Concepts of epithelial-mesenchymal interactions during development: tooth and lung organogenesis

Preferential FGF18/FGFR activity in pseudoglandular versus canalicular stage human lung fibroblasts

Fibroblast growth factor (FGF) signaling is necessary for proper lung branching morphogenesis, alveolarization, and vascular development. Dysregulation of FGF activity has been implicated in various lung diseases. Recently, we showed that FGF18 promotes human lung branching morphogenesis by regulating mesenchymal progenitor cells. However, the underlying mechanisms remain unclear. Thus, we aimed to determine the role of FGF18 and its receptors (FGFR) in regulating mesenchymal cell proliferation, migration, and differentiation from pseudoglandular to canalicular stage. We performed siRNA assays to identify the specific FGFR(s) associated with FGF18-induced biological processes. We found that FGF18 increased proliferation and migration in human fetal lung fibroblasts (HFLF) from both stages. FGFR2/FGFR4 played a significant role in pseudoglandular stage. HFLF proliferation, while FGFR3/FGFR4 were involved in canalicular stage. FGF18 enhanced HFLF migration through FGFR2 and FGFR4 in pseudoglandular and canalicular stage, respectively. Finally, we provide evidence that FGF18 treatment leads to reduced expression of myofibroblast markers (ACTA2 and COL1A1) and increased expression of lipofibroblast markers (ADRP and PPARγ) in both stages HFLF. However, the specific FGF18/FGFR complex involved in this process varies depending on the stage. Our findings suggest that in context of human lung development, FGF18 tends to associate with distinct FGFRs to initiate specific biological processes on mesenchymal cells.

Roles of Mesenchymal Cells in the Lung: From Lung Development to Chronic Obstructive Pulmonary Disease

Mesenchymal cells are an essential cell type because of their role in tissue support, their multilineage differentiation capacities and their potential clinical applications. They play a crucial role during lung development by interacting with airway epithelium, and also during lung regeneration and remodeling after injury. However, much less is known about their function in lung disease. In this review, we discuss the origins of mesenchymal cells during lung development, their crosstalk with the epithelium, and their role in lung diseases, particularly in chronic obstructive pulmonary disease.

In the absence of a basal lamina, ameloblasts absorb enamel in a serumless and chemically defined organ culture system

OBJECTIVES

Tooth organ development was examined in a serumless, chemically defined organ culture system to determine whether morphological and functional development was identical to that in in vivo and serum-supplemented organ cultures.

METHODS

Mouse mandibular first molar tooth organs at 16 days of gestation were cultured for up to 28 days in a Tronwell culture system using a serum-supplemented or serumless, chemically defined medium. After culture, specimens were processed for assessing tooth development using ultrastructural, immunohistochemical, and mRNA expression analyses.

RESULTS

In serum-supplemented conditions, inner enamel epithelial cells differentiated into secretory-stage ameloblasts, which formed enamel and reached the maturation stage after 14 and 21 days of culture, respectively. Ameloblasts deposited a basal lamina on immature enamel. Conversely, in serumless conditions, ameloblasts formed enamel on mineralized dentin after 21 days. Moreover, maturation-stage ameloblasts did not form basal lamina and directly absorbed mineralized enamel after 28 days of culture. RT-PCR analysis indicated that tooth organs, cultured in serumless conditions for 28 days, had significantly reduced expression levels of ODAM, amelotin, and laminin-322.

CONCLUSIONS

These results indicate that several differences were detected compared to the development in serum-supplemented conditions, such as delayed enamel and dentin formation and the failure of maturation-stage ameloblasts to form basal laminae. Therefore, our results suggest that some factors might be required for the steady formation of mineralized dentin, enamel, and a basal lamina. Additionally, our results indicate that a basal lamina is necessary for enamel maturation.

Novel tissue interactions support the evolution of placentation

Organ development occurs through the coordinated interaction of distinct tissue types. So, a question at the core of understanding the evolution of new organs is, how do new tissue-tissue signalling networks arise? The placenta is a great model for understanding the evolution of new organs, because placentas have evolved repeatedly, evolved relatively recently in some lineages, and exhibit intermediate forms in extant clades. Placentas, like other organs, form from the interaction of two distinct tissues, one maternal and one fetal. If each of these tissues produces signals that can be received by the other, then the apposition of these tissues is likely to result in new signalling dynamics that can be used as a scaffold to support placenta development. Using published data and examples, in this review I demonstrate that placentas are derived from hormonally active organs, that considerable signalling potential exists between maternal and fetal tissues in egg-laying vertebrates, that this signalling potential is conserved through the oviparity-viviparity transition, and that consequences of these interactions form the basis of derived aspects of placentation including embryo implantation. I argue that the interaction of placental tissues, is not merely a consequence of placenta formation, but that novel interactions form the basis of new placental regulatory networks, functions, and patterning mechanisms.

Bmp4 is an essential growth factor for the initiation of genital tubercle (GT) outgrowth

The external genitalia are appendage organs outgrowing from the posterior body trunk. Murine genital tubercle (GT), anlage of external genitalia, initiates its outgrowth from embryonic day (E) 10.5 as a bud structure. Several growth factors such as fibroblast growth factor (FGF), Wnt and Sonic hedgehog (Shh) are essential for the GT outgrowth. However, the mechanisms of initiation of GT outgrowth are poorly understood. We previously identified bone morphogenetic protein (Bmp) signaling as a negative regulator for GT outgrowth. We show here novel aspects of Bmp4 functions for GT outgrowth. We identified the Bmp4 was already expressed in cloaca region at E9.5, before GT outgrowth. To analyze the function of Bmp4 at early stage for the initiation of GT outgrowth, we utilized the Hoxa3-Cre driver and Bmp4 ^flox/flox mouse lines. Hoxa3 ^Cre/+ ; Bmp4 ^flox/flox mutant mice showed the hypoplasia of GT with reduced expression of outgrowth promoting genes such as Wnt5a, Hoxd13 and p63, whereas Shh expression was not affected. Formation of distal urethral epithelium (DUE) marked by the Fgf8 expression is essential for controlling mesenchymal genes expression in GT and subsequent its outgrowth. Furthermore, Fgf8 expression was dramatically reduced in such mutant mice indicating the defective DUE formation. Hence, current results indicate that Bmp4 is an essential growth factor for the initiation of GT outgrowth independent of Shh signaling. Thus, Bmp4 positively regulates for the formation of DUE. The current study provides new insights into the function of Bmp signaling at early stage for the initiation of GT outgrowth.

Spatial and temporal expression of Sox9 during murine incisor development

Mouse incisors are capable of continuously growing due to the renewal of dental epithelium stem cells and mesenchymal stem cells residing at the proximal ends. The transcription factor Sox9 plays important roles in maintaining the stem cells of hair follicles, retinal progenitor cells and neural crest stem cells. Whether Sox9 is involved during mouse incisor development is not reported yet. In this study, we examined the expression pattern of Sox9 during mouse incisor development by in situ hybridization and immunohistochemistry. Sox9 mRNA and protein showed similar expression pattern from embryonic day (E) 13.5 to postnatal (PN) day 10. At E13.5 and E14.5, Sox9 was strongly expressed in the dental epithelium. At E16.5, Sox9 started to be detected in the mesenchymal cells within the dental pulp, especially the dental pulp cells that adjacent to the labial cervical loop. Similarly with E14.5, Sox9 was strongly detected in the labial cervical loop, including the basal epithelium, the stellate reticulum and the outer enamel epithelium from E16.5 to PN10. The mesenchyme adjacent to the labial cervical loop also showed strong signal of Sox9. The spatiotemporal expression of Sox9 suggested its possible involvement during mouse incisor development.

Expression pattern of Sox2 during mouse tooth development

The transcription factor Sox2 plays important roles in maintaining the pluripotency of embryonic stem cells and adult progenitors. However, whether Sox2 is involved in odontogenesis has not been reported. In this study, we examined the expression pattern of Sox2 during mouse incisor and molar development using real-time PCR, in situ hybridization and immunohistochemistry. Sox2 mRNA was expressed in the dental epithelium and mesenchyme while Sox2 protein was mainly detected in the epithelium from embryonic day (E) 11.5 to postnatal (PN) day 20. In the case of incisor, Sox2 mRNA and protein were expressed in most of dental epithelial cells from E11.5 to E14.5, and they were both highly expressed in the labial cervical loop area from E16.5 to PN20. During molar development, we observed an asymmetrical distribution of Sox2 protein in the epithelium from E13.5 to E16.5, with stronger signals in the lingual side. From E18.5 to PN2, Sox2 was expressed within the cervical loop area, and the stellate intermediate layer. From PN6 to PN14, Sox2 expression was confined mainly to the apical end of hertwig's epithelium root sheath (HERS) cells. Sox2 was also detected within the perivascular region of the dental pulp at PN14 and PN20. Our results suggested that: (1) Sox2 was involved in mouse odontogenesis, and (2) it might participate in maintaining the pluripotency of the epithelial stem cells of labial cervical loop in mouse incisor development and the epithelium progenitors during molar development, (3) Sox2 might be regulated at post-transcription level during mouse odontogenesis.

Identification of human fibroblast cell lines as a feeder layer for human corneal epithelial regeneration

There is a great interest in using epithelium generated in vitro for tissue bioengineering. Mouse 3T3 fibroblasts have been used as a feeder layer to cultivate human epithelia including corneal epithelial cells for more than 3 decades. To avoid the use of xeno-components, we evaluated human fibroblasts as an alternative feeder supporting human corneal epithelial regeneration. Five human fibroblast cell lines were used for evaluation with mouse 3T3 fibroblasts as a control. Human epithelial cells isolated from fresh corneal limbal tissue were seeded on these feeders. Colony forming efficiency (CFE) and cell growth capacity were evaluated on days 5–14. The phenotype of the regenerated epithelia was evaluated by morphology and immunostaining with epithelial markers. cDNA microarray was used to analyze the gene expression profile of the supportive human fibroblasts. Among 5 strains of human fibroblasts evaluated, two newborn foreskin fibroblast cell lines, Hs68 and CCD1112Sk, were identified to strongly support human corneal epithelial growth. Tested for 10 passages, these fibroblasts continually showed a comparative efficiency to the 3T3 feeder layer for CFE and growth capacity of human corneal epithelial cells. Limbal epithelial cells seeded at 1×10⁴ in a 35-mm dish (9.6 cm²) grew to confluence (about 1.87–2.41×10⁶ cells) in 12–14 days, representing 187–241 fold expansion with over 7–8 doublings on these human feeders. The regenerated epithelia expressed K3, K12, connexin 43, p63, EGFR and integrin β1, resembling the phenotype of human corneal epithelium. DNA microarray revealed 3 up-regulated and 10 down-regulated genes, which may be involved in the functions of human fibroblast feeders. These findings demonstrate that commercial human fibroblast cell lines support human corneal epithelial regeneration, and have potential use in tissue bioengineering for corneal reconstruction.

Three dimensional dental epithelial-mesenchymal constructs of predetermined size and shape for tooth regeneration

While it is known that precise dental epithelial-mesenchymal (DE-DM) cell interactions provide critical functions in tooth development, reliable methods to establish proper DE-DM cell interactions for tooth regeneration have yet to be established. To address this challenge, and to generate bioengineered teeth of predetermined size and shape, in this study, we characterize three dimensional (3D) pre-fabricated DE-DM cell constructs. Human dental pulp cell seeded Collagen gel layers were co-cultured with porcine DE cells suspended in Growth Factor Reduced (GFR) Matrigel. The resulting 3D DE-DM cell layers were cultured in vitro, or implanted and grown subcutaneously in vivo in nude rats. Molecular, histological and immunohistochemical (IHC) analyses of harvested implants revealed organized DE-DM cell interactions, the induced expression of dental tissue-specific markers Amelogenin (AM) and Dentin Sialophosphoprotein (DSPP), and basement membrane markers Laminin 5 and collagen IV, and irregular mineralized tissue formation after 4 weeks. We anticipate that these studies will facilitate the eventual establishment of reliable methods to elaborate dental tissues, and full sized teeth of specified sized and shape.

Tracheal innervation is abnormal in rats with experimental congenital diaphragmatic hernia

BACKGROUND

Tracheobronchial motility influences lung development. Lung hypoplasia and lung sequelae accompany congenital diaphragmatic hernia (CDH) in which the vagus nerves and esophageal innervation are abnormal. As the vagus supplies tracheal innervation, this study tested the hypothesis that it might also be abnormal in rats with CDH.

MATERIAL AND METHODS

Intrinsic ganglia were counted and measured in whole mount acetylcholinesterase-stained tracheas from CDH and control E21 fetal rats. The relative surfaces occupied by neural structures were measured in tracheal sections immunostained for p75(NTR) and PGP 9.5. PGP 9.5 protein and mRNA expression were determined. Mann-Whitney tests were used for comparisons between groups using P < .05 as significant.

RESULTS

p75(NTR) staining showed the neural crest origin of tracheal innervation. Scarce neural structures and smaller ganglia were found in CDH fetuses. PGP 9.5 protein expression was decreased in CDH fetuses, whereas PGP 9.5 mRNA levels were increased in comparison with controls.

CONCLUSIONS

Decreased density of neural structures and size of intramural ganglia, reduced expression of neural tissue and PGP 9.5 protein, and increased levels of PGP 9.5 mRNA reveal deficient tracheal innervation in rats with CDH. If similar anomalies exist in the human condition, they could contribute to explaining the pathogenesis of lung hypoplasia and bronchopulmonary sequelae.

The sequential seeding of epithelial and mesenchymal cells for tissue-engineered tooth regeneration

Progress is being made toward regenerating teeth by seeding dissociated postnatal odontogenic cells onto scaffolds and implanting them in vivo, but tooth morphology remains difficult to control. In this study, we aimed to facilitate tooth regeneration using a novel technique to sequentially seed epithelial cells and mesenchymal cells so that they formed appropriate interactions in the scaffold. Dental epithelium and mesenchyme from porcine third molar teeth were enzymatically separated and dissociated into single cells. Mesenchymal cells were seeded onto the surface of the scaffold and epithelial cells were then plated on top so that the two cell types were in direct contact. The cell-scaffold constructs were evaluated in vitro and also implanted into immunocompromised rats for in vivo analysis. Control groups included constructs where direct contact between the two cell types was prevented. In scaffolds seed using the novel technique, alkaline phosphatase activity was significantly greater than controls, the tooth morphology in vivo was developed in similar to that of natural tooth, and only one tooth structure formed in each scaffold. These results suggest that the novel cell-seeding technique could be useful for regulating the morphology of regenerated teeth.

Regional regulation of palatal growth and patterning along the anterior-posterior axis in mice

Cleft palate is a congenital disorder arising from a failure in the multistep process of palate development. In its mildest form the cleft affects only the posterior soft palate. In more severe cases the cleft includes the soft (posterior) and hard (anterior) palate. In mice a number of genes show differential expression along the anterior-posterior axis of the palate. Mesenchymal heterogeneity is established early, as evident from Bmp4-mediated induction of Msx1 and cell proliferation exclusively in the anterior and Fgf8-specific induction of Pax9 in the posterior palate alone. In addition, the anterior palatal epithelium has the unique ability to induce Shox2 expression in the anterior mesenchyme in vivo and the posterior mesenchyme in vitro. Therefore, the induction and competence potentials of the epithelium and mesenchyme in the anterior are clearly distinct from those in the posterior. Defective growth in the anterior palate of Msx1-/- and Fgf10-/- mice leads to a complete cleft palate and supports the anterior-to-posterior direction of palatal closure. By contrast, the Shox2-/- mice exhibit incomplete clefts in the anterior presumptive hard palate with an intact posterior palate. This phenotype cannot be explained by the prevailing model of palatal closure. The ability of the posterior palate to fuse independent of the anterior palate in Shox2-/- mice underscores the intrinsic differences along the anterior-posterior axis of the palate. We must hitherto consider the heterogeneity of gene expression and function in the palate to understand better the aetiology and pathogenesis of non-syndromic cleft palate and the mechanics of normal palatogenesis.

Spatial and temporal distribution of nerves, ganglia, and smooth muscle during the early pseudoglandular stage of fetal mouse lung development

Neural tissue and smooth muscle appear early in the developing fetal lung, but little is known of their origin and subsequent distribution. To investigate the spatial and temporal distribution of nerves, ganglia, and airway smooth muscle during the early pseudoglandular stage, fetal mouse lungs at embryonic days (E) 11 to 14 were immunostained as whole-mounts and imaged by confocal microscopy. At E11, the primordial lung consisted of the future trachea and two budding epithelial tubules that were covered in smooth muscle to the base of the growing buds. The vagus and processes entering the lung were positive for the neural markers PGP 9.5 (protein gene product 9.5) and synapsin but no neurons were stained at this stage. An antibody to p75NTR revealed neural crest cells on the future trachea as well as in the vagus and in processes extending from the vagus to the lung. This finding indicates that even though neuronal precursors are already present at this stage, they are still migrating into the lung. By E12, neural tissue was abundant in the proximal part of the lung and nerves followed the smooth muscle-covered tubules to the base of the growing buds. At E13 and E14, a neural network of interconnected ganglia, innervated by the vagus, covered the trachea. The postganglionic nerves mainly followed the smooth muscle-covered tubules, but some extended out into the mesenchyme beyond the epithelial buds. Furthermore, we show in a model of cultured lung explants that neural tissue and smooth muscle persist and continue to grow and differentiate in vitro. By using fluorescent markers and confocal microscopy, we present the developing lung as a dynamic structure with smooth muscle and neural tissue in a prime position to influence growth and development.

Molecular analysis of external genitalia formation: the role of fibroblast growth factor (Fgf) genes during genital tubercle formation

The molecular mechanisms underlying the development of the external genitalia in mammals have been very little examined. Recent gene knockout studies have suggested that the developmental processes of its anlage, the genital tubercle (GT), have much in common with those of limb buds. The Fgf genes have been postulated as regulating several downstream genes during organogenesis. Fgf8 was expressed in the distal urethral plate epithelium of the genital tubercle (GT) together with other markers such as the Msx1, Fgf10, Hoxd13 and Bmp4 expressed in the mesenchyme. To analyze the role of the FGF system during GT formation, an in vitro organ culture system was utilized. It is suggested that the distal urethral plate epithelium of GT, the Fgf8-expressing region, regulates the outgrowth of GT. Ectopic application of FGF8 beads to the murine GT induced mesenchymal gene expression, and also promoted the outgrowth of the GT. Experiments utilizing anti-FGF neutralizing antibody suggested a growth-promoting role for FGF protein(s) in GT outgrowth. In contrast, despite its vital role during limb-bud formation, Fgf10 appears not to be primarily essential for initial outgrowth of GT, as extrapolated from Fgf10(−/−) GTs. However, the abnormal external genitalia development of Fgf10(−/−) perinatal mice suggested the importance of Fgf10 in the development of the glans penis and the glans clitoridis. These results suggest that the FGF system is a key element in orchestrating GT development.

In vitro reconstitution of the tracheal epithelium

We have developed a unique in vitro reconstitution system for tracheal epithelia of guinea pigs. In the system, a human amnion membrane was used as a basement membrane and the tracheal epithelial cells were cultured on the epithelial side of the membrane. Three weeks later, the tracheal fibroblasts were co-cultured on the serosal side of the amnion membrane and the culturing was continued for an additional 10 d. The morphology of the cultured epithelial cells consisted of a pseudostratified columnar ciliated epithelium from cuboidal ciliated epithelium during the last 10 d of the culture period. Epithelial cells included both goblet-like and basal cells. In addition, the frequency of each type of differentiated cells was almost identical to that of in vivo tracheas. Interestingly, the same results were obtained when the conditioned medium of the tracheal fibroblasts was used instead of the fibroblasts themselves. These results suggest that epithelial-mesenchymal interaction is likely involved in growth and differentiation of epithelial cells in vivo in a soluble factor(s)-mediated manner. As well as the epithelial cells, the fibroblasts also formed a multilayer during the last 10 d of co-culturing. This indicates that in vitro reconstitution of tracheal epithelia is achieved without addition of any exogenous growth or differentiation factors. The reconstitution system is shown to be useful for investigating the cellular and molecular interaction of epithelial and mesenchymal cells. Possible applications of the culture system and possible factors involved in growth and differentiation of epithelial cells are discussed.

Induction of precocious pepsinogen synthesis by glucocorticoids in fetal rat gastric epithelium in organ culture: importance of mesenchyme for epithelial differentiation

Glucocorticoids significantly affect both proliferation and differentiation of gastric epithelial cells in vivo. Here we examined the mechanism of action of glucocorticoids on the cells in vitro, with special reference to the epithelial-mesenchymal interaction. When 16.5-day fetal rat gastric explants were maintained in organ culture, the epithelial cells began to invaginate into mesenchyme on days 3 to 4, and formed glandular structures on days 5 to 6 in culture. Immunohistochemical analysis with specific antibodies revealed that pepsinogen-synthesizing cells first appeared on day 2, and they increased in number with epithelial morphogenesis to about 20%-30% of total epithelial cells on days 4 to 6, and that these cells were localized at the base of glandular structures in control media. When the explants were treated with hydrocortisone (1 microgram/ml), epithelial morphogenesis was mostly suppressed, but epithelial cytodifferentiation was significantly stimulated, indicating that epithelial morphogenesis is not necessary for their cytodifferentiation. In glucocorticoid-treated explants, pepsinogen-synthesizing cells first appeared on day 1, and more than 90% of the cells were positively stained with the antibodies from days 3 to 5 in culture. Biochemical analysis showed that much higher acid protease activity could be detected in glucocorticoid-treated explants than in controls from days 2 to 6 in culture, and analysis by zymography indicated that the synthesis of pepsinogen 1 but not cathepsin E was stimulated by the hormone. Northern blotting analysis showed that the level of pepsinogen 1 mRNA was greatly increased by glucocorticoids. Examination of the effect of the hormone on the epithelial proliferation showed that hydrocortisone (1 microgram/ml) significantly inhibited the epithelial growth from days 1 to 3 in culture. To investigate the role of epithelial-mesenchymal interaction in the glucocorticoid-induced differentiation of the gastric epithelial cells, effects of the hormone on the proliferation and differentiation of the cells in the absence of mesenchyme were examined, using a recently established primary culture system. The epithelial cells synthesized cathepsin E but not pepsinogen in cell culture, irrespective of glucocorticoid treatment, and the level of acid protease activity was not affected by the hormone, indicating that mesenchyme is necessary for the hormone to induce pepsinogen gene expression in the epithelial cells. In the cell culture system, glucocorticoids did not inhibit but significantly stimulated epithelial proliferation. This suggests that the hormone indirectly inhibited epithelial proliferation in organ culture, probably via mesenchyme. The mechanism of action of glucocorticoids on the epithelial-mesenchymal interaction in the fetal glandular stomach is discussed.

Expression of matrix proteins during the development of mineralized tissues

The specific properties of mineralized tissues are defined by the composition of the fraction of the noncollagenous matrix proteins. Because these proteins play a pivotal role in the processes of cell differentiation and activation and of mineralization, their temporal and spatial expression is tightly regulated. Within this study, the expression of the enamel protein amelogenin and of the bone matrix proteins osteopontin, bone sialoprotein, osteocalcin, and osteonectin was investigated by in situ hybridization. Two models that allow observation of the formation of mineralized tissues were chosen. The development of bone and cartilage was observed on murine metatarsals from 15-day-old embryos up to 1-day-old mice. This time covers the periods of initial bone formation as well as onset of resorption of mineralized cartilage and bone. To study gene expression in the mineralized tissues of the dental organ, enamel, dentin, and cementum, developing molars ranging in age from 16-day-old embryos to 14 days after delivery were chosen. Within this time frame, the molars develop from an immature state to the differentiated organ which erupts through the mandibular bone. In the developing metatarsals, osteopontin and bone sialoprotein mRNAs were detected in osteoblasts and hypertrophic chondrocytes at the onset of mineralization. In the tooth organ, only cementoblasts expressed transcripts encoding the two proteins; odontoblasts and ameloblasts did not express these genes. Osteonectin was expressed by osteoblasts and hypertrophic chondrocytes as well, whereas in the molars it was produced exclusively by odontoblasts. Osteocalcin was expressed specifically by osteoblasts in the developing metatarsals. In tooth, osteocalcin transcripts were detected in odontoblasts. Finally, amelogenin was a specific product of ameloblasts. Thus, a sequential and cell type-restricted expression of matrix proteins takes place during the development of the mineralized tissues. The expression patterns of the transcripts encoding the bone matrix proteins suggest different biological roles depending on the time and site of expression.

Differential intrastromal invasion by normal ocular surface epithelia is mediated by different fibroblasts

For most mucosal epithelia including the ocular surface, it is generally believed that wound healing is executed by epithelial migration on the plane of erosion or ulceration. In explant cultures, we incidentally observed the phenomenon of intrastromal invasion by corneal, limbal and conjunctival epithelial cells even when cell migration on plastics was promoted. Homotypic and heterotypic tissue recombinants between corneal and conjunctival epithelial cells and their stroma revealed that this phenomena was dependent on viable mesenchymal cells and was more active in conjunctival stroma than corneal stroma. Using organotypic cultures in which 3T3 fibroblasts were incorporated in collagen gel, we noted that this phenomenon was fibroblast-dependent and up-regulated by lifting the culture to the air-fluid interphase. The extent of intrastromal invasion was decreased if 3T3 fibroblasts were treated with increasing concentrations of mitomycin C. The invading epithelial islands retained the same basal and suprabasal epithelial phenotypes as those of the surface epithelial layers using several anti-keratin monoclonal antibodies. Using 5-fluorouracil (5-FU) to eliminate the rapid-cycling, i.e. transient amplifying progenitor basal cells, we further noted that this phenomenon could still be produced by 5-FU-resistant slow-cycling progenitor cells of corneal, limbal and conjunctival explants. In organotypic cultures, conjunctival fibroblasts were more active than corneal fibroblasts in inducing corneal or conjunctival epithelial invasion. As such intrastromal invasion can experimentally be produced by normal non-transformed adult epithelial cells and mediated by fibroblasts, this in vitro phenomenon may be useful for studying the epithelial-mesenchymal interactions operating during embryonic development and post-natal wound healing.

Three patterns of cytokine expression potentially involved in epithelial-fibroblast interactions of human ocular surface

Signals transmitted from mesenchyme to epithelia or vice versa constitute the basis of reciprocal epithelial-mesenchymal interactions. As a first step toward understanding epithelial-mesenchymal interactions on the ocular surface where the transit amplifying cell-containing corneal epithelium is anatomically separated from the stem cell-containing limbal epithelium, we sought to characterize the expression patterns of cytokines and their receptors by primary epithelial and early-passaged fibroblast cultures of human cornea and limbus. Northern hybridization with oligonucleotide and cDNA probes to a total of 25 cytokines and 12 of their receptors revealed that the positively expressed cytokines could be divided into the following four patterns. Type I: TGF-alpha, IL-1 beta, and PDGF-B were expressed exclusively by epithelial cells but their respective receptors EGFR and IL-1R were predominantly and PDGFR-beta was exclusively expressed by fibroblasts. Type II: IGF-I, TGF-beta 1, -beta 2, LIF, and bFGF, and their receptors were expressed by both epithelial cells and fibroblasts. FGFR-1 (flg) and FGFR-2 (bek) were expressed more by fibroblasts and bFGF was expressed more by corneal than limbal epithelial cells. Type III: keratinocyte growth factor (KGF) and hepatocyte growth factor (HGF) were expressed exclusively by fibroblasts and their respective receptors, KGFR and c-met, were predominantly expressed by epithelial cells. Combined with RT-PCR, the quantity of KGF and KGFR transcripts was highest in limbal fibroblasts and epithelial cells, respectively. In contrast, the quantity of HGF and HGFR (c-met) transcripts was highest in corneal fibroblasts and epithelial cells, respectively. Type IV: M-CSF and IL-8 were expressed by fibroblasts and/or epithelial cells but their receptors were not expressed by epithelial cells nor fibroblasts, but by immune or inflammatory cells. In addition to these potential paracrine actions, autocrine actions mediated by TGF-alpha/EGFR, IL-1 beta/IL1-R, and bFGF/FGFR-1 were more expressed by corneal than limbal epithelial cells. Immunofluorescence staining on human corneoscleral cryosections confirmed that EGFR and bFGF were not expressed by the limbal basal epithelium, but expressed strongly by the corneal epithelium, a pattern consistent with Northern hybridization. These results indicate that ocular surface epithelial cells and fibroblasts can express a myriad of cytokines, among which the first three patterns constitute the network of potential epithelial-mesenchymal cytokine dialogues. The difference of certain cytokine expression between corneal and limbal regions suggests that this network participates in normal epithelial growth and differentiation, and plays an important role in wound healing.

Production of fibroblast-pneumocyte-like factor by fetal rabbit lung fibroblasts: isolation and effects of it and related factors on fetal type II cells in vitro

Fetal lung fibroblasts interact with type II epithelial cells, inducing their maturation. This interaction arises by secretion of factors which alter fetal type II cell function. To analyze these factors, conditioned medium (CM) was produced by exposing serum-free minimum essential medium, with [35S]methionine (5 microCi/ml), to confluent cultures of fetal rabbit lung fibroblasts. This medium was tested for ability to stimulate [3H]choline incorporation by fetal type II cells and subsequently fractionated on molecular weight filtration columns P60 (2.5cm x 90cm; NMW cutoff, 60kd; 1M acetic acid) and A1.5m (2.5cm x 90cm; NMW cutoff, 1,500kd; Tris-buffered saline) and a hydroxyapatite column (HT) (1.5cm x 30cm; NaCl and 0.01-0.3M phosphate). Crude medium stimulated choline incorporation into phosphatidylcholine. [35S]methionine was resolved in void volume material and in material of apparent molecular weight of 6000 daltons on the P60 filtration column. Filtration on the A1.5m column showed two major fractions with radiolabel incorporation. Each of these was resolved into two subfractions on HT chromatography. The high molecular mass fraction contained material which stimulated [3H]choline incorporation by fetal type II cells. The low molecular mass fraction tended to inhibit [3H]choline incorporation. The second subfractions of both the first and second primary fractions inhibited [3H]thymidine incorporation into DNA by fetal type II cells. SDS-PAGE electrophoresis and autoradiography showed that under reducing conditions, each peak contained several proteins. However few of these displayed radioactivity. These results indicate that protein factors produced by fetal lung fibroblasts may be involved in regulating both differentiation and replication of fetal type II cells.

Epigenetic role of epidermal growth factor expression and signalling in embryonic mouse lung morphogenesis

A major unsolved problem in developmental biology is to determine when and how time- and position-restricted instructions are signaled and received during secondary embryonic inductions such as branching morphogenesis. The mouse embryonic lung rudiment was used to test the hypothesis that endogenous peptide growth factors, specifically epidermal growth factor (EGF), serve as instructive epigenetic signals for morphogenesis. The presence of EGF precursor mRNA transcripts was detected using the reverse-transcriptase-coupled polymerase chain reaction both in E11-E17-day mouse embryo lung tissues in vivo and in E11-day lung cultured for up to 7 days in vitro under chemically defined, serum-free conditions. Immunolocalization identified a position-restricted distribution of EGF in and around the primitive airways both during in vivo lung morphogenesis and in culture. EGF receptors (EGFR) coimmunolocalized with EGF in the primitive airways. Addition of exogenous EGF to lungs in culture resulted in significant concentration-dependent stimulation of branching morphogenesis, DNA, RNA, and protein content, and in [3H]thymidine incorporation into DNA. Conversely, the addition of tyrphostin (specific EGF receptor kinase antagonist) to lungs in culture resulted in concentration-dependent inhibition of branching morphogenesis, DNA, RNA, and protein content, and in [3H]thymidine incorporation into DNA without apparent cytotoxicity. The inhibition of the EGF signal by tyrphostin was confirmed by immunoprecipitation of tyrosine phosphoproteins. We conclude that early mouse embryo lungs express EGF transcripts and corresponding EGF peptides in a specific position-restricted distribution which coimmunolocalizes with EGFR in the primitive airways, while stimulatory and inhibitory studies indicate a functional role for the transduced EGF signal in the epigenetic regulation of lung branching morphogenesis. We speculate that the peptide growth factor EGF serves a function in secondary embryonic morphogenetic inductions, which may be modulated by interaction with other growth factors.

Rat endometrial stromal-epithelial response to estrogen infusion

Morphologic changes at the interface of rat endometrial luminal epithelial cells and the stromal cells immediately adjacent were examined and correlated with hypertrophy of the epithelial cells during estradiol (E2) infusion (1 microgram E2/24 h). While the lamina densa in castrate endometrium was thread-like, it became thicker and apparently more granular in some areas below the luminal epithelium during E2 infusion. However, no changes were seen in the intensity of laminin-like immunoreactivity at various time points up to 96 hours after beginning infusion, suggesting that these alterations were due to changes in nonlaminin components. The stromal cells adjacent to the basal lamina in the castrate state had cell processes extending toward the epithelium that terminated on the basal lamina. Under estrogen infusion, stromal cell bodies migrated close to and became oriented along the basal lamina. No interruptions were seen in the lamina densa or in the laminin-like immunoreactivity in the basal lamina. Thus, there were no direct morphologic interactions between epithelial and stromal cells induced by estrogen. Some of the stromal cells developed a dilated rough endoplasmic reticulum and some developed multiple elaborate processes within 41 hours after minipump implantation. Within 28 hours, nuclear hypertrophy had occurred in 15% of the epithelial cell layer. If interactions occur between stromal and epithelial cells, and morphologic evidence presented here suggests they do, then all such interactions are through an intact lamina densa-laminin layer, and any chemical mediators affecting cells on opposite sides of the lamina densa must migrate through it.

Ultrastructure of murine trisomy 1 optic cup

The optic cups of two gestational day 11 trisomy (ts) 1 mouse embryos and a normal littermate control were examined using transmission electron microscopy (TEM). One trisomic embryo had a small lens with a lens stalk; the other was aphakic. The resolution available with TEM allowed detailed evaluation of cell organelles, spatial relationships, and the intra- and extracellular structural environment of the optic cup in normal and abnormal mouse embryos. Differences between the normal littermate and the trisomic optic cups, as well as between the two ts 1 structures, included the following: 1) melanin granules in the retinal layer and intraretinal space as well as in the pigment layer, 2) neither pseudostratified nor cuboidal neuroepithelium in trisomic optic cups, 3) increasing cell lysis with severity of eye defect, 4) fusion between retinal and pigment layer cells and cells from the pigment layer and head mesoderm. This investigation not only confirmed some of the abnormal morphology found in light microscopic studies of ts 1 at this gestational age but also identified other anomalies in the ts 1 eye that may play a part in the dysgenesis of this organ. The roles of larger than normal intercellular lacunae, disorganized microtubules, and the connections between different cell types in the ts 1 optic cup require further investigation.

Fibroblast-mediated acceleration of human epithelial tumor growth in vivo

Transformed fibroblasts coinoculated with epithelial cells accelerated the growth and shortened the latency period of human epithelial tumors in athymic mice. Addition of NbF-1 fibroblasts caused epithelial tumors to grow from five marginally tumorigenic or "nontumorigenic" (nontumor-forming) human tumor cell lines or strains: PC-3 (prostate), WH (bladder), MDA-436 (breast), and cells derived from the ascites fluids of patients with metastatic renal pelvic or prostate cancers. Evidence for the human and epithelial nature of these experimental tumors was provided by histologic, immunohistochemical, Southern and dot-blot hybridization, and cytogenetic analyses. Transformed fibroblasts induced predominantly carcinosarcomas, whereas nontumorigenic fibroblasts (NIH 3T3) and lethally irradiated transformed fibroblasts induced exclusively carcinomas. The fibroblast-epithelial interaction appears to occur bidirectionally and does not result from cell fusion. Because coculture experiments in vitro did not demonstrate an increased cell proliferation, it appears that undefined host factors can influence tumor growth. This tumor model may be useful in drug-screening programs and in mechanistic studies of factors regulating human tumor growth and progression.

Lamellar body formation precedes pulmonary surfactant apoprotein expression during embryonic mouse lung development in vivo and in vitro

The purpose of this investigation was to determine whether lamellar inclusion body (LB) formation and surfactant apoprotein (SP-35) production are directly coordinated by temporal and positional information during development. In the present study we report a comparison between embryonic B10.A mouse lung morphogenesis and cytodifferentiation in vivo with that observed during organ culture in serumless medium. Precursor LB were first detected at embryonic day 12 (E12d), and progressively larger numbers and forms were produced during subsequent differentiation of respiratory alveolar duct epithelium. SP-35 was first detected during the canalicular period (E16.5d). Lung cultures (E12d) showed pseudoglandular and canalicular periods of morphogenesis, and both ciliated epithelial and type II cell differentiation. Nonciliated cells produced increasing numbers of lamellar inclusion bodies throughout the culture period. SP-35 was detected at 9 days in vitro (d.i.v.). These observations indicate (i) precursor LB formation precedes SP-35 expression and is not dependent on apoprotein synthesis; (ii) E12d lung development in vitro using serumless medium proceeds at a rate equivalent to 0.5 days in vivo through 11 d.i.v.; and (iii) morphogenesis and differentiation occur in the absence of exogenous hormones and growth factors. The cell-cell interactions that play a role in morphogenesis and cell differentiation appear to be intrinsic to the developmental program for embryonic lung development and are likely to be mediated by autocrine and/or paracrine factors.

Splice of life: toward understanding genetic determinants of oral diseases

The introduction of recombinant DNA technology has led to a rapid advancement of our knowledge of genes and genomic structure. Such technology, applied to the human genome, has provided valuable information concerning the nature and possible treatment of inherited disorders throughout the human life span. A number of oral diseases can be attacked by use of molecular biological techniques in conjunction with genetic linkage analysis. It will be possible to identify carriers of inherited diseases, to utilize prenatal and antenatal diagnosis when appropriate, and to pursue studies on the mode of inheritance of specific oral diseases. Present and projected advances in this dimension of dental research provide the knowledge and strategies for approaching and solving a number of major problems as we continue to pave the way for improvements in the diagnosis, treatment, and prevention of many oral and craniofacial diseases. This position paper examines a few of the promising areas and suggests future goals toward establishing a knowledge of the genetic determinants for oral diseases.

Immunological characterization, developmental pattern and vitamin-D-dependency of calbindin D-28 K in rat teeth ameloblasts

It has been suggested that vitamin D is involved in the process of cell differentiation and extracellular mineralization during tooth development. One of the best-defined molecular markers of the action of vitamin D is a calcium-binding protein of Mr 28,000 called calbindin D-28 K (CaBP 28 K). Since this protein is present in growing teeth, we have examined its synthesis in teeth from vitamin D-replete and -deplete rats by Western blotting and immunocytochemistry with an antiserum to CaBP 28 K purified from rat kidney. The CaBP 28 K present in the enamel organ is a single molecular species migrating near 30 k Da, similarly to the kidney protein. The differentiation and maturation of odontogenic cells were followed during early postnatal development (2-12 days) in rat molars. At the light-microscope level, CaBP 28 K was only found in a single cell-type, the ameloblasts. The expression of this protein appeared to be developmentally controlled, since its distribution varied with the cell stage and the functional steps of amelogenesis. The protein was localized in the basal compartment of ameloblasts from the presecretory stage. During the early secretory stage, the concentration of cytoplasmic CaBP 28 K formed a gradient from the apical to the basal pole of the ameloblasts. Staining appeared homogeneous in the cytoplasm of later secretory ameloblasts. CaBP 28 K was discontinuously distributed during the maturation stage. This discontinuity might be related to cyclical changes in mature ameloblasts. In all stages, ameloblasts from vitamin-D-deficient rats appeared depleted of CaBP 28 K.

Cellular interactions promote tissue-specific function, biomatrix deposition and junctional communication of primary cultured hepatocytes

Hepatocytes, prepared from normal adult rat liver, were seeded onto a collagen substratum and cultured alone or in the presence of rat liver endothelial cells. When hepatocytes were cultured alone in a hormonally defined serum-free medium, decreased albumin production and rapid morphological deterioration of bile canaliculi structures and gap junctions occurred within 4 to 5 days. In contrast, hepatocytes cocultured with liver mesenchymal cells remained morphologically intact and biochemically functional for at least 4 weeks. They reorganized into small islands, continued to secrete high levels of albumin, did not express alpha-fetoprotein (a fetal marker), and remained strongly dye coupled. All of the hepatocytes synthesized albumin and retained their gap junctional channels. No junctional communication was observed between hepatocytes and endothelial cells. Long fibers containing fibronectin, Type I collagen and laminin distributed over the hepatocytes were induced in coculture but never appeared in hepatocytes cultured alone. Moreover, supplementation of the hormonally defined medium with phenobarbital and dimethyl sulfoxide, both of which improve the life span and functional activities of cultured hepatocytes, failed to induce reticulin fiber formation in pure culture of hepatocytes. The modulation of albumin secretion, biomatrix deposition and junctional communication observed in hepatocytes cultured with sinusoidal liver cells was also obtained when hepatocytes were in association with various epithelial or mesenchymal cells [rat liver epithelial cells (T51B), mouse embryonic fibroblasts (NIH 3T3), human or rat dermal fibroblasts and bovine aorta endothelial cells (AG 4762)].

Embryonic mouse lung morphogenesis and type II cytodifferentiation in serumless, chemically defined medium using prolonged in vitro cultures

The timing, position and mechanism(s) for determining type II cytodifferentiation during mammalian lung development are not known. To approach this problem, we have cultured Theiler stage 16 embryonic B10.A strain mouse lung primordia (12-days gestation, E12) in serumless, chemically defined medium in the presence or absence of dexamethasone (DEX) for periods up to 27 days in vitro. Morphogenesis and cytodifferentiation were evaluated by light and transmission electron microscopy and immunochemical techniques. Pulmonary surfactant-associated apoproteins (PSAP) were initially expressed by type II cells at 16.5-day gestation in vivo. DEX-supplementation to the culture medium resulted in the accelerated expression of PSAP; the apoprotein isoforms (A1, A2, and A3) produced in vitro were comparable to those synthesized during fetal and postnatal in vivo development by high resolution, two-dimensional gel electrophoresis coupled with immunoblot staining. Cultures without DEX produced PSAP A2 and A3 isoforms, but did not produce A1 (26-31 kDa, pI 5.2-5.3). DEX-treated cultures produced more lamellar bodies within type II cells than non-treated controls. The results demonstrate that long-term cultures of embryonic lung primordia express morphogenesis, cytodifferentiation and the synthesis and secretion of PSAP in the absence of exogenous hormones or growth factors. The data set further supports the hypothesis that morphogenesis and type II cytodifferentiation are regulated by autocrine and paracrine factors intrinsic to the embryonic lung developmental program and independent of exogenous hormone controls.

Ultrastructural evidence of stromal/epithelial interactions in the human endometrial cycle

We found ultrastructural evidence of interactions between glandular epithelium and superficial stromal cells of the human endometrium during phases of the menstrual cycle. Four significant changes were observed in the transition from early proliferative (days 5 to 9) to early secretory (days 15 to 19) phases. These changes included: (1) an increase in the number and size of lamina densa disruptions, (2) an increase in the number and size of gap junctions, (3) an increase in the number and complexity of epithelial cell projections that extended through the lamina densa, and (4) an increase in close contacts between stromal and epithelial cells. The complex epithelial cell projections that extended through the lamina densa were in close proximity to stromal cells. These interactions were seen primarily in the early secretory phase. After that time (days 20 to 28) the interactions were less frequent. These morphologic results reveal complex physical interactions between epithelial and stromal cells of the adult endometrium. The interactions reach maximal development during the preimplantation phase of the endometrial cycle.

Molecular determinants of cranial neural crest-derived odontogenic ectomesenchyme during dentinogenesis

Positional information on tooth morphogenesis is investigated by the identification of when and where phenotypic markers are expressed during odontogenesis. This temporal and positional information is correlated with the instructive and permissive signaling required for both dentinogenesis and amelogenesis. Of particular interest is the establishment of a map for the cranial neural crest-derived dental papilla ectomesenchyme and the odontoblast cell lineages. The expression of ectomesenchyme-derived cytotactin, dentin phosphoprotein, and epithelial-derived enamel proteins was studied in mice using embryonic, fetal, and postnatal mandibular first molar tooth organ development. This review summarizes the observations in the context of instructive epithelial-mesenchymal interactions and suggests that amelogenesis imperfecta and dentinogenesis imperfecta may in part be explained by alterations in these differentiation markers. Recombinant DNA methods should facilitate future investigations of these inherited dental disorders.

Lung organoid culture

An organoid culture system for lung cells is described in which morphogenesis of lung histotypic structures and differentiation of both pneumocytes type II and mesenchyme occur. The principle of this technique is the culture of mouse fetal lung cells at high density on a membrane filter at the medium/air interface. In the course of cultivation, cell sorting-out, epithelial cell aggregation, formation of an alveolar-like lumen in the organoids and formation of a basal lamina occur. Epithelial differentiation culminates in the production of lamellar bodies, and the mesenchyme develops into mature connective tissue. Morphogenesis and differentiation depend on the stage of fetal development from which the lung cells were derived but appear independent of the formation of a basal lamina. Various drugs have been tested for their effects on morphogenesis and differentiation in this lung organoid culture: some of them inhibit differentiation or damage the mesenchyme, others stimulate surfactant production. Due to the quite complex morphogenetic and cellular events occurring in lung organoid culture, it may be an applicable tool for alternative in vitro screening methods.