Hedgehog peptide promotes cell polarization and lumen formation in developing mouse submandibular gland

Submandibular gland epithelial development and the importance of junctions

Salivary glands consist of highly specialized epithelial cells that secrete the fluid, saliva, and/or transport saliva into the oral cavity. Saliva is essential to lubricate the oral cavity for food consumption and to maintain the hygiene of the oral cavity. In this review, we will focus on the formation of the epithelial cell lineage and the cell junctions that are essential for formation of saliva and maintenance of the epithelial barrier between the ducts that transport saliva and the extracellular environment.

Role of the hedgehog signaling pathway in rheumatic diseases: An overview

Hedgehog (Hh) signaling pathway is an evolutionarily conserved signal transduction pathway that plays an important regulatory role during embryonic development, cell proliferation, and differentiation of vertebrates, and it is often inhibited in adult tissues. Recent evidence has shown that Hh signaling also plays a key role in rheumatic diseases, as alterations in their number or function have been identified in rheumatoid arthritis, osteoarthritis, ankylosing spondylitis, systemic sclerosis, and Sjogren's Syndrome. As a result, emerging studies have focused on the blockade of this pathogenic axis as a promising therapeutic target in several autoimmune disorders; nevertheless, a greater understanding of its contribution still requires further investigation. This review aims to elucidate the most recent studies and literature data on the pathogenetic role of Hh signaling in rheumatic diseases.

Midbody-Localized Aquaporin Mediates Intercellular Lumen Expansion During Early Cleavage of an Invasive Freshwater Bivalve

Intercellular lumen formation is a crucial aspect of animal development and physiology that involves a complex interplay between the molecular and physical properties of the constituent cells. Embryos of the invasive freshwater mussel Dreissena rostriformis are ideal models for studying this process due to the large intercellular cavities that readily form during blastomere cleavage. Using this system, we show that recruitment of the transmembrane water channel protein aquaporin exclusively to the midbody of intercellular cytokinetic bridges is critical for lumenogenesis. The positioning of aquaporin-positive midbodies thereby influences the direction of cleavage cavity expansion. Notably, disrupting cytokinetic bridge microtubules impairs not only lumenogenesis but also cellular osmoregulation. Our findings reveal a simple mechanism that provides tight spatial and temporal control over the formation of luminal structures and likely plays an important role in water homeostasis during early cleavage stages of a freshwater invertebrate species.

The role of salivary gland macrophages in infection, disease and repair

Macrophages are mononuclear innate immune cells which have become of increasing interest in the fields of disease and regeneration, as their non-classical functions have been elucidated in addition to their classical inflammatory functions. Macrophages can regulate tissue remodeling, by both mounting and reducing inflammatory responses; and exhibit direct communication with other cells to drive tissue turnover and cell replacement. Furthermore, macrophages have recently become an attractive therapeutic target to drive tissue regeneration. The major salivary glands are glandular tissues that are exposed to pathogens through their close connection with the oral cavity. Moreover, there are a number of diseases that preferentially destroy the salivary glands, causing irreversible injury, highlighting the need for a regenerative strategy. However, characterization of macrophages in the mouse and human salivary glands is sparse and has been mostly determined from studies in infection or autoimmune pathologies. In this review, we describe the current literature around salivary gland macrophages, and speculate about the niches they inhabit and how their role in development, regeneration and cancer may inform future therapeutic advances.

Expression patterns of genes critical for SHH, BMP, and FGF pathways during the lumen formation of human salivary glands

Sjögren's syndrome or radiotherapy for head and neck cancer leads to the irreversible hypofunction of salivary gland (SG). The stem/progenitor cell-based regenerative strategy has been proven to be the most promising approach to repair the function of SG. The molecular mechanisms that regulate SG morphogenesis, especially during lumen formation, provide valuable hints for establishment of such regenerative strategies. It has been demonstrated that numerous growth factors particularly belonging to SHH, BMP, and FGF signaling pathway are involved in the regulation of lumen formation and have shown protective effects on the SG from irradiation in mouse models. However, it remains elusive whether the expression pattern and function of these signaling molecules are conserved in humans. In this study, we examined the expression patterns of the molecules critical for SHH, BMP, and FGF signaling cascades from the canalicular stage to the terminal bud stage, the key stages for lumen formation, in human SG and compared them with the expression data observed in mice. Our results manifested that genes involved in SHH signaling pathway showed identical expression patterns, while genes involved in BMP as well as FGF pathway exhibited similar but distinct expression patterns in humans to those in the mouse. We concluded that the expression patterns of genes involved in SHH, BMP, and FGF pathways in the development of human SG exhibit high similarity to that in the development of mouse SG during lumen formation, suggesting that the molecular mechanism regulating the morphogenesis of SG during lumen formation may be conserved in mice and humans. Our results will have an implication in the future establishment of stem-cell based approaches for the repair of SG function.

A novel role for cilia-dependent sonic hedgehog signaling during submandibular gland development

BACKGROUND

Submandibular glands (SMGs) are specialized epithelial structures which generate saliva necessary for mastication and digestion. Loss of SMGs can lead to inflammation, oral lesions, fungal infections, problems with chewing/swallowing, and tooth decay. Understanding the development of the SMG is important for developing therapeutic options for patients with impaired SMG function. Recent studies have suggested Sonic hedgehog (Shh) signaling in the epithelium plays an integral role in SMG development; however, the mechanism by which Shh influences gland development remains nebulous.

RESULTS

Using the Kif3a^f/f ;Wnt1-Cre ciliopathic mouse model to prevent Shh signal transduction by means of the loss of primary cilia in neural crest cells, we report that mesenchymal Shh activity is necessary for gland development. Furthermore, using a variety of murine transgenic lines with aberrant mesenchymal Shh signal transduction, we determine that loss of Shh activity, by means of loss of the Gli activator, rather than gain of Gli repressor, is sufficient to cause the SMG aplasia. Finally, we determine that loss of the SMG correlates with reduced Neuregulin1 (Nrg1) expression and lack of innervation of the SMG epithelium.

CONCLUSIONS

Together, these data suggest a novel mechanistic role for mesenchymal Shh signaling during SMG development. Developmental Dynamics 247:818-831, 2018. © 2018 Wiley Periodicals, Inc.

Regulatory mechanisms of branching morphogenesis in mouse submandibular gland rudiments

Branching morphogenesis is an important developmental process for many organs, including the salivary glands. Whereas epithelial–mesenchymal interactions, which are cell-to-cell communications, are known to drive branching morphogenesis, the molecular mechanisms responsible for those inductive interactions are still largely unknown. Cell growth factors and integrins are known to be regulators of branching morphogenesis of salivary glands. In addition, functional microRNAs (miRNAs) have recently been reported to be present in the developing submandibular gland. In this review, the authors describe the roles of various cell growth factors, integrins and miRNAs in branching morphogenesis of developmental mouse submandibular glands.

Shh/Ptch and EGF/ErbB cooperatively regulate branching morphogenesis of fetal mouse submandibular glands

The hedgehog family includes Sonic hedgehog (Shh), Desert hedgehog, and Indian hedgehog, which are well known as a morphogens that play many important roles during development of numerous organs such as the tongue, pancreas, kidney, cartilage, teeth and salivary glands (SMG). In Shh null mice, abnormal development of the salivary gland is seen after embryonic day 14 (E14). Shh also induced lobule formation and lumen formation in acini-like structures in cultured E14 SMG. In this study, we investigated the relationship between Shh and epidermal growth factor (EGF)/ErbB signaling in developing fetal mouse SMG. Administration of Shh to cultured E13 SMG stimulated branching morphogenesis (BrM) and induced synthesis of mRNAs for EGF ligands and receptors of the ErbB family. Shh also stimulated activation of ErbB signaling system such as ERK1/2. AG1478, a specific inhibitor of ErbB receptors, completely suppressed BrM and activation of EGF/ErbB/ERK1/2 cascade in E13 SMGs cultured with Shh. The expressions of mRNA for Egf in mesenchyme and mRNA for Erbb1, Erbb2 and Erbb3 in epithelium of E13 SMG were specifically induced by administration of Shh. These results show that Shh stimulates BrM of fetal mouse SMG, at least in part, through activation of the EGF/ErbB/ERK1/2 signaling system.

Salivary gland development and disease

Mammalian salivary glands synthesize and secrete saliva via a vast interconnected network of epithelial tubes attached to secretory end units. The extensive morphogenesis required to establish this organ is dependent on interactions between multiple cell types (epithelial, mesenchymal, endothelial, and neuronal) and the engagement of a wide range of signaling pathways. Here we describe critical regulators of salivary gland development and discuss how mutations in these impact human organogenesis. In particular, we explore the genetic contribution of growth factor pathways, nerve-derived factors and extracellular matrix molecules to salivary gland formation in mice and humans.

Parasympathetic innervation regulates tubulogenesis in the developing salivary gland

A fundamental question in development is how cells assemble to form a tubular network during organ formation. In glandular organs, tubulogenesis is a multistep process requiring coordinated proliferation, polarization and reorganization of epithelial cells to form a lumen, and lumen expansion. Although it is clear that epithelial cells possess an intrinsic ability to organize into polarized structures, the mechanisms coordinating morphogenetic processes during tubulogenesis are poorly understood. Here, we demonstrate that parasympathetic nerves regulate tubulogenesis in the developing salivary gland. We show that vasoactive intestinal peptide (VIP) secreted by the innervating ganglia promotes ductal growth, leads to the formation of a contiguous lumen, and facilitates lumen expansion through a cyclic AMP/protein kinase A (cAMP/PKA)-dependent pathway. Furthermore, we provide evidence that lumen expansion is independent of apoptosis and involves the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-regulated Cl(-) channel. Thus, parasympathetic innervation coordinates multiple steps in tubulogenesis during organogenesis.

Rewiring cell polarity signaling in cancer

Disrupted cell polarity is a feature of epithelial cancers. The Crumbs, Par and Scribble polarity complexes function to specify and maintain apical and basolateral membrane domains, which are essential to organize intracellular signaling pathways that maintain epithelial homeostasis. Disruption of apical-basal polarity proteins facilitates rewiring of oncogene and tumor suppressor signaling pathways to deregulate proliferation, apoptosis, invasion and metastasis. Moreover, apical-basal polarity integrates intracellular signaling with the microenvironment by regulating metabolic signaling, extracellular matrix remodeling and tissue level organization. In this review, we discuss recent advances in our understanding of how polarity proteins regulate diverse signaling pathways throughout cancer progression from initiation to metastasis.

Molecular cues for development and regeneration of salivary glands

The hypofunction of salivary glands caused by Sjögren's Syndrome or radiotherapy for head and neck cancer significantly compromises the quality of life of millions patients. Currently no curative treatment is available for the irreversible hyposalivation, whereas regenerative strategies targeting salivary stem/progenitor cells are promising. However, the success of these strategies is constrained by the lack of insights on the molecular cues of salivary gland regeneration. Recent advances in the molecular controls of salivary gland morphogenesis provided valuable clues for identifying potential regenerative cues. A complicated network of signaling molecules between epithelia, mesenchyme, endothelia, extracellular matrix and innervating nerves orchestrate the salivary gland organogenesis. Here we discuss the roles of several cross-talking intercellular signaling pathways, i.e., FGF, Wnt, Hedgehog, Eda, Notch, Chrm1/HB-EGF and Laminin/Integrin pathways, in the development of salivary glands and their potentials to promote salivary regeneration.

Recombinant human sonic hedgehog protein regulates the expression of ZO-1 and occludin by activating angiopoietin-1 in stroke damage

This study examines the regulating effect of Sonic Hedgehog (Shh) on the permeability of the blood-brain barrier (BBB) in cerebral ischemia. By employing permanent middle cerebral artery occlusion (pMCAO) model, we find that Shh significantly decreases brain edema and preserves BBB permeability. Moreover, Shh increases zonula occludens-1 (ZO-1), occludin and angiopiotetin-1 (Ang-1) expression in the ischemic penumbra. Blockage of Shh with cyclopamine abolishes the effects of Shh on brain edema, BBB permeability and ZO-1, occludin, Ang-1 expression. Primary brain microvessel endothelial cells (BMECs) and astrocytes were pre-treated with Shh, cyclopamine, Ang-1-neutralizing antibody, and subjected to oxygen-glucose deprivation (OGD). Results show that the Ang-1 protein level in the culture medium of Shh-treated astrocytes is significantly higher. Shh also increased ZO-1, occludin and Ang-1 expression in BMECs, while cyclopamine and Ang-1-neutralizing antibody inhibited the effects of Shh on the ZO-1 and occludin expression, respectively. This study suggests that, under ischemic insults, Shh triggers Ang-1 production predominantly in astrocytes, and the secreted Ang-1 acts on BMECs, thereby upregulating ZO-1 and occludin to repair the tight junction and ameliorate the brain edema and BBB leakage.

Quantitative single cell analysis of cell population dynamics during submandibular salivary gland development and differentiation

Epithelial organ morphogenesis involves reciprocal interactions between epithelial and mesenchymal cell types to balance progenitor cell retention and expansion with cell differentiation for evolution of tissue architecture. Underlying submandibular salivary gland branching morphogenesis is the regulated proliferation and differentiation of perhaps several progenitor cell populations, which have not been characterized throughout development, and yet are critical for understanding organ development, regeneration, and disease. Here we applied a serial multiplexed fluorescent immunohistochemistry technology to map the progressive refinement of the epithelial and mesenchymal cell populations throughout development from embryonic day 14 through postnatal day 20. Using computational single cell analysis methods, we simultaneously mapped the evolving temporal and spatial location of epithelial cells expressing subsets of differentiation and progenitor markers throughout salivary gland development. We mapped epithelial cell differentiation markers, including aquaporin 5, PSP, SABPA, and mucin 10 (acinar cells); cytokeratin 7 (ductal cells); and smooth muscle α-actin (myoepithelial cells) and epithelial progenitor cell markers, cytokeratin 5 and c-kit. We used pairwise correlation and visual mapping of the cells in multiplexed images to quantify the number of single- and double-positive cells expressing these differentiation and progenitor markers at each developmental stage. We identified smooth muscle α-actin as a putative early myoepithelial progenitor marker that is expressed in cytokeratin 5-negative cells. Additionally, our results reveal dynamic expansion and redistributions of c-kit- and K5-positive progenitor cell populations throughout development and in postnatal glands. The data suggest that there are temporally and spatially discreet progenitor populations that contribute to salivary gland development and homeostasis.

The Hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence

The blood-brain barrier (BBB) is composed of tightly bound endothelial cells (ECs) and perivascular astrocytes that regulate central nervous system (CNS) homeostasis. We showed that astrocytes secrete Sonic hedgehog and that BBB ECs express Hedgehog (Hh) receptors, which together promote BBB formation and integrity during embryonic development and adulthood. Using pharmacological inhibition and genetic inactivation of the Hh signaling pathway in ECs, we also demonstrated a critical role of the Hh pathway in promoting the immune quiescence of BBB ECs by decreasing the expression of proinflammatory mediators and the adhesion and migration of leukocytes, in vivo and in vitro. Overall, the Hh pathway provides a barrier-promoting effect and an endogenous anti-inflammatory balance to CNS-directed immune attacks, as occurs in multiple sclerosis.

Targeted expression of GLI1 in the salivary glands results in an altered differentiation program and hyperplasia

Hedgehog (Hh) signaling is a regulator of salivary gland morphogenesis, but its role in postnatal glands has only recently begun to be addressed. To examine the effects of deregulated Hh signaling in the salivary gland, we expressed the Hh effector protein GLI1, in salivary epithelial cells using both cytokeratin 5 and mouse mammary tumor virus (MMTV) transgenic systems. Ectopic pathway activation resulted in restrained acinar differentiation, formation of cystic lesions, and prominent appearance of ductal structures. Moreover, induced expression of GLI1 aids the formation of hyperplastic lesions, which closely resemble GLI1-induced changes in murine skin and mammary glands, suggesting that GLI1 targets cells with similar characteristics in different tissues. Furthermore, GLI1-expressing salivary epithelial cells are actively dividing, and GLI1-induced lesions are proliferative, an incident accompanied by enhanced expression of the Hh target genes, cyclin D1, and Snail. GLI1-induced salivary lesions regress after transgene withdrawal and become histologically normalized. Taken together, our data reveal the ability of GLI1 to modulate salivary acinar differentiation and to promote proliferation of ductal epithelial cells.

Ectodysplasin and Wnt pathways are required for salivary gland branching morphogenesis

The developing submandibular salivary gland (SMG) is a well-studied model for tissue interactions and branching morphogenesis. Its development shares similar features with other ectodermal appendages such as hair and tooth. The ectodysplasin (Eda) pathway is essential for the formation and function of several ectodermal organs. Mutations in the signaling components of the Eda pathway lead to a human syndrome known as hypohidrotic ectodermal dysplasia (HED), which is characterized by missing and malformed teeth, sparse hair and reduced sweating. Individuals with HED suffer also from dry mouth because of reduced saliva flow. In order to understand the underlying mechanism, we analyzed salivary gland development in mouse models with altered Eda pathway activities. We have found that Eda regulates growth and branching of the SMG via transcription factor NF-κB in the epithelium, and that the hedgehog pathway is an important mediator of Eda/NF-κB. We also sought to determine whether a similar reciprocal interplay between the Eda and Wnt/β-catenin pathways, which are known to operate in other skin appendages, functions in developing SMG. Surprisingly and unlike in developing hair follicles and teeth, canonical Wnt signaling activity did not colocalize with Edar/NF-κB in salivary gland epithelium. Instead, we observed high mesenchymal Wnt activity and show that ablation of mesenchymal Wnt signaling either in vitro or in vivo compromised branching morphogenesis. We also provide evidence suggesting that the effects of mesenchymal Wnt/β-catenin signaling are mediated, at least in part, through regulation of Eda expression.

Coordination of epithelial branching and salivary gland lumen formation by Wnt and FGF signals

Branching morphogenesis is a molecularly conserved mechanism that is adopted by several organs, such as the lung, kidney, mammary gland and salivary gland, to maximize the surface area of a tissue within a small volume. Branching occurs through repetitive clefting and elongation of spherical epithelial structures, called endbuds, which invade the surrounding mesenchyme. In the salivary gland, lumen formation takes place alongside branching morphogenesis, but in a controlled manner, so that branching is active at the distal ends of epithelial branches while lumen formation initiates at the proximal ends, and spreads distally. We present here data showing that interaction between FGF signaling and the canonical (β-catenin dependent) and non-canonical branches of Wnt signaling coordinates these two processes. Using the Axin2(lacZ) reporter mice, we find Wnt/β-catenin signaling activity first in the mesenchyme and later, at the time of lumen formation, in the ductal epithelium. Gain and loss of function experiments reveal that this pathway exerts an inhibitory effect on salivary gland branching morphogenesis. We have found that endbuds remain devoid of Wnt/β-catenin signaling activity, a hallmark of ductal structures, through FGF-mediated inhibition of this pathway. Our data also show that FGF signaling has a major role in the control of lumen formation by preventing premature hollowing of epithelial endbuds and slowing down the canalization of presumptive ducts. Concomitantly, FGF signaling strongly represses the ductal marker Cp2l1, most likely via repression of Wnt5b and non-canonical Wnt signaling. Inhibition of canonical and non-canonical Wnt signaling in endbuds by FGF signaling occurs at least in part through sFRP1, a secreted inhibitor of Wnt signaling and downstream target of FGF signaling. Altogether, these findings point to a key function of FGF signaling in the maintenance of an undifferentiated state in endbud cells by inhibition of a ductal fate.

Localization of AQP5 during development of the mouse submandibular salivary gland

Aquaporin 5 (AQP5) is known to be central for salivary fluid secretion. A study of the temporal-spatial distribution of AQP5 during submandibular gland (SMG) development and in adult tissues might offer further clues to its unknown role during development. In the present work, SMGs from embryonic day (E) 14.5-18.5 and postnatal days (P) 0, 2, 5, 25, and 60 were immunostained for AQP5 and analyzed using light microscopy. Additional confocal and transmission electron microscopy were performed on P60 glands. Our results show that AQP5 expression first occurs in a scattered pattern in the late canalicular stage and becomes more prominent and organized in the terminal tubuli/pro-acinar cells towards birth. Additional apical membrane staining in the entire intralobular duct is found just prior to birth. During postnatal development, AQP5 is expressed in both the luminal and lateral membrane of pro-acinar/acinar cells. AQP5 is also detected in the basal membrane of acinar cells at P25 and P60. In the intercalated ducts at P60, the male glands show apical staining in the entire segment, while only the proximal region is positive in the female glands. These results demonstrate an evolving distribution of AQP5 during pre- and postnatal development in the mouse SMGs.

Wnt/β-catenin signaling regulates postnatal development and regeneration of the salivary gland

Regenerative therapy of the salivary gland (SG) is a promising therapeutic approach for irreversible hyposalivation in patients with head and neck cancer treated by radiotherapy. However, little is known about the molecular regulators of stem/progenitor cell activity and regenerative processes in the SG. Wnt/β-catenin signaling regulates the function of many adult stem cell populations, but its role in SG development and regeneration is unknown. Using BAT-gal Wnt reporter transgenic mice, we demonstrate that in the submandibular glands (SMGs) of newborn mice Wnt/β-catenin signaling is active in a few cells at the basal layer of intercalated ducts, the putative location of salivary gland stem/progenitor cells (SGPCs). Wnt activity decreases as mice age, but is markedly enhanced in SG ducts during regeneration of adult SMG after ligation of the main secretory duct. The Hedgehog (Hh) pathway is also activated after duct ligation. Inhibition of epithelial β-catenin signaling in young Keratin5-rtTA/tetO-Dkk1 mice impairs the postnatal development of SMG, particularly affecting maturation of granular convoluted tubules. Conversely, forced activation of epithelial β-catenin signaling in adult Keratin5-rtTA/tetO-Cre/Ctnnb1((Ex3)fl) mice promotes proliferation of ductal cells, expansion of the SGPC compartment, and ectopic activation of Hh signaling. Taken together, these results indicate that Wnt/β-catenin signaling regulates the activity of SGPCs during postnatal development and regeneration upstream of the Hh pathway, and suggest the potential of modulating Wnt/β-catenin and/or Hh pathways for functional restoration of SGs after irradiation.

Branching morphogenesis in the fetal mouse submandibular gland is codependent on growth factors and extracellular matrix

Branching morphogenesis (BrM) is a basic developmental process for the formation of the lung, kidney, and all exocrine glands, including the salivary glands. This process proceeds as follows. An epithelial downgrowth invaginates into underlying mesenchyme, and forms a cleft at its distal end, which is the site of dichotomous branching and elongation; this process of clefting and elongation is repeated many times at the distal ends of the invading epithelium until the desired final extent of branching is reached. The distal ends of the epithelium differentiate into the secretory endpieces, and the elongated segments become the ducts. This presentation is a brief historical review of studies on BrM during the development of the submandibular gland (SMG).

The developmental roles of the extracellular matrix: beyond structure to regulation

Cells in multicellular organisms are surrounded by a complex three-dimensional macromolecular extracellular matrix (ECM). This matrix, traditionally thought to serve a structural function providing support and strength to cells within tissues, is increasingly being recognized as having pleiotropic effects in development and growth. Elucidation of the role that the ECM plays in developmental processes has been significantly advanced by studying the phenotypic and developmental consequences of specific genetic alterations of ECM components in the mouse. These studies have revealed the enormous contribution of the ECM to the regulation of key processes in morphogenesis and organogenesis, such as cell adhesion, proliferation, specification, migration, survival, and differentiation. The ECM interacts with signaling molecules and morphogens thereby modulating their activities. This review considers these advances in our understanding of the function of ECM proteins during development, extending beyond their structural capacity, to embrace their new roles in intercellular signaling.

Salivary gland development

Salivary glands provide an excellent model for the study of epithelial-mesenchymal interactions and branching morphogenesis. This review will discuss the anatomy of different types of glands, in a range of different organisms. Then, concentrating on the mouse submandibular gland, the stages of salivary gland development will be reviewed and the relative role of the mesenchyme and the epithelium will be discussed. Finally, the genes thought to play a role in development of the glands from initiation to differentiation will be reviewed.

Salivary gland branching morphogenesis

Salivary gland branching morphogenesis involves coordinated cell growth, proliferation, differentiation, migration, apoptosis, and interaction of epithelial, mesenchymal, endothelial, and neuronal cells. The ex vivo analysis of embryonic mouse submandibular glands, which branch so reproducibly and beautifully in culture, is a powerful tool to investigate the molecular mechanisms regulating epithelium-mesenchyme interactions during development. The more recent analysis of genetically modified mice provides insight into the genetic regulation of branching morphogenesis. The review begins, as did the field historically, focusing on the role of the extracellular matrix (ECM), and its components such as glycosaminoglycans, collagens, and laminins. Following sections describe the modification of the ECM by proteases and the role of cell-matrix and cell-cell receptors. The review then focuses on two major families of growth factors implicated in salivary gland development, the fibroblast growth factors (FGFs) and the epidermal growth factors (EGFs). The salivary gland phenotypes in mice with genetic modification of FGFs and their receptors highlight the central role of FGFs during salivary gland branching morphogenesis. A broader section mentions other molecules implicated from analysis of the phenotypes of genetically modified mice or organ culture experiments. The review concludes with speculation on some future areas of research.