Branching Morphogenesis of Mouse Embryonic Submandibular Epithelia Cultured under Three Different Conditions. (mouse submandibular gland/epithelial branching/morphogenesis/collagenase/heparitinase/heparin/Matrigel)

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).

Branched organs: mechanics of morphogenesis by multiple mechanisms

Branching morphogenesis is ubiquitous and important in creating bulk transport systems. Branched ducts can be generated by several different mechanisms including growth, cell rearrangements, contractility, adhesion changes, and other mechanisms. We have developed several models of the mechanics of cleft formation, which we review. We discuss the implications of several candidate mechanisms and review what has been found in models and in experiments.

Heparanase cleavage of perlecan heparan sulfate modulates FGF10 activity during ex vivo submandibular gland branching morphogenesis

Heparan sulfate proteoglycans are essential for biological processes regulated by fibroblast growth factors (FGFs). Heparan sulfate (HS) regulates the activity of FGFs by acting as a coreceptor at the cell surface, enhancing FGF-FGFR affinity, and being a storage reservoir for FGFs in the extracellular matrix (ECM). Here we demonstrate a critical role for heparanase during mouse submandibular gland (SMG) branching morphogenesis. Heparanase, an endoglycosidase, colocalized with perlecan in the basement membrane and in epithelial clefts of SMGs. Inhibition of heparanase activity in organ culture decreased branching morphogenesis, and this inhibition was rescued specifically by FGF10 and not by other FGFs. By contrast, exogenous heparanase increased SMG branching and MAPK signaling and, surprisingly, when isolated epithelia were cultured in a three-dimensional ECM with FGF10, it increased the number of lateral branches and end buds. In a solid-phase binding assay, an FGF10-FGFR2b complex was released from the ECM by heparanase. In addition, surface plasmon resonance (SPR) analysis showed that FGF10 and the FGF10-FGFR2b complex bound to purified perlecan HS and could be released by heparanase. We used the FGF10-FGFR2b complex as a probe for HS in SMGs, and it colocalized with perlecan in the basement membrane and partly colocalized with syndecan 1 in the epithelium, and binding was reduced by treatment with heparanase. In summary, our results show heparanase releases FGF10 from perlecan HS in the basement membrane, increasing MAPK signaling, epithelial clefting, and lateral branch formation, which results in increased branching morphogenesis.

Mechanics of mesenchymal contribution to clefting force in branching morphogenesis

Branching morphogenesis is ubiquitous and may involve several different mechanisms. Glandular morphogenesis is affected by growth, cell rearrangements, changes in the basal lamina, changes in the stromal ECM, changes in cell-cell and cell-ECM adhesions, mesenchymal contractility, and possibly other mechanisms. We have developed a 3D model of the mechanics of clefting, focusing in this paper solely on the potential role of mesenchyme-generated traction forces. The tissue mechanics are assumed to be those of fluids, and the hypothesized traction forces are modeled as advected by the deformations which they generate. We find that mesenchymal traction forces are sufficient to generate a cleft of the correct size and morphology, in the correct time frame. We find that viscosity of the tissues affects the time course of morphogenesis, and also affects the resulting form of the organ. Morphology is also strongly dependent on the initial distribution of contractility. We suggest an in vitro method of examining the role of mesenchyme in branching morphogenesis.

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.

Involvement of heparin-binding EGF-like growth factor and its processing by metalloproteinases in early epithelial morphogenesis of the submandibular gland

In the present study, the role of a member of the epidermal growth factor (EGF) family, heparin-binding EGF-like growth factor (HB-EGF), in organ development was investigated by using developing mouse submandibular gland (SMG), in which the EGF receptor signaling and heparan sulfate chains have been implicated. HB-EGF mRNA was detected in developing SMG by RT-PCR analysis and was expressed mainly in epithelium and weakly in mesenchyme of the embryonic SMG. Epithelial morphogenesis was inhibited by a synthetic peptide corresponding to the heparin-binding domain of HB-EGF and by anti-HB-EGF neutralizing antibody. An in vitro assay using an EGF receptor ligand-dependent cell line, EP170.7 cells, allowed us to detect the growth factor activity in SMG-conditioned media, which was significantly reduced by anti-HB-EGF antibody. Furthermore, treatment of SMG rudiments with the hydroxamate-based metalloproteinase inhibitor OSU8-1, which inhibits processing of EGFR ligands including HB-EGF, markedly diminished the growth factor activity in conditioned media and resulted in almost complete inhibition of SMG morphogenesis. The inhibitory effects on morphogenesis were reversed, though partially, by adding the soluble form of HB-EGF. Our results provide the first evidence that HB-EGF is a crucial regulator of epithelial morphogenesis during organ development, highlighting the importance of its processing by metalloproteinases.

EGF-dependent lobule formation and FGF7-dependent stalk elongation in branching morphogenesis of mouse salivary epithelium in vitro

When supplemented with appropriate growth factors, salivary gland epithelial explants isolated from mouse embryos undergo branching morphogenesis in vitro in the absence of mesenchyme. Epidermal growth factor (EGF) induces lobule formation, while fibroblast growth factor 7 (FGF7) promotes stalk elongation. A mixture of EGF and FGF7 produces an intermediate morphology, which resembles the branching pattern of salivary epithelium observed in vivo. To investigate how lobule formation and stalk elongation are related to the pattern of epithelial cell proliferation induced by EGF and FGF7, we performed a bromodeoxyuridine labeling study in whole-mount preparations. During the initial steps of lobule formation in EGF cultures, cleft and non-cleft regions had similar proliferative activity. However, once clefts had fully deepened, cells with low proliferative activity appeared at the bottom of the clefts. In contrast, during stalk elongation in FGF7 cultures, distal regions of the explants always showed higher proliferative activity than proximal regions. These results suggest that stalk elongation, but not cleft formation, may result from differential cell proliferation.

Effects of mesenchyme on epithelial tissue architecture revealed by tissue recombination experiments between the submandibular gland and lung of embryonic mice

Lung epithelium during morphogenesis maintains a sheet structure of polarized cells lining a lumen, in which E-cadherin, beta-catenin and tight junctional proteins are localized at the cell-cell contact sites. On the other hand, the submandibular gland epithelium at early stages of development forms a non-cavitated mass of cells where E-cadherin/beta-catenin are present on the entire cell surfaces and tight junctional proteins are almost absent or weakly scattered. In the present study, tissue recombination experiments were performed between the two organs to explore roles of mesenchyme in the architectural development of the epithelium. Homotypic recombinants of both submandibular gland and lung showed the tissue architecture as observed in the intact organs. In contrast, 11-day lung epithelium cultured with 13-day submandibular mesenchyme formed multilayers of cells with the lumen being less visible. It was accompanied by redistribution of E-cadherin/beta-catenin along the entire cell surfaces and by an irregular distribution of tight junctional proteins. A similar redistribution of these molecules was observed in 15-day lung epithelium cultured with the submandibular mesenchyme, although the epithelial sheet structure lining the lumen was formed. On the other hand, the tissue architecture of submandibular gland epithelium was little affected by lung mesenchyme, although the epithelium was flattened and showed branching morphogenesis.

Epidermal growth factor system is a physiological regulator of development of the mouse fetal submandibular gland and regulates expression of the alpha6-integrin subunit

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) regulate branching morphogenesis of fetal mouse submandibular gland (SMG) rudiments in vitro. The EGF system (EGF, TGF-alpha, and their shared receptor, EGFR) also regulates expression of integrins and their ligands in the extracellular matrix. We show here that inhibition of EGFR tyrosine-kinase activity by a tyrphostin retards in vitro development of SMGs. Using total RNA isolated from pooled SMGs taken from intact mouse fetuses, mRNA transcripts for EGF, TGF-alpha, and EGFR were detected by reverse transcription-polymerase chain reaction (RT-PCR), and age-dependent variations in the levels of these mRNA were quantitatively determined by nuclease protection assays. These findings suggest that the EGF system is operative in the in vivo development of this gland. alpha6-Integrin subunit was localized by immunofluorescence at the basal surface of epithelial cells. Branching morphogenesis of cultured SMG rudiments was inhibited by anti-alpha6 antibodies. Synthesis of alpha6-subunit in cultured SMGs, detected by metabolic labeling and immunoprecipitation, was increased by EGF and drastically reduced by tyrphostin. RT-PCR revealed that mRNAs for alpha6- and beta1- and beta4-integrin subunits are expressed at all ages between embryonic day 13 and postnatal day 7. These findings suggest that 1) the EGF system is a physiologic regulator of development of fetal mouse SMG, and 2) one mechanism by which it acts may be by regulating expression of integrins, which in turn control interaction of epithelial cells with the extracellular matrix.

Epithelial morphogenesis in mouse embryonic submandibular gland: its relationships to the tissue organization of epithelium and mesenchyme

Epithelial tissues in various organ rudiments undergo extensive shape changes during their development. The processes of epithelial shape change are controlled by tissue interactions with the surrounding mesenchyme which is kept in direct contact with the epithelium. One of the organs which has been extensively studied is the mouse embryonic submandibular gland, whose epithelium shows the characteristic branching morphogenesis beginning with the formation of narrow and deep clefts as well as changes in tissue organization. Various molecules in the mesenchyme, including growth factors and extracellular matrix components, affect changes of epithelial shape and tissue organization. Also, mesenchymal tissue exhibits dynamic properties such as directional movements in groups and rearrangement of collagen fibers coupled with force-generation by mesenchymal cells. The epithelium, during early branching morphogenesis, makes a cell mass where cell-cell adhesion systems are less developed. Such properties of both the mesenchyme and epithelium are significant for considering how clefts, which first appear as unstable tiny indentations on epithelial surfaces, are formed and stabilized.

Mouse embryonic submandibular gland epithelium loses its tissue integrity during early branching morphogenesis

During the development of the mouse submandibular gland, the epithelium undergoes not only shape changes to produce extensively branched lobules and stalk, but also changes in cell arrangement from a cell mass to a cavitated cell sheet. The present study examined the organization in the developing epithelium of intercellular adhesion systems and of actin-containing microfilaments. E-cadherin and beta-catenin, which are components of cell-to-cell adherens junctions in epithelial cells, were distributed along the cell periphery of almost the entire epithelium of the submandibular gland at all stages examined and were mainly localized at the apical region of the oral epithelium. Actin-containing microfilaments, which are associated with cell-to-cell adherens junctions, showed a distribution similar to that of those molecules. In contrast, although the distributions of desmoplakins I/II, major desmosomal proteins, and ZO-1 (a tight junction protein) were seen in the oral epithelium and proximal stalk of the submandibular gland epithelium, signals representing these molecules were absent from or much reduced in the submandibular gland epithelium of the cell mass at the 12- and 13-day stages. In the 14-day gland, they strongly appeared in the cells facing the appearing lumens, whereas they were weakly scattered within the terminal lobules that were still a part of the cell mass. These findings suggest that cell-to-cell adhesion systems are differentially regulated during the epithelial morphogenesis of the submandibular gland and that the integrity of the submandibular gland epithelium is lost during the early stages of development, indicating the tissue to be a rather plastic structure.