Effects of papaverine and calcium-free medium on salivary gland morphogenesis

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.

Norman K. Wessells: a life in science

"In its triple role as locomotory organelle, as a site of deposition of new surface material for the elongating axon, and a source of microspikes (sensory probes), the growth cone becomes the key to axon elongation" Yamada et al. (1971).

Control of basement membrane remodeling and epithelial branching morphogenesis in embryonic lung by Rho and cytoskeletal tension

Local alterations in the mechanical compliance of the basement membrane that alter the level of isometric tension in the cell have been postulated to influence tissue morphogenesis. To explore whether cell tension contributes to tissue pattern formation in vivo, we modulated cytoskeletal force generation in embryonic mouse lung (embryonic days 12-14) rudiments using inhibitors of Rho-associated kinase (ROCK), myosin light chain kinase, myosin ATPase, and microfilament integrity, or a Rho stimulator (cytotoxic necrotizing factor-1). Tension inhibition resulted in loss of normal differentials in basement membrane thickness, inhibition of new terminal bud formation, and disorganization of epithelial growth patterns as well as disruption of capillary blood vessels. In contrast, increasing cell tension through Rho activation, as confirmed by quantitation of myosin light chain phosphorylation and immunohistocytochemical analysis of actin organization, accelerated lung branching and increase capillary elongation. These data suggest that changes in cytoskeletal tension mediated by Rho signaling through ROCK may play an important role in the establishment of the spatial differentials in cell growth and extracellular matrix remodeling that drive embryonic lung development.

Cancer as a disease of epithelial-mesenchymal interactions and extracellular matrix regulation

Carcinogenesis - the process of cancer formation - is commonly discussed in terms of genetic alterations that lead to deregulation of cell growth. Recently, there has been a resurgence of interest in epigenetic factors and, in particular, the role of the stromal microenvironment and angiogenesis in tumor formation. In this article, cancer is presented as a disease of the developmental processes that govern how cells organize into tissues and tissues into organs. This histogenetic perspective raises the possibility that epithelial-mesenchymal interactions and the extracellular matrix (basement membrane) that is deposited through these interactions may actively contribute to the carcinogenic process. Experimental work is reviewed that confirms that extracellular matrix plays a key role in normal histodifferentiation during both epitheliogenesis and angiogenesis, and that epigenetic deregulation of cell-matrix interactions may actively promote tumor initiation and progression. The contributions of integrins, cytoskeleton, tensegrity and local variations in extracellular matrix mechanics to these processes are discussed, as are the implications of this work for future studies on cancer formation.

Control of embryonic lung branching morphogenesis by the Rho activator, cytotoxic necrotizing factor 1

BACKGROUND

Lung development is sensitive to physiological stresses, and its development may be impaired by physical distortion, as in patients with congenital diaphragmatic hernia. Yet, little is known about how mechanical forces can influence lung morphogenesis. Studies with cultured cells suggest that cytoskeletal tension may play a key role in growth control. Since the small GTPase Rho plays an important role in the control of cell tension generation, we carried out studies to test the hypothesis that changes in Rho-mediated cell tension may influence branching morphogenesis.

METHODS

Embryonic lung buds from timed pregnant Swiss Webster mice were microdissected on Embryonic Day 12 (E12), and whole organs were cultured in serum-free medium in the presence of the Rho activator cytotoxic necrotizing factor 1 (CNF-1) for 48 h. Serial measurements of the degree of epithelial branch formation and tissue maturation were performed using light microscopy and computerized image analysis.

RESULTS

At 48 h, embryonic lungs treated with 2 ng/ml CNF-1 increased their terminal bud count by 236 +/- 18% (P = 0.01) compared with 132 +/- 2% for untreated controls. However, dose-response experiments revealed biphasic behavior: at a higher dose of CNF-1 (200 ng/ml), bud number was actually decreased relative to controls (43 +/- 1%, P < 0.001). Histological analysis revealed that individual glands appeared to be more highly developed at low-dose CNF-1, whereas the high dose produced gland contraction.

CONCLUSIONS

These data support a potential role for Rho and cytoskeletal tension in control of epithelial pattern formation during lung development.

Fork head prevents apoptosis and promotes cell shape change during formation of the Drosophila salivary glands

The secretory tubes of the Drosophila salivary glands are formed by the regulated, sequential internalization of the primordia. Secretory cell invagination occurs by a change in cell shape that includes basal nuclear migration and apical membrane constriction. In embryos mutant for fork head (fkh), which encodes a transcription factor homologous to mammalian hepatocyte nuclear factor 3beta (HNF-3beta), the secretory primordia are not internalized and secretory tubes do not form. Here, we show that secretory cells of fkh mutant embryos undergo extensive apoptotic cell death following the elevated expression of the apoptotic activator genes, reaper and head involution defective. We rescue the secretory cell death in the fkh mutants and show that the rescued cells still do not invaginate. The rescued fkh secretory cells undergo basal nuclear migration in the same spatial and temporal pattern as in wild-type secretory cells, but do not constrict their apical surface membranes. Our findings suggest at least two roles for fkh in formation of the embryonic salivary glands: an early role in promoting survival of the secretory cells, and a later role in secretory cell invagination, specifically in the constriction of the apical surface membrane.

The structural and mechanical complexity of cell-growth control

Tight control of cell proliferation is required to ensure normal tissue patterning and prevent cancer formation. The analysis of cultured cells has led to an explosion in our understanding of the molecules that trigger growth and mediate cell-cycle progression. However, the mechanism by which the local growth differentials that drive morphogenesis are established and maintained still remains unknown. Here we review recent work that reveals the importance of cell binding to the extracellular matrix, and associated changes in cell shape and cytoskeletal tension, to the spatial control of cell-cycle progression. These findings change the paradigm of cell-growth control, by placing our understanding of molecular signalling cascades in the context of the structural and mechanical complexity of living tissues.

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.

Microfilaments, cell shape changes, and the formation of primary mesenchyme in sea urchin embryos

Primary mesenchyme formation in sea urchin embryos occurs when a subset of epithelial cells of the blastula move from the epithelial layer into the blastocoel. The role of microfilaments in producing the cell shape changes that characterize this process, referred to as ingression, was investigated in this study. f-Actin was localized by confocal microscopy using labeled phalloidin. The distribution of f-actin was observed before, during, and after ingression and was correlated with cellular movements. Prior to the onset of ingression, staining became intense in the apical region of putative primary mesenchyme and disappeared following the completion of mesenchyme formation. The apical end of these cells constricted coincidentally with the appearance of the intensified staining, indicating that f-actin may be involved in this constriction. In addition, papaverine, a smooth muscle cell relaxant that interferes with microfilament-based contraction, and that was shown in this study to inhibit cytokinesis, diminished apical constriction and delayed ingression. Despite this interference with apical constriction, the basal surface of ingressing cells protruded into the blastocoel. It is suggested that apical constriction, while not necessary for ingression, does contribute to the efficient production of mesenchyme and that protrusion of the basal surface results from changes that occur independent of apical constriction.

Calcium dependence of rhythmic contractions of the Oryzias latipes blastoderm

1. The blastoderm of the Oryzias latipes (medaka, Teleostei) embryo begins to contract rhythmically, about once per min at 25 degrees C, during epiboly. When the blastoderm was mechanically detached from the rest of the egg, it contracted into a pear-shaped ball and also continued to contract rhythmically. 2. The optimal [Ca2+] for the rhythmic contractions was approximately 1 mM. 3. The contractions stopped in media containing La3+, Ni2+, Mn2+, Co2+ or Ba2+. 4. A number of organic calcium antagonists--cinnarizine, D600, diltiazem, nifedipine, TMB-8 and verapamil--had no apparent effect on the contractions. However, the contractions were inhibited by papaverine, caffeine, and a mixture of TMB-8 and verapamil. 5. The contractions stopped in a medium containing 25 mM K+ or cytochalasin D. 6. We conclude that microfilaments cause the contractions, that each rhythmic contraction is preceded or accompanied by an increase in cytoplasmic free [Ca2+], and that Ca2+ enters the cytoplasm from both an extracellular and an intracellular pool.

Control of cell shape and locomotion by external calcium

Dependence of locomotion of Xenopus laevis epidermal cells on calcium influx from the external medium was investigated. Inhibition of Ca2+ influx by 2 mM La3+ or 4 mM Tb3+ in the culture medium causes an immediate stop to locomotion and a loss of motion at the outer margin of the lamella; microcolliculi disappear and the entire lamella becomes flat and very thin. The cell body region enlarges by spreading into the lamella to an extent approximately coincident with the distribution of myosin. The increase in thickness of this area is the result. The cytoskeletal elements actin, alpha-actinin and myosin become homogeneously distributed throughout the cell and a great number of straight microtubules extend to the margin after 20 min in La3+-containing media. Prekeratin distribution does not change. Reduction of calcium concentration in the external medium by EGTA leads to cessation of cell locomotion. Sr2+ (1-4 mM) is also able to replace calcium for triggering locomotion. These findings point to a control of Ca2+-activated contractions of actomyosin by influx of external Ca2+. According to our model of cell locomotion [14] the contractions generate a hydrostatic pressure extending the lamella by flow of hyaloplasm towards the margin. Small swellings (microcolliculi) appearing thereby will be dislocated by a calcium-dependent sol-gel transformation in this area, which contains actin but not myosin.

The role of cell proliferation and cellular shape change in branching morphogenesis of the embryonic mouse lung: analysis using aphidicolin and cytochalasins

The formation of induced supernumerary buds in the embryonic mouse tracheal epithelium has been used as a model system to analyse the respective roles of cell proliferation and microfilament-mediated cell shape change during branching morphogenesis. In order to analyse the mitotic events associated with the formation of epithelial buds, the induction of supernumerary tracheal buds by mesenchymal grafts was carried out with the inhibitor of DNA synthesis, aphidicolin, present in the culture medium for varying intervals of time during the 16-hour inductive process. The presence of aphidicolin for 10 to 16 hours of the inductive period blocks the formation of induced tracheal buds, whereas the presence of the inhibitor for half of that time (either the first 8 hours or the last 8 hours) does not prevent this morphogenetic event from taking place, although smaller buds resulted from induction under these conditions. Both the inhibition of DNA synthesis and the recovery from 10 microM aphidicolin treatment, as measured by 3H-thymidine incorporation, were found to occur rapidly. The addition of 2 microM dihydrocytochalasin B (or cytochalasin B) together with aphidicolin during the second half of the inductive period inhibits the formation of supernumerary buds and upon removal of the cytochalasin rapid formation of buds takes place. We conclude that the formation of epithelial buds during branching morphogenesis occurs as a result of enhanced localized cell proliferation coupled with epithelial cell shape change (or preservation of cell morphology) mediated by microfilaments, which have been observed in both the apical and basal cytoplasm of the epithelial cells in the region where branching of the trachea is taking place.

Calcium, microfilaments and morphogenesis

Morphogenesis, the generation of tissue form, is important not only in the embryogenesis of a new individual, but also because a change in morphogenesis may be involved in the establishment of differences between individuals during evolution. Morphogenetic movements are effected in part by coordinated changes in the shapes of individual cells and over the past decade the cellular organelles responsible for cell shape have been identified as microfilaments and microtubules. In non-embryonic systems the contraction of microfilaments is controlled by the level of intracellular free calcium, and so calcium is implicated as an intermediate control mechanism in morphogenisis. Through techniques which perturb the calcium balance of cells, or which measure calcium ion concentration directly, evidence is accumulating that calcium is involved in morphogenetic movements such as gastrulation and neurulation, and related phenomena such as wound healing. Thus fundamental questions about the control of morphogenesis in embryogenesis and evolution may now be couched in more precise terms of the control of intracellular calcium ion balance.

Optic cup formation: a calcium-regulated process

Invagination of the optic vesicle to form the optic cup is an important event in the formation of the eye in the early embryo. To obtain support for earlier conclusions that a contractile process is involved, calcium dependency of optic cup formation was tested. Heads were excised from chicken embryos at the optic vesicle stage of development (stage 13) and incubated in nutrient medium containing antagonists or agonists of calcium transport. Invagination was reversibly inhibited by the Ca2+ antagonists verapamil and papaverine. It was initiated in a precocious fashion by the Ca2+ ionophore A23187 but only in the presence of external Ca2+. Neither caffeine, theophylline, nor A23187 (in the absence of external Ca2+) were able to initiate precocious optic cup formation. Trifluoperazine and chlorpromazine reversibly inhibited optic cup formation while chlorpromazine sulfoxide had no effect at the concentrations used. The binding of [3H]trifluoperazine to isolated stage 13 heads revealed a class of Ca2+-dependent binding sites having a Kd similar to that of calmodulin. These results indicate a Ca2+-dependence for optic cup formation and that the source of the Ca2+ may be extracellular. This Ca2+ dependence probably is conferred to the system by calmodulin.

Calcium dependence and contraction in somite formation

The existence of a calcium-dependent contractile process in the formation of somites from segmental plate mesoderm was investigated using a Ca2+ agonist and Ca2+ and calmodulin antagonists. The contribution of cell movement and apical constriction in the segmentation process were assessed using SEM of normal and drug-treated somite and segmental plate tissue. Explants that contained segmental plates of stage 14-15 chick embryos were cultured on vitelline membranes in calcium- and magnesium-free (CMF) Hands' solution, liquid culture medium, and medium containing drugs. Ca2+ ionophore A23187 promoted the rapid completion of one new somite pair. CMF halted segmentation. The Ca2+ antagonists verapamil and papaverine reversibly inhibited segmentation. Theophylline did not inhibit segmentation, suggesting that the effects of the Ca2+ antagonists are not due to inhibition of phosphodiesterase activity. These results suggest that somitogenesis is Ca2+-dependent. Two drugs that inhibit the binding of calmodulin, chlorpromazine and trifluoperazine (TFP), halted segmentation. The inhibitory effect of TFP was reversible. The effects of TFP on somites were compared with those of cytochalasin D. The contribution of microtubules to cell shape and movement in somitogenesis was examined by incubation with nocodazole, a reversible inhibitor of tubulin polymerization. Cell elongation and somitogenesis were inhibited.

Calcium in epithelial cell contraction

Epithelial morphogenesis in many organs involves asymmetric microfilament-mediated cellular contractions. Similar contractions, in terms of ultrastructure and cytochalasin B sensitivity, can be induced in the carcinoma cell line C-4II in culture. This line was used to compare total intracellular calcium levels ([Ca]i) in contracted monolayer fragments and in control cultures, and to determine whether epithelial cell contraction depends on influx of extracellular Ca. [Ca]i, defined as Ca not displaceable by lanthanum, was measured by atomic absorption spectrophotometry. Degrees of contraction were determined from shape changes of monolayer fragments. Detachment from the growth surface initiated cellular contractions and caused an immediate increase in [Ca]i, from 1.0 to 4.0-5.0 micrograms Ca/mg protein in early confluent cultures, and from 0.3 to 1.0-2.0 micrograms Ca/mg protein in crowded cultures. This increase was followed by a gradual decline in [Ca]i, though Ca levels remained higher than in controls and contraction progressed for 30 min. Contraction was inhibited completely by cold (7 degrees C) and by Ca-free medium, and in a dose-dependent manner by papaverine (2.5 x 10(-6) M-2.5 x 10(-4) M), lanthanum (1.0 x 10(-6) M-1.0 x 10(-4) M); and D-600 (1.0-2.0 x 10(-4) M). The Ca ionophore A23187 had no effect at 5.0 x 10(-6) M and was inhibitory at higher concentrations. The results provided direct evidence for increased [Ca]i in contracting epithelial cells, and suggest that Ca influx is required for such contraction to take place.

Neural tube closure defects caused by papaverine in explanted early chick embryos

Papaverine (50 micrograms/ml) preferentially inhibited uplifting of neural folds in explanted stage 8 chick embryos. Affected neuroepithelial cells often lost their wedge-shaped and elongated appearance. Also, luminal surfaces of most affected cells were smoother than usual as evidenced by the marked decrease in the number of cytoplasmic extensions, but the integrity of other structures (including cytoskeletal components) was not noticeably affected. The observed changes in cell surface topography were due, at least in part, to the imparied ability of apical microfilaments to contract and their eventual relaxation. The "relaxing" effect of papaverine on neural folds could be reversed by subsequent treatment with ionophore A23187. Since papaverine and ionophore A23187 are known to alter the normal distribution of intracellular Ca2+ and changes in cell surface topography are correlated with contractile activities of apical microfilaments, papaverine elicits neural tube closure defects by lowering intracellular free Ca2+ levels, thereby relaxing contracted apical microfilaments in neuroepithelial cells.

Macrophage polykarya

Multinucleated giant cells are commonly found in a wide variety of inflammatory reactions. They are formed at sites of tissue injury by fusion of freshly exuded monocytes, the rate of fusion being dependent on a range of extracellular and intracellular factors. Electron miscroscopy shows that the pooled components of the fused monocytes are not randomly dispersed in the syncytium, but are highly reorganized into a functioning unit. In addition, histochemical and biochemical profiles of cell populations containing these polykarya display a range of metabolic activities, including DNA synthesis, which, on occasions, is followed by successful mitotic division and the formation of polyploid daughter cells. Fusion results in the loss of some surface receptors which in turn interferes with the phagocytic performance of polykarya, which is generally less pronounced than their mononuclear precurses. In addition, polykarya are not as actively motile as macrophages although phenomena of contact inhibition are less obvious. On the other hand, the multinucleate giant cells display prominent exocytosis which may aid in the degradation of extracellular material. The properties of macrophage polykarya contrast with macrophage homokarya produced in vitro. The latter are actively phagocytic, do not synthesize DNA, and have a longer half-life than the syncytia produced in chronic inflammatory reactions. It may well be that the polykarya in such reactions are not true homokarya.

Basal lamina of embryonic salivary epithelia. Production by the epithelium and role in maintaining lobular morphology

The role of the basal lamina in maintaining the normal morphology of mouse embryo submandibular epithelia was assessed by examining its production as well as the cellular and organ culture changes associated with its removal and replacement. The lamina was removed from epithelia isolated free of mesenchyme by brief treatment with testicular hyaluronidase in the absence of calcium. The treatment causes rounding-up of the cells, loss of cellular cohesion, appearance of microvilli, and changes in the organization of cytoskeletal structures. The lamina is not removed and the cellular alterations do not occur in the absence of hyaluronidase in calcium-free medium or when both enzyme and calcium are present, possibly because digestion of chondroitin sulfate, a component of the lamina, is inhibited by calcium. Within 2 h after treatment, in the absence of mesenchyme or biological substrata, the epithelia deposits a new lamina, which is identical by several criteria to the preexisting lamina, and reverses the cellular alterations. Epithelia treated with hyaluronidase lose lobular morphology during culture with mesenchyme. Delaying culture with mesenchyme, to allow restoration of the lamina and of normal cellular architecture, prevents the loss of lobular morphology. The results indicate that the basal lamina imposes morphologic stability on the epithelium, while the mesenchyme apparently affects processes involved in changes in morphology, possibly by selective degradation of the basal lamina.

The locomotory behaviour of the multinucleate giant cells of foreign body reactions

Cinemicrographic observations have shown that multinucleate giant cells are motile but the degree of translocational movement decreases with increasing size. Generally these polykaryocytes are slower than macrophages exhibiting a speed translocation of 0-3-1-0 micron/min for small polykaryocytes (2-4 nuclei) to 0-04 micron/min for large polykaryocytes (greater than 15 nuclei). Such behaviour would tend to maintain these cells near the site of their formation. The direction of translocation of small syncytia, correlates with the direction of the single leading lamellipodium. Slightly larger cells (5-15 nuclei) show loss of polarity with multiple lamellipodia, none of which is dominant, while the very large syncytia possess a single, large, circumferential lamellipodium whose movement leads to little displacement. These latter polykarya also display reduced contact inhibition which may favour fusion phenomena. Glycolysis appears to be the main source of energy for cellular movement which is also dependent on physiological calcium concentration and an intact microtubular-microfilamentous system. Apposition to the substratum is not complete, occurring in some instances by means of flaps and microvilli which may also be involved in cellular locomotion. Loss of glycocalyceal material probably occurs during locomotion.

Cell differentiation in the terminal tubule of fetal rat submandibular gland in organ culture

Submandibular glands from 17-day-old rat fetuses were maintained in organ culture for five days in a medium consisting of Eagle's MEM (87%), horse serum (10%), and chick embryo extract (3%). Each day of the culture period explants were incubated for the demonstration of peroxidase activity and processed for light and electron microscopic observations. In some experiments cultures were exposed to 3H-thymidine one hour prior to fixation and incubation for the demonstration of peroxidase activity. Labelling index was determined using radioautographs of 1 mu Epon-embedded sections. At the time of explantation the submandibular gland rudiment consisted of undifferentiated epithelial cells arranged in cords. On day 3 of culture two additional cell types could be distinguished: terminal tubule cells and proacinar cells. The proacinar cells were characterized by peroxidase activity in their granules and cytoplasm. By day 4 acinar cells begin to appear. On the fifth day of culture the four cell types of the terminal tubule were present in the following proportions: undifferentiated cells, 44%; terminal tubule cells, 19%; proacinar cells, 31%; acinar cells, 6%. These results indicate that the cytodifferentiation of the secretory unit of rat submandibular gland in vitro is comparable to the differentiation in vivo.