Extracellular matrix promotes mammary epithelial growth and differentiation in vitro

Decellularization and Their Significance for Tissue Regeneration in the Era of 3D Bioprinting

Three-dimensional bioprinting is an emerging technology that has high potential application in tissue engineering and regenerative medicine. Increasing advancement and improvement in the decellularization process have led to an increase in the demand for using a decellularized extracellular matrix (dECM) to fabricate tissue engineered products. Decellularization is the process of retaining the extracellular matrix (ECM) while the cellular components are completely removed to harvest the ECM for the regeneration of various tissues and across different sources. Post decellularization of tissues and organs, they act as natural biomaterials to provide the biochemical and structural support to establish cell communication. Selection of an effective method for decellularization is crucial, and various factors like tissue density, geometric organization, and ECM composition affect the regenerative potential which has an impact on the end product. The dECM is a versatile material which is added as an important ingredient to formulate the bioink component for constructing tissue and organs for various significant studies. Bioink consisting of dECM from various sources is used to generate tissue-specific bioink that is unique and to mimic different biometric microenvironments. At present, there are many different techniques applied for decellularization, and the process is not standardized and regulated due to broad application. This review aims to provide an overview of different decellularization procedures, and we also emphasize the different dECM-derived bioinks present in the current global market and the major clinical outcomes. We have also highlighted an overview of benefits and limitations of different decellularization methods and various characteristic validations of decellularization and dECM-derived bioinks.

A review of regulated self-organizing approaches for tissue regeneration

Tissue and organ regeneration is the dynamic process by which a population of cells rearranges into a specific form with specific functions. Traditional tissue regeneration utilizes tissue grafting, cell implantation, and structured scaffolds to achieve clinical efficacy. However, tissue grafting methods face a shortage of donor tissue, while cell implantation may involve leakage of the implanted cells without a supportive 3D matrix. Cell migration, proliferation, and differentiation in structured scaffolds may disorganize and frustrate the artificially pre-designed structures, and sometimes involve immunogenic reactions. To overcome this limitation, the self-organizing properties and innate regenerative capability of tissue/organism formation in the absence of guidance by structured scaffolds has been investigated. This review emphasizes the growing subfield of the regulated self-organizing approach for neotissue formation and describes advances in the subfield using diverse, cutting-edge, inter-disciplinarity technologies. We cohesively summarize the directed self-organization of cells in the micro-engineered cell-ECM system and 3D/4D cell printing. Mathematical modeling of cellular self-organization is also discussed for providing rational guidance to intractable problems in tissue regeneration. It is envisioned that future self-organization approaches integrating biomathematics, micro-nano engineering, and gene circuits developed from synthetic biology will continue to work in concert with self-organizing morphogenesis to enhance rational control during self-organizing in tissue and organ regeneration.

Bearing My Heart: The Role of Extracellular Matrix on Cardiac Development, Homeostasis, and Injury Response

The extracellular matrix (ECM) is an essential component of the heart that imparts fundamental cellular processes during organ development and homeostasis. Most cardiovascular diseases involve severe remodeling of the ECM, culminating in the formation of fibrotic tissue that is deleterious to organ function. Treatment schemes effective at managing fibrosis and promoting physiological ECM repair are not yet in reach. Of note, the composition of the cardiac ECM changes significantly in a short period after birth, concurrent with the loss of the regenerative capacity of the heart. This highlights the importance of understanding ECM composition and function headed for the development of more efficient therapies. In this review, we explore the impact of ECM alterations, throughout heart ontogeny and disease, on cardiac cells and debate available approaches to deeper insights on cell–ECM interactions, toward the design of new regenerative therapies.

Distinct phenotypes of cancer cells on tissue matrix gel

BACKGROUND

Breast cancer cells invading the connective tissues outside the mammary lobule or duct immerse in a reservoir of extracellular matrix (ECM) that is structurally and biochemically distinct from that of their site of origin. The ECM is a spatial network of matrix proteins, which not only provide physical support but also serve as bioactive ligands to the cells. It becomes evident that the dimensional, mechanical, structural, and biochemical properties of ECM are all essential mediators of many cellular functions. To better understand breast cancer development and cancer cell biology in native tissue environment, various tissue-mimicking culture models such as hydrogel have been developed. Collagen I (Col I) and Matrigel are the most common hydrogels used in cancer research and have opened opportunities for addressing biological questions beyond the two-dimensional (2D) cell cultures. Yet, it remains unclear whether these broadly used hydrogels can recapitulate the environmental properties of tissue ECM, and whether breast cancer cells grown on CoI I or Matrigel display similar phenotypes as they would on their native ECM.

METHODS

We investigated mammary epithelial cell phenotypes and metabolic profiles on animal breast ECM-derived tissue matrix gel (TMG), Col I, and Matrigel. Atomic force microscopy (AFM), fluorescence microscopy, acini formation assay, differentiation experiments, spatial migration/invasion assays, proliferation assay, and nuclear magnetic resonance (NMR) spectroscopy were used to examine biological phenotypes and metabolic changes. Student's t test was applied for statistical analyses.

RESULTS

Our data showed that under a similar physiological stiffness, the three types of hydrogels exhibited distinct microstructures. Breast cancer cells grown on TMG displayed quite different morphologies, surface receptor expression, differentiation status, migration and invasion, and metabolic profiles compared to those cultured on Col I and Matrigel. Depleting lactate produced by glycolytic metabolism of cancer cells abolished the cell proliferation promoted by the non-tissue-specific hydrogel.

CONCLUSION

The full ECM protein-based hydrogel system may serve as a biologically relevant model system to study tissue- and disease-specific pathological questions. This work provides insights into tissue matrix regulation of cancer cell biomarker expression and identification of novel therapeutic targets for the treatment of human cancers based on tissue-specific disease modeling.

Effectiveness of a novel, completely biomaterial valved pulmonary arterial conduit: An in vivo study

As a pre-clinical assessment, the present study aimed to investigate the safety and effectiveness of a novel valved pulmonary arterial conduit constructed entirely from biomaterials by transplanting it in the outflow tract of the right ventricle in sheep. Under extracorporeal circulation, the valved pulmonary arterial conduit was used to replace the pulmonary artery of sheep with a beating heart. The performance was assessed at 30, 90 and 180 days post-surgery. Hemodynamic and structural changes were evaluated, and safety was assessed after 180 postoperative days. The hemodynamic effect and biosafety of the implant were further evaluated by observing the changes in various pressure indicators of the heart, echocardiographic results, anatomical and pathological examination results, liver and kidney functions, routine blood tests, a blood coagulation test, and other test results following implantation of the purely biotic valved conduit. The conduit was successfully implanted in 12 sheep and no mortality occurred postoperatively. During the 180-day follow-up, there was no obvious stenosis or regurgitation of the right ventricular outflow tract and pulmonary valve after valved conduit implantation. The findings of autopsy, pathology and laboratory examinations were unremarkable. The implantation of this biosynthetic vascular graft into animals meets the safety and effectiveness requirements for clinical application. This pulmonary arterial conduit has potential clinical application for children with complex congenital heart disease who require pulmonary artery reconstruction to achieve a radical cure.

Evaluation of Different Decellularization Protocols on the Generation of Pancreas-Derived Hydrogels

Different approaches have investigated the effects of different extracellular matrices (ECMs) and three-dimensional (3D) culture on islet function, showing encouraging results. Ideally, the proper scaffold should mimic the biochemical composition of the native tissue as it drives numerous signaling pathways involved in tissue homeostasis and functionality. Tissue-derived decellularized biomaterials can preserve the ECM composition of the native tissue making it an ideal scaffold for 3D tissue engineering applications. However, the decellularization process may affect the retention of specific components, and the choice of a proper detergent is fundamental in preserving the native ECM composition. In this study, we evaluated the effect of different decellularization protocols on the mechanical properties and biochemical composition of pancreatic ECM (pECM) hydrogels. Fresh porcine pancreas tissue was harvested, cut into small pieces, rinsed in water, and treated with two different detergents (sodium dodecyl sulfate [SDS] or Triton X-100) for 1 day followed by 3 days in water. Effective decellularization was confirmed by PicoGreen assay, Hoescht, and H&E staining, showing no differences among groups. Use of a protease inhibitor (PI) was also evaluated. Effective decellularization was confirmed by PicoGreen assay and hematoxylin and eosin (H&E) staining, showing no differences among groups. Triton-treated samples were able to form a firm hydrogel under appropriate conditions, while the use of SDS had detrimental effects on the gelation properties of the hydrogels. ECM biochemical composition was characterized both in the fresh porcine pancreas and all decellularized pECM hydrogels by quantitative mass spectrometry analysis. Fibrillar collagen was the major ECM component in all groups, with all generated hydrogels having a higher amount compared with fresh pancreas. This effect was more pronounced in the SDS-treated hydrogels when compared with the Triton groups, showing very little retention of other ECM molecules. Conversely, basement membrane and matricellular proteins were better retained when the tissue was pretreated with a PI and decellularized in Triton X-100, making the hydrogel more similar to the native tissue. In conclusion, we showed that all the protocols evaluated in the study showed effective tissue decellularization, but only when the tissue was pretreated with a PI and decellularized in Triton detergent, the biochemical composition of the hydrogel was closer to the native tissue ECM. Impact Statement The article compares different methodologies for the generation of a pancreas-derived hydrogel for tissue engineering applications. The biochemical characterization of the newly generated hydrogel shows that the material retains all the extracellular molecules of the native tissue and is capable of sustaining functionality of the encapsulated beta-cells.

Regenerative Materials for Surgical Reconstruction: Current Spectrum of Materials and a Proposed Method for Classification

Chronic wound management is an enormous economic strain and quality-of-life issue for patients. Current treatments are ineffective or expensive and invasive. Materials (native and artificial) can act as the basis to enhance wound repair but often fall short of complete healing. The therapeutic index of materials have often been enhanced by combining them with drug or biologic elution technologies. Combination of materials with living drugs (cells) presents a new paradigm for enhancing therapy. Cell material interaction and therapeutic output will depend on variables ascribed to the living drug as well as variables ascribed to the underlying matrix. In this article, we review medical matrices currently approved by the US Food and Drug Administration (FDA) that would likely be the first generation of materials to be used in this manner. Currently there are hundreds of different materials on the market. Identification of the right combinations would benefit from a classification scheme to group materials with similar composition or derivation. We provide a classification scheme and FDA documentation references that should provide researchers and clinicians a starting point for testing these materials in the laboratory and rapidly transitioning cell therapies to the bedside.

Species-specific models in toxicology: in vitro epithelial barriers

Species-specific in vitro epithelial barriers represent interesting predictive tools for risk assessment evaluation in toxicological studies. Moreover, these models could be applied either as stand-alone methods for the study of absorption, bioavailability, excretion, transport, effects of xenobiotics, or through an Integrated Testing Strategy. The aim of this review is to give a comprehensive overview of in vitro species-specific epithelial barrier models from bovine, dog and swine. Bovine mammary epithelial barrier as a fundamental instrument for the evaluation of the toxicant excretion, the blood brain barrier as a useful first approach in toxicological and pharmacological studies, the porcine intestinal barrier, the canine skin barrier, and finally the pulmonary barrier from bovine and swine species are described in this review.

Acellular Cauda Equina Allograft as Main Material Combined with Biodegradable Chitin Conduit for Regeneration of Long-Distance Sciatic Nerve Defect in Rats

Autologous nerve grafting (ANG), the gold standard treatment for peripheral nerve defects, still has many restrictions. In this study, the acellular cauda equina allograft (ACEA), which consists of biodegradable chitin conduit and acellular cauda equina, is developed. The cauda equina is able to complete decellularization more quickly and efficiently than sciatic nerves under the same conditions, and it is able to reserve more basal lamina tube. In vitro, ACEA shows superior guidance capacity for the regeneration of axons and migration of Schwann cells compared to acellular sciatic nerve allograft (ASNA) in dorsal root ganglion culture. In vivo, ACEA is used to bridge 15 mm long-distance defects in rat sciatic nerves. On day 21 after transplantation, the regenerative distance of neurofilaments in the grafting segment is not significantly different between the ACEA and ANG groups. At week 12, ACEA group shows better sciatic nerve repair than chitin conduit only and ASNA groups, and the effect is similar to that in the ANG group as determined by gait analysis, neural electrophysiological, and histological analyses. The above results suggest that the ACEA has the potential to become a new biological material as a replacement for autografting in the treatment of long-distance nerve defects.

Different approaches to heart valve decellularization: A comprehensive overview of the past 30 years

Xenogeneic decellularized heart valve scaffolds have the potential to overcome the limitations of existing bioprosthetic heart valves that have limited duration due to calcification and tissue degeneration phenomena. This article presents a review of 30 years of decellularization approaches adopted in cardiovascular tissue engineering, with a focus on the use, either individually or in combination, of different detergents. The safety and efficacy of cell-removal procedures are specifically reported and discussed, as well as the structure and biomechanics of the treated extracellular matrix (ECM). Detergent residues within the ECM, production of hyaluronan fragments, safe removal of cellular debris, and the persistence of the alpha-Gal epitope after the decellularization treatments are of particular interest as parameters for the identification of the best tissue for the manufacture of bioprostheses. Special attention has also been given to key factors that should be considered in the manufacture of the next generation of xenogeneic bioprostheses, where tissues must retain the ability to be remodeled and to grow in weight along with body reshaping.

Stem Cells in the Treatment of Insulin-Dependent Diabetes Mellitus

Diabetes affects over 350 million people worldwide, with the figure projected to rise to nearly 500 million over the next 20 years, according to the World Health Organization. Insulin-dependent diabetes mellitus (type 1 diabetes) is an endocrine disorder caused by an autoimmune reaction that destroys insulin-producing β-cells in the pancreas, which leads to insulin deficiency. Administration of exogenous insulin remains at the moment the treatment mainstay. This approach helps to regulate blood glucose levels and significantly increases the life expectancy of patients. However, type 1 diabetes is accompanied by long-term complications associated with the systemic nature of the disease and metabolic abnormalities having a profound impact on health. Of greater impact would be a therapeutic approach which would overcome these limitations by better control of blood glucose levels and prevention of acute and chronic complications. The current efforts in the field of regenerative medicine are aimed at finding such an approach. In this review, we discuss the time-honored technique of donor islets of Langerhans transplantation. We also focus on the use of pluripotent stem and committed cells and cellular reprogramming. The molecular mechanisms of pancreatic differentiation are highlighted. Much attention is devoted to the methods of grafts delivery and to the materials used during its creation.

Tissue stiffness dictates development, homeostasis, and disease progression

Tissue development is orchestrated by the coordinated activities of both chemical and physical regulators. While much attention has been given to the role that chemical regulators play in driving development, researchers have recently begun to elucidate the important role that the mechanical properties of the extracellular environment play. For instance, the stiffness of the extracellular environment has a role in orienting cell division, maintaining tissue boundaries, directing cell migration, and driving differentiation. In addition, extracellular matrix stiffness is important for maintaining normal tissue homeostasis, and when matrix mechanics become imbalanced, disease progression may ensue. In this article, we will review the important role that matrix stiffness plays in dictating cell behavior during development, tissue homeostasis, and disease progression.

Current Bioengineering Methods for Whole Kidney Regeneration

Kidney regeneration is likely to provide an inexhaustible source of tissues and organs for immunosuppression-free transplantation. It is currently garnering considerable attention and might replace kidney dialysis as the ultimate therapeutic strategy for renal failure. However, anatomical complications make kidney regeneration difficult. Here, we review recent advances in the field of kidney regeneration, including (i) the directed differentiation of induced pluripotent stem cells/embryonic stem cells into kidney cells; (ii) blastocyst decomplementation; (iii) use of a decellularized cadaveric scaffold; (iv) embryonic organ transplantation; and (v) use of a nephrogenic niche for growing xenoembryos for de novo kidney regeneration from stem cells. All these approaches represent potentially promising therapeutic strategies for the treatment of patients with chronic kidney disease. Although many obstacles to kidney regeneration remain, we hope that innovative strategies and reliable research will ultimately allow the restoration of renal function in patients with end-stage kidney disease.

Histologic characterization of acellular dermal matrices in a porcine model of tissue expander breast reconstruction

BACKGROUND

Acellular dermal matrices (ADMs) have been commonly used in expander-based breast reconstruction to provide inferolateral prosthesis coverage. Although the clinical performance of these biologic scaffold materials varies depending on a number of factors, an in-depth systematic characterization of the host response is yet to be performed. The present study evaluates the biochemical composition and structure of two ADMs, AlloDerm(®) Regenerative Tissue Matrix and AlloMax™ Surgical Graft, and provides a comprehensive spatiotemporal characterization in a porcine model of tissue expander breast reconstruction.

METHODS

Each ADM was characterized with regard to thickness, permeability, donor nucleic acid content, (residual double-stranded DNA [dsDNA]), and growth factors (basic fibroblast growth factor [bFGF], vascular endothelial growth factor [VEGF], and transforming growth factor-beta 1 [TGF-β1]). Cytocompatibility was evaluated by in vitro cell culture on the ADMs. The host response was evaluated at 4 and 12 weeks at various locations within the ADMs using established metrics of the inflammatory and tissue remodeling response: cell infiltration, multinucleate giant cell formation, extent of ADM remodeling, and neovascularization.

RESULTS

AlloMax incorporated more readily with surrounding host tissue as measured by earlier and greater cell infiltration, fewer foreign body giant cells, and faster remodeling of ADM. These findings correlated with the in vitro composition and cytocompatibility analysis, which showed AlloMax to more readily support in vitro cell growth.

CONCLUSIONS

AlloMax and AlloDerm demonstrated distinct remodeling characteristics in a porcine model of tissue expander breast reconstruction.

Regeneration and bioengineering of the kidney: current status and future challenges

The prevalence of chronic kidney disease continues to outpace the development of effective treatment strategies. For patients with advanced disease, renal replacement therapies approximate the filtration functions of the kidney at considerable cost and inconvenience, while failing to restore the resorptive and endocrine functions. Allogeneic transplantation remains the only restorative treatment, but donor shortage, surgical morbidity and the need for lifelong immunosuppression significantly limit clinical application. Emerging technologies in the fields of regenerative medicine and tissue engineering strive to address these limitations. We review recent advances in cell-based therapies, primordial allografts, bio-artificial organs and whole-organ bioengineering as they apply to renal regeneration. Collaborative efforts across these fields aim to produce a bioengineered kidney capable of restoring renal function in patients with end-stage disease.

Extracellular Matrix Scaffold Technology for Bioartificial Pancreas Engineering: State of the Art and Future Challenges

Emergent technologies in regenerative medicine may soon overcome the limitations of conventional diabetes therapies. Collaborative efforts across the subfields of stem cell technology, islet encapsulation, and biomaterial carriers seek to produce a bioengineered pancreas capable of restoring endocrine function in patients with insulin-dependent diabetes. These technologies rely on a robust understanding of the extracellular matrix (ECM), the supportive 3-dimensional network of proteins necessary for cellular attachment, proliferation, and differentiation. Although these functions can be partially approximated by biosynthetic carriers, novel decellularization protocols have allowed researchers to discover the advantages afforded by the native pancreatic ECM. The native ECM has proven to be an optimal platform for recellularization and whole-organ pancreas bioengineering, an exciting new field with the potential to resolve the dire shortage of transplantable organs. This review seeks to contextualize recent findings, discuss current research goals, and identify future challenges of regenerative medicine as it applies to diabetes management.

The extracellular matrix locally regulates asynchronous concurrent lactation in tammar wallaby (Macropus eugenii)

Asynchronous concurrent lactation (ACL) is an extreme lactation strategy in macropod marsupials including the tammar wallaby, that may hold the key to understanding local control of mammary epithelial cell function. Marsupials have a short gestation and a long lactation consisting of three phases; P2A, P2B and P3, representing early, mid and late lactation respectively and characterised by profound changes in milk composition. A lactating tammar is able to concurrently produce phase 2A and 3 milk from adjacent glands in order to feed a young newborn and an older sibling at heel. Physiological effectors of ACL remain unknown and in this study the extracellular matrix (ECM) is investigated for its role in switching mammary phenotypes between phases of tammar wallaby lactation. Using the level of expression of the genes for the phase specific markers tELP, tWAP, and tLLP-B representing phases 2A, 2B and 3 respectively we show for the first time that tammar wallaby mammary epithelial cells (WallMECs) extracted from P2B acquire P3 phenotype when cultured on P3 ECM. Similarly P2A cells acquire P2B phenotype when cultured on P2B ECM. We further demonstrate that changes in phase phenotype correlate with phase-specific changes in ECM composition. This study shows that progressive changes in ECM composition in individual mammary glands provide a local regulatory mechanism for milk protein gene expression thereby enabling the mammary glands to lactate independently.

Porcine pancreas extracellular matrix as a platform for endocrine pancreas bioengineering

Emergent technologies of regenerative medicine have the potential to overcome the limitations of organ transplantation by supplying tissues and organs bioengineered in the laboratory. Pancreas bioengineering requires a scaffold that approximates the biochemical, spatial and vascular relationships of the native extracellular matrix (ECM). We describe the generation of a whole organ, three-dimensional pancreas scaffold using acellular porcine pancreas. Imaging studies confirm that our protocol effectively removes cellular material while preserving ECM proteins and the native vascular tree. The scaffold was seeded with human stem cells and porcine pancreatic islets, demonstrating that the decellularized pancreas can support cellular adhesion and maintenance of cell functions. These findings advance the field of regenerative medicine towards the development of a fully functional, bioengineered pancreas capable of establishing and sustaining euglycemia and may be used for transplantation to cure diabetes mellitus.

Rapidly quantifying drug sensitivity of dispersed and clumped breast cancer cells by mass profiling

Live cell mass profiling is a promising new approach for rapidly quantifying responses to therapeutic agents through picogram-scale changes in cell mass over time. A significant barrier in mass profiling is the inability of existing methods to handle pleomorphic cellular clusters and clumps, which are more commonly present in patient-derived samples or tissue cultures than are isolated single cells. Here we demonstrate automated Live Cell Interferometry (LCI) as a rapid and accurate quantifier of the sensitivity of single cell and colony-forming human breast cancer cell lines to the HER2-directed monoclonal antibody, trastuzumab (Herceptin). The relative sensitivities of small samples (<500 cells) of four breast cancer cell lines were determined tens-to-hundreds of times faster than is possible with traditional proliferation assays. These LCI advances in clustered sample assessment and speed open up the possibility for therapeutic response testing of patient-derived solid tumor samples, which are viable only for short periods ex vivo and likely to be in the form of cell aggregates and clusters.

Effect of tissue specificity on the performance of extracellular matrix in improving endothelialization of cardiovascular implants

Natural extracellular matrix (ECM) deposited in situ by cultured endothelial cells (ECs) has been proven effective in accelerating endothelialization of titanium (Ti) cardiovascular implants (CVIs) in our previous studies. In this study, the ECM deposited by smooth muscle cells (SMCs) was used in comparison to investigate the effects of tissue specificity of the ECM on the ability to accelerate endothelialization of CVIs. The results demonstrated that the ECM deposited by ECs and SMCs (EC-ECM, SMC-ECM, respectively) differed considerably in components and fibril morphology. Surface modification of Ti CVIs with both types of natural ECM was effective in improving their in vitro hemocompatibility and cytocompatibility simultaneously. However, the endothelialization of ECM-modified Ti CVIs in a canine model demonstrated a high tissue specificity of the ECM. Although the ECM deposited by SMCs (SMC-ECM) induced fewer platelet adhesion and sustained better growth and viability of ECs in vitro, its performance in accelerating in vivo endothelialization of Ti CVIs was extremely poor. In contrast, the ECM deposited by ECs (EC-ECM) led to complete endothelium formation in vivo.

On the shoulders of giants: a historical perspective of unique experimental methods in mammary gland research

While most organs undergo development in utero, the mouse mammary gland orchestrates five major developmental stages following birth: pre-puberty, puberty, pregnancy, lactation, and involution. Induced by both local and systemic factors, these five developmental stages transpire with dramatic alterations in glandular morphology and cellular function. As an experimental system, the mammary gland provides remarkable accessibility to processes regulating stem cell function, hormone response, and epithelial-stromal-extracellular matrix interactions. This review will provide a historical perspective of the unique in vitro and in vivo techniques used to study the mammary gland and how these methods have provided valuable insight into the biology of this organ.

Biologic scaffolds composed of central nervous system extracellular matrix

Acellular biologic scaffolds are commonly used to facilitate the constructive remodeling of three of the four traditional tissue types: connective, epithelial, and muscle tissues. However, the application of extracellular matrix (ECM) scaffolds to neural tissue has been limited, particularly in the central nervous system (CNS) where intrinsic regenerative potential is low. The ability of decellularized liver, lung, muscle, and other tissues to support tissue-specific cell phenotype and function suggests that CNS-derived biologic scaffolds may help to overcome barriers to mammalian CNS repair. A method was developed to create CNS ECM scaffolds from porcine optic nerve, spinal cord, and brain, with decellularization verified against established criteria. CNS ECM scaffolds retained neurosupportive proteins and growth factors and, when tested with the PC12 cell line in vitro, were cytocompatible and stimulated proliferation, migration, and differentiation. Urinary bladder ECM (a non-CNS ECM scaffold) was also cytocompatible and stimulated PC12 proliferation but inhibited migration rather than acting as a chemoattractant over the same concentration range while inducing greater rates of PC12 differentiation compared to CNS ECM. These results suggest that CNS ECM may provide tissue-specific advantages in CNS regenerative medicine applications and that ECM scaffolds in general may aid functional recovery after CNS injury.

Three-dimensional cultures of mouse mammary epithelial cells

The mammary gland is an ideal "model organism" for studying tissue specificity and gene expression in mammals: it is one of the few organs that develop after birth and it undergoes multiple cycles of growth, differentiation and regression during the animal's lifetime in preparation for the important function of lactation. The basic "functional differentiation" unit in the gland is the mammary acinus made up of a layer of polarized epithelial cells specialized for milk production surrounded by myoepithelial contractile cells, and the two-layered structure is surrounded by basement membrane. Much knowledge about the regulation of mammary gland development has been acquired from studying the physiology of the gland and of lactation in rodents. Culture studies, however, were hampered by the inability to maintain functional differentiation on conventional tissue culture plastic. We now know that the microenvironment, including the extracellular matrix and tissue architecture, plays a crucial role in directing functional differentiation of organs. Thus, in order for culture systems to be effective experimental models, they need to recapitulate the basic unit of differentiated function in the tissue or organ and to maintain its three-dimensional (3D) structure. Mouse mammary culture models evolved from basic monolayers of cells to an array of complex 3D systems that observe the importance of the microenvironment in dictating proper tissue function and structure. In this chapter, we focus on how 3D mouse mammary epithelial cultures have enabled investigators to gain a better understanding of the organization, development and function of the acinus, and to identify key molecular, structural, and mechanical cues important for maintaining mammary function and architecture. The accompanying chapter of Vidi et al. describes 3D models developed for human cells. Here, we describe how mouse primary epithelial cells and cell lines--essentially those we use in our laboratory--are cultured in relevant 3D microenvironments. We focus on the design of functional assays that enable us to understand the intricate signaling events underlying mammary gland biology, and address the advantages and limitations of the different culture settings. Finally we also discuss how advances in bioengineering tools may help towards the ultimate goal of building tissues and organs in culture for basic research and clinical studies.

Mechanisms by which the extracellular matrix and integrin signaling act to regulate the switch between tumor suppression and tumor promotion

Cell adhesion to the extracellular matrix (ECM) is necessary for development of the mammary gland, and to maintain the normal architecture and function of the gland. Cells adhere to the ECM via the integrin family of trans-membrane receptors, which signal to control mammary-specific gene expression and regulate cell proliferation and survival. During tumor formation, the ECM is extensively remodeled and signaling through integrins is altered such that cells become proliferative and invasive. A key regulator of whether integrin-mediated adhesion will promote tumor suppression or tumor formation is the stiffness of the stromal ECM. The normal mammary gland is typically surrounded by a loose collagenous stroma. An increase in the deposition of collagen and other stromal components is associated with mammographic density, which is one of the greatest risk factors for developing breast carcinoma. Several groups have demonstrated that increased stromal ECM density results in a matrix that is stiffer. Cells sense the stiffness of their surrounding ECM by Rho-mediated contraction of the actin-myosin cytoskeleton. If the surrounding ECM is stiffer than the cell's ability to contract it, then the tensile forces that result are able to drive the clustering of integrins and assemble adhesion signaling complexes. The result is subsequent activation of signaling pathways including FAK, ERK, and PI3K that drive cell proliferation and survival. In contrast, focal complexes are not formed in a compliant matrix, and activation of FAK and pERK is diminished, resulting in control of proliferation. Signaling from FAK moreover regulates p53 and miR-200 members, which control apoptosis and epithelial phenotype, such that a compliant matrix is predicted to promote normal mammary gland architecture and suppress tumor formation.

An overview of tissue and whole organ decellularization processes

Biologic scaffold materials composed of extracellular matrix (ECM) are typically derived by processes that involve decellularization of tissues or organs. Preservation of the complex composition and three-dimensional ultrastructure of the ECM is highly desirable but it is recognized that all methods of decellularization result in disruption of the architecture and potential loss of surface structure and composition. Physical methods and chemical and biologic agents are used in combination to lyse cells, followed by rinsing to remove cell remnants. Effective decellularization methodology is dictated by factors such as tissue density and organization, geometric and biologic properties desired for the end product, and the targeted clinical application. Tissue decellularization with preservation of ECM integrity and bioactivity can be optimized by making educated decisions regarding the agents and techniques utilized during processing. An overview of decellularization methods, their effect upon resulting ECM structure and composition, and recently described perfusion techniques for whole organ decellularization techniques are presented herein.

Isolation of mammary-specific extracellular matrix to assess acute cell-ECM interactions in 3D culture

Studies of mammary epithelial cells (MECs) cultured with reconstituted basement membrane proteins derived from EHS tumors have contributed greatly to the understanding of both normal physiology and transformation. Only when plated on such biologically relevant substratum are MECs able to form morphologically correct, differentiated structures, highlighting a critical role for extracellular matrix (ECM) proteins in MEC organization and function. Here, we describe methods modified from the original EHS matrix protocol for isolating tissue-specific ECM from rat mammary glands, and for subsequent use in short-term 3D cell culture models designed to assess acute cell-ECM interactions. Using this protocol, the final matrix is enriched up to 58-fold for ECM proteins such as fibronectin and laminin, while cellular proteins such as GAPDH are reduced 98-fold. We have previously shown that MECs plated in mammary-specific ECM form more elaborate duct-like and alveolar-like structures compared to MECs plated in Matrigel™, demonstrating the biological relevance of tissue-specific ECM. Use of mammary-specific ECM in 3D cell culture models will further our ability to study the intricate interplay between a cell and its microenvironment, and permit identification of modifying factors.

Mechanical control of tissue and organ development

Many genes and molecules that drive tissue patterning during organogenesis and tissue regeneration have been discovered. Yet, we still lack a full understanding of how these chemical cues induce the formation of living tissues with their unique shapes and material properties. Here, we review work based on the convergence of physics, engineering and biology that suggests that mechanical forces generated by living cells are as crucial as genes and chemical signals for the control of embryological development, morphogenesis and tissue patterning.

Perlecan domain IV peptide stimulates salivary gland cell assembly in vitro

Treatment of xerostomia would benefit from development of a functional implantable artificial salivary gland. Salivary gland tissue from surgical patients was assessed by histology and immunohistochemistry to establish the phenotype of normal salivary gland cells including the native basement membranes. Ductal and acinar cells were identified in tissue and cultured cells from dispersed tissue. High levels of laminin and perlecan/HSPG2 (heparan sulfate proteoglycan 2) were noted in basement membranes, and perlecan also was secreted and organized by cultured acinar populations, which formed lobular structures that mimicked intact glands when cultured on Matrigel or a bioactive peptide derived from domain IV of perlecan. On either matrix, large acini-like lobular structures grew and formed connections between the lobes. alpha-Amylase secretion was confirmed by staining and activity assay. Biomarkers, including tight junction protein E-cadherin and water channel protein aquaporin 5 found in tissue, were expressed in cultured acinar cells. Cells cultured on Matrigel or domain IV of perlecan peptide organized stress fibers and activated focal adhesion kinase. We report a novel technique to isolate acinar cells from human salivary gland and identify a human peptide sequence in perlecan that triggers differentiation of salivary gland cells into self-assembling acini-like structures that express essential biomarkers and which secrete alpha-amylase.

Modeling dynamic reciprocity: engineering three-dimensional culture models of breast architecture, function, and neoplastic transformation

In order to understand why cancer develops as well as predict the outcome of pharmacological treatments, we need to model the structure and function of organs in culture so that our experimental manipulations occur under physiological contexts. This review traces the history of the development of a prototypic example, the three-dimensional (3D) model of the mammary gland acinus. We briefly describe the considerable information available on both normal mammary gland function and breast cancer generated by the current model and present future challenges that will require an increase in its complexity. We propose the need for engineered tissues that faithfully recapitulate their native structures to allow a greater understanding of tissue function, dysfunction, and potential therapeutic intervention.

Hepatocyte growth factor and neuregulin in mammary gland cell morphogenesis

Organ culture and transplantation experiments in the early 1960s and 1970s have demonstrated that growth and morphogenesis of the epithelium of the mammary gland are controlled by mesenchymal-epithelial interactions. The identification of molecules that provide the essential signals exchanged in mesenchymal-epithelial interactions is an area of active research. Recent evidence suggests that morphogenic programs of epithelia can be triggered by mesenchymal factors that signal via tyrosine kinase receptors. This review concentrates on the effects of two mesenchymal factors, Hepatocyte Growth Factor/Scatter Factor and neuregulin, on morphogenesis and differentiation of mammary epithelial cells in vitro and signalling pathways involved during morphogenesis of mammary epithelial cells.

Elastin gene expression in benign prostatic hyperplasia

BACKGROUND

Benign prostatic hyperplasia (BPH) is mainly a stromal process, showing an increased ratio of stromal to epithelial elements, a collagen type III downregulation, and a collagen types I and IV upregulation. Little is known about elastin gene expression in BPH tissues due to difficulties related to extensive alternative splicing of the elastin gene. Therefore, we analyzed and quantified elastin gene expression in BPH.

METHODS

A competitive reverse transcriptase-polymerase chain reaction (competitive RT-PCR) quantitative technique was used, and a quantitative elastin mRNA analysis with normal (n = 10) and BPH (n = 12) tissues was performed with two newly designed elastin primers. Small tissue samples (4-8 mg) were homogenized and sonicated, and cDNA was synthesized from mRNA using a RT reaction. Various target (wild-type) elastin cDNAs with unknown concentrations were competitively coamplified with known serial dilutions of the control mutant template, differing from the target cDNA by a short deletion. Gel fractions and computerized densitometry, were performed and cDNA concentration was calculated by linear regression.

RESULTS

The primers identified in our study (BOB-1 and BOB-2) accurately amplified a consistent length of the elastin cDNA, avoiding areas of alternative splicing. The average elastin mRNA concentration in BPH tissues was 53 attomole/mg +/- 11.6 vs. 140.6 attomole/mg +/- 19.6 in normal prostatic tissue (P = 0.019). The variation within every sample was less than 10%.

CONCLUSIONS

Our observations suggest a significant downregulation (70%) of the elastin mRNA gene in the transition zone of BPH patients.

Reconstitution of mammary gland development in vitro: requirement of c-met and c-erbB2 signaling for branching and alveolar morphogenesis

We have established a cell culture system that reproduces morphogenic processes in the developing mammary gland. EpH4 mouse mammary epithelial cells cultured in matrigel form branched tubules in the presence of hepatocyte growth factor/scatter factor (HGF/SF), the ligand of the c-met tyrosine kinase receptor. In contrast, alveolar structures are formed in the presence of neuregulin, a ligand of c-erbB tyrosine kinase receptors. These distinct morphogenic responses can also be observed with selected human mammary carcinoma tissue in explant culture. HGF/SF-induced branching was abrogated by the PI3 kinase inhibitors wortmannin and LY294002. In contrast, neuregulin- induced alveolar morphogenesis was inhibited by the MAPK kinase inhibitor PD98059. The c-met-mediated response could also be evoked by transfection of a c-met specific substrate, Gab1, which can activate the PI3 kinase pathway. An activated hybrid receptor that contained the intracellular domain of c-erbB2 receptor suffices to induce alveolar morphogenesis, and was observed in the presence of tyrosine residues Y1028, Y1144, Y1201, and Y1226/27 in the substrate-binding domain of c-erbB2. Our data demonstrate that c-met and c-erbB2 signaling elicit distinct morphogenic programs in mammary epithelial cells: formation of branched tubules relies on a pathway involving PI3 kinase, whereas alveolar morphogenesis requires MAPK kinase.

Control of fibronectin receptor expression by fibronectin: Antisense fibronectin RNA downmodulates the induction of fibronectin receptor by transforming growth factor β1

The results of our previous studies of mouse embryo fibroblasts showed that fibronectin expression and fibronectin receptor expression are tightly coregulated and that fibronectin modulates expression of its receptor in response to treatment with the differentiation-inducing agent N,N,-dimethylformamide (Varani and Chakrabarty, 1990, J. Cell. Physiol., 143:445-454; Huang et al., 1994, J. Cell. Physiol., 161:470-482). We also found that transforming growth factor beta1 (TGFbeta1) induces a more differentiated phenotype in the epithelium-derived human colon carcinoma cell line Moser and upregulates the expression of both fibronectin and its receptor (Huang and Chakrabarty, 1994, Int. J. Cancer, 57:742-746). By expressing antisense fibronectin RNA in Moser cells, we have downregulated fibronectin mRNA expression and thus blocked the ability of TGFbeta1 to induce fibronectin expression (Huang and Chakrabarty, 1994, J. Biol. Chem., 269:28764-28768). In this study, we examined the effect of antisense fibronectin RNA expression on the induction of fibronectin receptor by TGFbeta1 and tested the hypothesis that the induction of fibronectin expression by TGFbeta1 is required for the induction of fibronectin receptor expression. Blocking fibronectin induction by TGFbeta1 attenuated the ability of TGFbeta1 to upregulate the expression of cell-surface fibronectin receptors, alpha5beta1 integrin expression, and adhesion to extracellular matrix fibronectin. We therefore conclude that induction of fibronectin expression is required for optimal upregulation of fibronectin receptor expression by TGFbeta1.

Three-dimensional mammary primary culture model systems

Model systems have been developed to investigate the complex and coordinated regulation of mammary gland development and transformation. Primary cultures, using newly isolated cells or tissue, are optimal for such studies since, in comparison to immortalized cell lines, the normal signal transduction pathways are presumed to be intact. Three such models are described, including whole organ culture, mammary epithelial cell (MEC) organoids, and MEC-stromal cocultures. Studies using whole-organ culture have the advantage that the normal glandular architecture remains intact, the MEC can undergo lobuloalveolar development and express milk proteins in a hormone dependent manner, and, following hormonal withdrawal, undergo involution. Moreover, transformation of the MEC is readily accomplished. Culture of isolated MEC organoids within an EHS-derived reconstituted basement membrane permits extensive proliferation, branching end bud and alveolar morphogenesis, and accumulation of milk protein and lipid in a physiologically relevant hormone- and growth factor-dependent manner. This model can thus be utilized to investigate the mechanism by which various modulators exert their direct effects on the epithelium. Finally, in view of compelling evidence for stromal-epithelial interactions during normal mammary gland development, and potentially also during the development of malignancy, models in which MEC can be cocultured with enriched populations of stroma offer considerable potential as a tool to understand the nature and mechanisms of the interactions that occur during the various developmental states, and how such interactions may go awry during carcinogenesis.

Collagen matrices attenuate the collagen-synthetic response of cultured fibroblasts to TGF-beta

Transforming growth factor-beta, a potent modulator of cell function, induces fibroblasts cultured on plastic to increase collagen synthesis. In 5- and 7-day porcine skin wounds, which have minimal to moderate collagen matrix, respectively, transforming growth factor-beta and type I procollagen were coordinately expressed throughout the granulation tissue. However, in 10-day collagen-rich granulation tissue type I procollagen expression diminished despite persistence of transforming growth factor-beta. To investigate whether collagen matrix attenuates the collagen-synthetic response of fibroblasts to transforming growth factor-beta, we cultured human dermal fibroblasts in conditions that simulate collagen-rich granulation tissue. Therefore, human dermal fibroblasts were suspended in attached collagen gels and collagen and noncollagen production was assayed in the absence and presence of transforming growth factor-beta. Although transforming growth factor-beta stimulated collagen synthesis by fibroblasts cultured in the collagen gels, these fibroblasts consistently produced less collagen than similarly treated fibroblasts cultured on plastic. This phenomenon was not secondary to nonspecific binding of transforming growth factor-beta to the collagen matrix. Fibroblasts cultured in a fibrin gel responded to transforming growth factor-beta similarly to fibroblasts cultured on plastic. Using immunofluorescence probes to type I procollagen, we observed that transforming growth factor-beta increased type I procollagen expression in most fibroblasts cultured on plastic, but only in occasional fibroblasts cultured in collagen gels. From these data we conclude that collagen matrices attenuate the collagen synthetic response of fibroblast to transforming growth factor-beta in vitro and possibly in vivo.

HH2A, an immortalized bovine mammary epithelial cell line, expresses the gene encoding mammary derived growth inhibitor (MDGI)

We have established and partially characterized a spontaneously immortalized bovine mammary epithelial cell line, designated HH2a. The cells express the gene encoding for mammary derived growth inhibitor (MDGI) when grown on released collagen gels in the presence of lactogenic hormones. This is the first report of a cell line that expresses MDGI. Immunohistochemical studies showed that HH2a cells contain keratin intermediate filaments and desmosomes. When plated on confluent monolayer of live fibroblasts, HH2a cells extensively contacted with fibroblasts. When embedded in the collagen gels, they rearranged themselves to produce three-dimensional duct-like outgrowths extending into the matrix. The HH2a cell line should be useful in investigations of the roles of cell-cell and cell-extracellular interactions in regulation of breast epithelial cell proliferation, and of the hormonal regulation of MDGI gene expression.

Control of AKR fibroblast phenotype by fibronectin: regulation of cell-surface fibronectin binding receptor by fibronectin

Results of previous studies show that the expression of fibronectin and its cell-surface fibronectin binding receptor is coregulated in 3-methylchloranthrene transformation of normal AKR-2B cells to form AKR-MCA cells and in N,N,-dimethylformamide (DMF) induction of differentiation of transformed AKR-MCA cells (1990, J. Cell. Physiol., 143:445). In this study, we tested the coregulation hypothesis by transfection experiments using an antisense fibronectin expression vector. We determined the effect of antisense fibronectin RNA expression on untransformed AKR-2B cells, and on the responses of transformed AKR-MCA cells to DMF treatment. Expression of antisense fibronectin RNA in AKR-2B cells down-modulated fibronectin production, reduced adhesion to extracellular fibronectin, and altered cellular morphology. Saturation binding and Scatchard analyses using radiolabelled fibronectin revealed a concurrent down-modulation of cell-surface fibronectin binding sites, but the binding affinity of the receptor for the ligand was not affected. Immunoblotting and immunostaining revealed down-modulation of the expression of alpha 5 beta 1 integrins. Expression of antisense fibronectin RNA in AKR-MCA cells down-modulated the ability of DMF to restore normal fibronectin production, cell-surface fibronectin binding receptor, adhesion to extracellular fibronectin, and cellular morphology. These studies show that both fibronectin and its cell-surface fibronectin binding receptor were tightly regulated during transformation and induction of differentiation in these cells, that the ligand and its cell-surface fibronectin binding receptor worked together to bring about phenotypic changes, and that fibronectin production regulated the expression of its cell-surface fibronectin binding receptor.

Influence of collagen gel substrata on certain biochemical activities of hepatocytes in primary culture

In order to study the influence of cell shape as modulated by the extracellular matrix on the cellular activity, hepatocytes isolated from liver were maintained on collagen I coated plastic substrata and collagen I gel substrata and certain hepatocyte specific functions were investigated. The incorporation of 3[H]-leucine into total proteins and albumin secreted by cells maintained on collagen gel was found to be significantly higher compared to those maintained on a collagen coated plastic substrata, indicating that hepatocytes on collagen gel have an enhanced albumin synthesizing capacity. Increased incorporation of 35[S]-sulphate into total proteoglycans (PG) and a relatively higher fraction of the 35[S]-PG in the extracellular space showed an increased rate of synthesis and secretion of sulphated PGs by cells on collagen gels. But in contrast to the above results, the incorporation of 3[H]-leucine into cytokeratins C8, C18 and actin were significantly low in cells maintained on collagen gel. The tyrosine amino transferase activity exhibited by hepatocytes preincubated with dexamethasone on collagen gel was also significantly low. The different forms of collagen substrata appeared to have no effect on the amino acid transport by hepatocytes, further suggesting that the various hepatocyte specific functions are not uniformly altered when hepatocytes are maintained on three-dimensional collagen gel substrata. These results indicate that the shape of the cell as determined by the nature of the matrix substratum influences the synthetic activity of secretory proteins and those remaining intracellularly, differently.

Transcriptional activation by viral enhancers: critical dependence on extracellular matrix-cell interactions in mammary epithelial cells

Extracellular matrix (ECM)-cell interactions are essential for the regulation of many genes in differentiated cell types. A number of expression vectors that work well in cells cultured on tissue-culture plastic appear to be inactive or sporadically active in vivo. We reasoned that these responses also may be influenced by the ECM. We therefore examined three commonly used viral enhancers and found that they all responded either positively or negatively to the presence of exogenous ECM. Using mouse mammary epithelial cells, we found that a mouse mammary tumor virus enhancer linked to its own promoter or to a truncated (and by itself inactive) beta-casein promoter drove transcription efficiently only when the cells were in contact with an ECM (more than a 100-fold induction over tissue-culture plastic). Similarly, the cytomegalovirus enhancer was more active in cells in contact with ECM. In contrast, the simian virus 40 enhancer, linked to the beta-casein promoter, was 12-fold more active in cells on tissue-culture plastic. This activity was strongly reduced when the cells interacted with ECM. Thus, we conclude that different enhancers can respond to ECM by either activating or suppressing transcription. This observation has important implications for understanding the mechanisms of promoter action and for designing expression systems for use in gene therapy.

Lipid synthesis and secretion by primary cultures of rat mammary epithelial cells

Lipid synthesis and secretion was measured in primary rat mammary epithelial cells cultured on basement matrix in medium supplemented with lactogenic hormones. The cells grew and differentiated to form alveolar-like structures reminiscent of lactating mammary gland. They synthesized abundant triacylglycerol, containing fatty acids characteristic of rat milk (C10:0-C14:0), using 14C-glucose, 14C-oleic acid or 14C-glycerol as precursors. Basal levels of triacylglycerol secretion were measured using 14C-oleic acid labeling; 1.3 +/- 0.3% of the labeled cellular triacylglycerol was secreted into the medium in 24 hours. Secreted lipid droplets were surrounded by a bilayer membrane with an electron-dense inner coat characteristic of fat globules secreted by the mammary gland. The rate of triglycerol secretion was increased to 998 +/- 98% of control (P < 0.01) by the addition of phorbol 12-myristate 13-acetate (PMA) in combination with staurosporine, a protein kinase inhibitor. Several other protein kinase inhibitors, when combined with PMA, also markedly stimulated secretion. Effective protein kinase inhibitors included sphingosine (has diverse cellular effects including the inhibition of protein kinase C; 13-fold increase in secretion), and KT5823 (a cGMP dependent protein kinase inhibitor; 5-fold increase). KT5720 (a cAMP-dependent protein kinase inhibitor) did not alter secretion. Kinase inhibitors were effective only in the presence of a phorbol ester. 4 alpha-phorbol-12,13-didecanoate, a phorbol ester which does not activate protein kinase C (PKC), could substitute for PMA. Lipid release was not mediated by disruption of cell-cell tight junctions, as EGTA did not release lipid. Based on these observations we suggest that two signals are needed to enable or stimulate lipid secretion in cultured rat mammary epithelial cells: 1) inhibition of a protein kinase and 2) a PKC-independent effect of phorbol ester. We have, for the first time, characterized a cell culture model suitable for studying lipid synthesis and secretion by mammary epithelial cells.

Vacuolar apical compartment (VAC) in breast carcinoma cell lines (MCF-7 and T47D): failure of the cell-cell regulated exocytosis mechanism of apical membrane

We have previously shown that an integral plasma membrane glycoprotein (AP2) is highly polarized to the apical domain in confluent Madin-Darby canine kidney (MDCK) epithelial cells. However, when the monolayers are prevented from forming intercellular contacts, approximately 60% of the AP2 cellular content is stored in the intracellular vacuolar apical compartment (VAC). In the current work we found that AP2 was present in the non-tumorigenic human mammary epithelial cell line MCF-10A, in the breast carcinoma cell lines MCF-7 and T47D, and in breast ductal carcinomas in vivo. By radioimmunoassay, an intracellular compartment of AP2 was identified in the mammary cell lines in culture. In MCF-10A, this compartment behaved as in MDCK cells; namely it was observed only when the cells cannot form cell-cell contacts. However, in the carcinoma cell lines MCF-7 and T47D, a significant AP2 intracellular compartment was observed also under conditions permissive for the formation of intercellular contacts. These results were confirmed by immunofluorescence and immunoelectron microscopy experiments that showed VACs in MCF-7 and T47D, even in cells with extensive intercellular contacts. In MCF-7 cells, the addition of serum caused a partial decrease of the AP2 intracellular compartment. The exocytosis of VACs occurred towards the center of multi-cellular groups, forming intercellular lumens, similar to those transiently observed in MDCK cells and to structures described by others during embryo development. Altogether, these results suggest that VAC exocytosis is controlled by cell-cell contact signalling, which may be defective in carcinoma cells.