial cancer: first evidence for efficacy

ObjectiveTo evaluate the feasibility and efficacy of embryologically based compartmental surgery for locoregional tumor control in intermediate and high risk endometrial cancer: peritoneal mesometrial resection with therapeutic pelvic and para-aortic lymphadenectomy by robotically assisted laparoscopy.Methods75 consecutive surgically treated patients with uterine malignancies have been analyzed. 68 patients with histologically proven endometrial cancer and complete robotically assisted surgery have been included in this study on morbidity and oncological outcome. 56 % of the patients were at intermediate/high risk with either stage IAG3 or IB (n = 22) or stage II–IV (n = 16). Adjuvant EBRT was offered to three patients only (4 %), whereas five received isolated vaginal brachytherapy (7 %). Indocyanine-green (ICG) fluorescence lymphography is demonstrated being useful for additional intraoperative visualization of the compartment borders and lymphatic drainage to the postponed lymph compartments.ResultsAfter a mean follow-up of 32 months, there were only two loco-regional recurrences (2.9 %). Both recurrences were apparently cured by salvage therapy. 9 patients died; 6 (8.8 %) from metastatic disease (5) or unknown cause (1), 3 (4.4 %) from intercurrent disease without evidence of disease. One patient (1.4 %) experienced a peritoneal carcinosis and is alive. There were 8/68 perioperative complications (12 %). No perioperative mortality was observed.ConclusionsEmbryologically defined compartmental surgery by robotically assisted laparoscopy seems to be feasible and safe in endometrial cancer. The low loco-regional recurrence rate of 2.9 % in spite of a very low percentage of adjuvant radiotherapy and 56 % of intermediate/high risk tumors should stimulate to initiate a multicentre trial to evaluate the value of compartmental surgery for prevention of locoregional recurrence in endometrial cancer.

Giant sacral chondrosarcoma in an elderly male : a case report.

Primary sacral tumours are rare, therefore experience of managing their associated complications are very limited. Effective surgical treatment of pelvic chondrosarcoma remains a major challenge for orthopaedic surgeons, due to the complex anatomic structure of the pelvis, the lack of defined compartment borders, the close vicinity to vital structures, and the risk of jeopardizing pelvic structural stability. We report a rare case of a giant sacral chondrosarcoma (100cm x 80cm) in an elderly male who successfully underwent tumour resection with good functional outcome and recovery. Long term follow up is essential in view of the possibility of local tumour recurrence. Giant Chondrosarcoma, Sacrum, Surgery, Elderly Male.

Distinct morphological features of NADPH diaphorase neurons across rodent's primary cortices

Nitric oxide (NO) is a versatile gaseous molecule involved in several pathological and physiological functions in nervous system (Calabrese et al., 2007; Steinert et al., 2010; Freire, 2012). Its biosynthesis occurs after the stoichiometric conversion of L-arginine to L-citrulline in a process requiring the presence of NADPH and O2 as co-substrates (Bredt and Snyder, 1994). A group of enzymes known as nitric oxide synthases (NOS) is responsible to synthesize NO (Stuehr et al., 2004). In the brain, neuronal NOS is strictly co-localized with NADPH diaphorase (NADPH-d), an oxidative enzyme present in a discrete population of interneurons (Dawson et al., 1991). NADPH-d histochemistry has been used to reveal the presence of NO dispersed in neuropil and to evaluate the presence and distribution of NO-synthesizing neurons across several species (Vincent et al., 1983; Sandell, 1986; Mizukawa et al., 1989; Yan et al., 1996; Franca et al., 2000; Freire et al., 2010, 2011). Previous studies concerning NADPH-d/NOS reactive neurons focused mainly in its distribution around brain. Differences in morphology were described qualitatively. More recently, a growing amount of information regarding differences in morphometric parameters of this cell group, based in neuronal reconstructions, has emerged (Freire et al., 2007, 2012; Rocha et al., 2012). In light of this, an interesting question arises: could morphological differences of nitrergic neurons among distinct cortical areas or throughout subdivisions of a given area in rodents be associated to their distinct physiologies, as observed to pyramidal neurons (Elston et al., 2006)? In rodents, primary somatosensory cortex (S1) is a suitable area to be explored since the complete body's representation delineates a topologically correlated cortical map (Rocha et al., 2007; Santiago et al., 2007) commonly revealed by NADPH-d histochemistry (Freire et al., 2012). The general pattern of NADPH-d-reactive neuropil is markedly similar to observed with cytochrome oxidase histochemistry, since both are oxidative enzymes involved in the energetic metabolism (Wong-Riley et al., 1998). The main representation of S1 in rats and mice corresponds to the posterior medial barrel subfield (PMBSF), the region where mystacial vibrissae are represented and the largest and more defined barrels are found. Previous studies have shown that, in a tangential view, NADPH-d neurons concentrate along the regions of PMBSF that separate barrels from each other, known as septa (Franca and Volchan, 1995; Freire et al., 2004). Nevertheless, other S1 regions were not explored in these studies. In this context, a comprehensive work published by Nogueira-Campos et al. (2012) in Frontiers in Neural Circuits contributes to cover this gap. In that study the authors evaluated the entire population of NADPH-d neurons found across rat's S1 in order to quantify the distribution of these cells throughout all body's representation and also evaluate the morphological aspects and relationship of these neurons with barrels and septal regions. Similar to previously described to PMBSF (Franca and Volchan, 1995; Freire et al., 2004), NADPH-d neurons were not limited by compartment borders across S1. Besides, cell density in septal regions was higher than inside barrels. Regarding morphometric measurements, only in one case evaluated by the authors there was an evident difference between neurons found in septa and barrels, as previously reported (Franca and Volchan, 1995; Freire et al., 2004), what can be explained by the plan of section adopted (coronal vs. tangential). But what might these morphological differences represent, in terms of physiology, along distinct regions across rodent's S1? Septal and barrel regions are physiologically/functionally distinct (Alloway, 2008) since septal neurons display significant differences in response latencies related to variations in the frequency of whisker stimulation when compared to barrel neurons (Chakrabarti and Alloway, 2009). In addition both regions receive projections from separated thalamic nuclei (Killackey and Sherman, 2003). Accordingly, barrels and septa would require a segregated circuitry in these different zones, what may be correlated to morphometric differences observed in NADPH-d neurons in both regions. These structural differences could reflect specializations in the processing of sensory information across distinct cortical areas, meaning a difference in their capacity for synaptic integration because dendritic coverage is directly related to the amount of synaptic contacts a cell may receive. Accordingly, neurons possessing a smaller dendritic arbor cover a small cortical area and potentially establish fewer synaptic contacts than more ramified cells. Because septal region is thought to be a segregated cortex from barrels (Killackey and Sherman, 2003), both regions represent two different processing streams that analyze distinct information (Alloway, 2008). So, more complex interneurons in septa could integrate a broader range of synaptic inputs than those found inside barrels. In conclusion, data provided by Nogueira-Campos et al. (2012) offer new insights about the organization of interneuron circuitry across rat's S1. A further quantification of NADPH-d neuronal morphology in non-primary/association areas may contribute to a more complete notion of cortical differences in the inhibitory circuitry.

Determinants of the Drosophila Odorant Receptor Pattern

In most olfactory systems studied to date, neurons that express the same odorant receptor (Or) gene are scattered across sensory epithelia, intermingled with neurons that express different Or genes. In Drosophila, olfactory sensilla that express the same Or gene are dispersed on the antenna and the maxillary palp. Here we show that Or identity is specified in a spatially stereotyped pattern by the cell-autonomous activity of the transcriptional regulators Engrailed and Dachshund. Olfactory sensilla then become highly motile and disperse beneath the epidermis. Thus, positional information and cell motility underlie the dispersed patterns of Drosophila Or gene expression.

Cancer Permeates Locally within Ontogenetic Compartments: Clinical Evidence and Implications for Cancer Surgery

The pathophysiologic process of local tumor spread is regarded as an isotropic infiltration of microscopic extensions of the malignant lesion irrespective of tissue boundaries. By contrast, the ontogenetic compartment theory states that malignant solid tumors are locally confined, for a relatively long phase during their natural course, to a permissive compartment derived from a common primordium in embryonic development. Tumor permeation is isotropic within the permissive ontogenetic compartment, but it is suppressed at the compartment borders. The validity of the ontogenetic compartment theory has been shown for cancer of the rectum and of the female lower genital tract. It is hypothesized that ontogenetic compartment resection, the translation of the theory into cancer surgery, holds a great potential to improve oncologic treatment results.

Effect of mutations in Drosophila melanogaster tumor suppressor Merlin on proliferation and differentiation of wing cells

Experiments on transplantation of wing imaginal discs homozygous for a mutation in the tumor suppressor gene Merlin have demonstrated that this mutation does not induce malignant tumors. Marking of the wing disc compartment borders by specific antibodies showed the absence of essential compartment border defects in case of the Merlin mutation. Drosophila melanogaster cells mutant for Merlin have shorter cell cycle than normal cells. Proliferation of imaginal discs lasts longer in case of the mutation. It is known that beginning from some moment of development, wing veins serve as clonal restriction lines that cannot be crossed by growing mosaic clones. We showed that the Merlin mutation leads to depression of vein clonal restriction property. This means that this gene is involved not only in the control of cell proliferation, but also in the control of cell mobility and adhesion.

Comparing the composition of leg muscles in young, old, and very old men

Cross‐sectional studies report varying degrees of muscle atrophy in different muscles but usually involve the comparison of one young with one old age group. Using serial sections obtained by MR imaging, the cross‐sectional areas (CSA) of contractile and non‐contractile tissues in the anterior (ANT), lateral (LAT), and posterior (POST) compartments of the leg were measured in 38 men with ages averaging in their 3rd, 7th or 9th decades (young, old, and very old, respectively). Tissue type and compartment borders were distinguished visually and traced manually. Contractile CSA for each compartment was similar between the young and old groups, but was significantly reduced between 15–28% for all compartments in the very old. There was a significant increase in the non‐contractile tissue in all compartments across the age groups which were 1–5% from young to old and a further 5–10% between the old and very old. The greatest change was observed in the POST compartment and the smallest change in the ANT compartment. With two groups of older adults, these data demonstrate that skeletal muscle CSA in the leg is moderately well preserved until about 65y of age when there is an increased rate of deterioration. Previous studies have not compared changes in these 3 compartments of the leg. It is clear that not all muscles show similar degrees of age‐related changes which likely has functional implications.

Cell tracing reveals a dorsoventral lineage restriction plane in the mouse limb bud mesenchyme

Regionalization of embryonic fields into independent units of growth and patterning is a widespread strategy during metazoan development. Compartments represent a particular instance of this regionalization, in which unit coherence is maintained by cell lineage restriction between adjacent regions. Lineage compartments have been described during insect and vertebrate development. Two common characteristics of the compartments described so far are their occurrence in epithelial structures and the presence of signaling regions at compartment borders. Whereas Drosophila compartmental organization represents a background subdivision of embryonic fields that is not necessarily related to anatomical structures, vertebrate compartment borders described thus far coincide with, or anticipate, anatomical or cell-type discontinuities. Here, we describe a general method for clonal analysis in the mouse and use it to determine the topology of clone distribution along the three limb axes. We identify a lineage restriction boundary at the limb mesenchyme dorsoventral border that is unrelated to any anatomical discontinuity, and whose lineage restriction border is not obviously associated with any signaling center. This restriction is the first example in vertebrates of a mechanism of primordium subdivision unrelated to anatomical boundaries. Furthermore, this is the first lineage compartment described within a mesenchymal structure in any organism, suggesting that lineage restrictions are fundamental not only for epithelial structures, but also for mesenchymal field patterning. No lineage compartmentalization was found along the proximodistal or anteroposterior axes, indicating that patterning along these axes does not involve restriction of cell dispersion at specific axial positions.

Dependence of Drosophila wing imaginal disc cytonemes on Decapentaplegic

The anterior/posterior (A/P) and dorsal/ventral (D/V) compartment borders that subdivide the wing imaginal discs of Drosophila third instar larvae are each associated with a developmental organizer. Decapentaplegic (Dpp), a member of the transforming growth factor-beta (TGF-beta) superfamily, embodies the activity of the A/P organizer. It is produced at the A/P organizer and distributes in a gradient of decreasing concentration to regulate target genes, functioning non-autonomously to regulate growth and patterning of both the anterior and posterior compartments. Wingless (Wg) is produced at the D/V organizer and embodies its activity. The mechanisms that distribute Dpp and Wg are not known, but proposed mechanisms include extracellular diffusion, successive transfers between neighbouring cells, vesicle-mediated movement, and direct transfer via cytonemes. Cytonemes are actin-based filopodial extensions that have been found to orient towards the A/P organizer from outlying cells. Here we show that in the wing disc, cytonemes orient towards both the A/P and D/V organizers, and that their presence and orientation correlates with Dpp signalling. We also show that the Dpp receptor, Thickveins (Tkv), is present in punctae that move along cytonemes. These observations are consistent with a role for cytonemes in signal transduction.

Developmental Compartments and Planar Polarity in Drosophila

Background: Planar polarity refers to the asymmetry of a cell within the plane of the epithelium; for example, cells may form hairs that point in a posterior direction, or cilia may beat in one way. This property implies that cells have information about their orientation; we wish to understand the nature of this information. Relevant also is the body plan of insects, which, in the ectoderm and somatic mesoderm, consists of a chain of alternating anterior and posterior compartments — basic units of development with independent cell lineage and subject to independent genetic control.Results: Using the abdomen of adult Drosophila, we have taken genes required for normal polarity and either removed the gene or constitutively expressed it in small clones of cells and observed the effects on polarity. Hitherto, all such studies of polarity genes have not found any difference of behavior between the different compartments. We report here that the three genes, four-jointed, dachsous, and fat, cause opposite effects in anterior and posterior compartments. For example, in anterior compartments, clones ectopically expressing four-jointed reverse the polarity of cells in front of the clone, while, in posterior compartments, they reverse behind the clone. These three genes have been reported by others to be functionally linked.Conclusions: This discovery impacts on models of how cells read polarity. At the heart of one class of models is the hypothesis that cell polarity is determined by the vector of a morphogen gradient. Here, we present evidence that cell polarity in the abdomen depends on at least two protein gradients (Fj and Ds), each of which is reflected at compartment borders. Consequently, these gradients have opposing slopes in the two compartments. Because all polarized structures made by abdominal cells point posteriorly, we surmise that cells in each compartment are programmed to interpret these protein gradients with opposite signs, pointing up the gradient in one compartment and down the gradient in the other.

Hedgehog and engrailed: pattern formation and polarity in the Drosophila abdomen.

Like the Drosophila embryo, the abdomen of the adult consists of alternating anterior (A) and posterior (P) compartments. However the wing is made by only part of one A and part of one P compartment. The abdomen therefore offers an opportunity to compare two compartment borders (A/P is within the segment and P/A intervenes between two segments), and ask if they act differently in pattern formation. In the embryo, abdomen and wing P compartment cells express the selector gene engrailed and secrete Hedgehog protein whilst A compartment cells need the patched and smoothened genes in order to respond to Hedgehog. We made clones of cells with altered activities of the engrailed, patched and smoothened genes. Our results confirm (1) that the state of engrailed, whether 'off' or 'on', determines whether a cell is of A or P type and (2) that Hedgehog signalling, coming from the adjacent P compartments across both A/P and P/A boundaries, organises the pattern of all the A cells. We have uncovered four new aspects of compartments and engrailed in the abdomen. First, we show that engrailed acts in the A compartment: Hedgehog leaves the P cells and crosses the A/P boundary where it induces engrailed in a narrow band of A cells. engrailed causes these cells to form a special type of cuticle. No similar effect occurs when Hedgehog crosses the P/A border. Second, we look at the polarity changes induced by the clones, and build a working hypothesis that polarity is organised, in both compartments, by molecule(s) emanating from the A/P but not the P/A boundaries. Third, we show that both the A and P compartments are each divided into anterior and posterior subdomains. This additional stratification makes the A/P and the P/A boundaries fundamentally distinct from each other. Finally, we find that when engrailed is removed from P cells (of, say, segment A5) they transform not into A cells of the same segment, but into A cells of the same parasegment (segment A6).

Cell proliferation in the attainment of constant sizes and shapes: the Entelechia model.

The Entelechia model is a generative model of morphogenesis where individual cells exhibit surface labels that express scalar difference and planar polarity along two orthogonal axes X and Y. The amount of surface label depends on the level of Martial (M) gene product within each cell. The model assumes that the confrontation of cells on both sides of compartment borders causes an increase in their level of M gene expression. The resulting disparity between the M value of border cells and that of their neighbors induces the latter to divide. After each division the daughter cells increase their own M value, and allocate to the best matching value position. The increase in M value at the borders therefore extends through the anlage in a cascade of proliferation. The Entelechia condition is reached when the border cells attain the species-specific maximal M values, and the value differences between adjacent cells become indistinguishable. Computer simulations reveal that this model accounts for a variety of observations made on imaginal discs, e.g., 1) each disc attains a constant size in terms of number of cells, independently of the growing conditions; 2) clonal restrictions separate populations of cells which proliferate by intercalar growth; 3) dissociated cells are capable of reconstructing original patterns upon reaggregation, and 4) genetic mosaics of morphogenetic mutations show local effects that may differ depending on the position of the mutant cells in the growing anlage.

Proteolysis That Is Inhibited by Hedgehog Targets Cubitus interruptus Protein to the Nucleus and Converts It to a Repressor

Cell–cell communication at anterior/posterior compartment borders in Drosophila involves Hedgehog (Hh), a protein secreted by posterior cells, and Cubitus interruptus (Ci), a protein in the Hh response pathway in anterior cells. Although Ci is thought to have roles as a transcription factor repressing hh expression and activating target genes, it localizes in the cytoplasm of anterior cells. We report here the identification of a domain that tethers Ci in the cytoplasm and show that in some anterior cells, Ci is cleaved to generate a form that lacks the tethering domain. This form translocates to the nucleus where it represses hh and other target genes. Hh inhibits proteolysis of Ci, and we suggest that this inhibition leads to the observed patterns of expression of key target genes at the compartment border.

Creating a Drosophila wing de novo, the role of engrailed, and the compartment border hypothesis

Anterior/posterior compartment borders bisect every Drosophila imaginal disc, and the engrailed gene is essential for their function. We analyzed the role of the engrailed and invected genes in wing discs by eliminating or increasing their activity. Removing engrailed/invected from posterior wing cells created two new compartments: an anterior compartment consisting of mutant cells and a posterior compartment that grew from neighboring cells. In some cases, these compartments formed a complete new wing. Increasing engrailed activity also affected patterning. These findings demonstrate that engrailed both directs the posterior compartment pathway and creates the compartment border. These findings also establish the compartment border as the pre-eminent organizational feature of disc growth and patterning.

Hedgehog is a signaling protein with a key role in patterning Drosophila imaginal discs

The segment polarity genes hedgehog and engrailed are expressed in identical posterior-compartment-specific patterns in both Drosophila embryos and imaginal discs. We show here that the hedgehog protein is secreted, and it can cross embryo parasegment borders and the anterior-posterior compartment border of imaginal discs to neighboring cells that express neither engrailed nor hedgehog. In these cells, it is localized in discrete punctate structures that are sequestered within the polarized epithelium. Analysis of animals that have expressed hedgehog ectopically, or of a mutant that expresses hedgehog abnormally in the anterior compartment of the wing disc, indicates that hedgehog is involved in regulating patched. In the embryo, hedgehog regulation of patched apparently facilitates patched and wingless expression. In the discs, hedgehog regulation of patched and other genes in the anterior compartment helps to establish the proximodistal axis. We propose that the cell-cell communication mediated by hedgehog links the special properties of compartment borders with specification of the proximodistal axis in imaginal development.