Odontogenic epithelium induces similar molecular responses in chick and mouse mandibular mesenchyme

Previous observations have shown that, during the initiation phase of odontogenesis, signals from mouse odontogenic epithelium can elicit teeth in non-odontogenic but neural crest-derived mesenchyme isolated from ectopic sites including chick mandibular mesenchyme. In the present study the formation of ectopic tooth buds and dental mesenchyme in chick mandibular mesenchyme was examined using heterospecific recombinations between E11 mouse odontogenic epithelium and stage 23 chick lateral mandibular mesenchyme. Both morphological criteria and chick-specific probes for Msx-1, Msx-2, and Bmp-4 mRNAs were used as markers for early dental mesenchyme. Our results demonstrated that interactions of mouse odontogenic epithelium with chick mandibular mesenchyme induce early changes in the chick mandibular mesenchyme including the appearance of a translucent zone, cell proliferation, and induction of expression of Msx-1, Msx-2, and Bmp-4, which have been shown to be associated with the formation of dental mesenchyme. In addition, tooth bud-like structures that resemble E13 tooth buds in vivo both morphologically and in their patterns of gene expression formed after 6 days in the heterospecific recombinations. The tooth bud-like structures consist of invaginated mouse mandibular epithelium and condensed chick mandibular mesenchyme expressing high levels of Msx-1 and Bmp-4, but undetectable levels of Msx-2. Unlike the induction of Msx-1, Msx-2, and Bmp-4 in the underlying mesenchyme, which is specific for signals derived from odontogenic epithelium, the induction of a translucent zone and cellular proliferation in the underlying mesenchyme may be related to the growth-promoting potential of embryonic epithelia and not be specific to signals derived from the odontogenic epithelium. Similar to mouse odontogenic epithelium, agarose beads soaked in recombinant BMP-4 induced a translucent zone, cellular proliferation, and expression of Msx-1, Msx-2, and Bmp-4 in chick mandibular mesenchyme after 24 hours. These observations together showed that avian mandibular mesenchyme has odontogenic potential that is expressed upon interactions with inductive signals from mouse odontogenic epithelium. Similar to odontogenesis in vivo, formation of dental mesenchyme in chick mandibular mesenchyme is mediated by the activation of Msx-1, Msx-2, and Bmp-4.     

Expression patterns of bone morphogenetic proteins (Bmps) in the developing mouse tooth suggest roles in morphogenesis and cell differentiation

Bone morphogenetic proteins (BMP) are secretory signal molecules which have a variety of regulatory functions during morphogenesis and cell differentiation. Teeth are typical examples of vertebrate organs in which development is controlled by sequential and reciprocal signaling between the epithelium and mesenchyme. In addition, tooth development is characterized by formation of mineralized tissues: the bone-like dentin and cementum as well as epithelially derived enamel. We have performed a comparative in situ hybridization analysis of the expression of six different Bmps (Bmp-2 to Bmp-7) starting from initiation of tooth development to completion of crown morphogenesis when dentine and enamel matrices are being deposited. Bmps-2, -4, and -7 were frequently codistributed and showed marked associations with epithelial-mesenchymal interactions. Their expression shifted between the epithelium and mesenchyme starting from the stage of tooth initiation. They were subsequently expressed in the enamel knot, the putative signaling center regulating tooth shape. Their expression domains prior to and during the differentiation of the dentine-forming odontoblasts and enamel-forming ameloblasts was in line with functions in regulation of cell differentiation and/or secretory activities of the cells. The expression of Bmp-3 was confined to mesenchymal cells, in particular to the dental follicle cells which give rise to the cementoblasts, forming the hard tissue covering the roots of teeth. Bmp-5 was expressed only in the epithelial ameloblasts. It was upregulated as the cells started to polarize and intense expression continued in the secretory ameloblasts. Bmp-6 was expressed only weakly in the dental mesenchyme during bud and cap stages. Our results are in line with regulatory functions of Bmps at all stages of tooth morphogenesis. Bmps-2, -4, and -7 are conceivably parts of signaling networks regulating tooth initiation and shape development. They as well as Bmp-5 may be involved in the induction and formation of dentine and enamel, and Bmp-3 in the development of cementum. The remarkable overlaps in the expression domains of different Bmp genes may implicate functional redundancy and/or formation of active heterodimers between different BMPs.     

Mesenchyme specifies epithelial differentiation in reciprocal recombinants of embryonic lung and trachea

Normal lung morphogenesis and cytodifferentiation require interactions between epithelium and mesenchyme. We have previously shown that distal lung mesenchyme (LgM) is capable of reprogramming tracheal epithelium (TrE) from day 13-14 rat fetuses to branch in a lung-like pattern and express a distal lung epithelial phenotype. In the present study, we have assessed the effects of tracheal mesenchyme (TrM) on branching and cytodifferentiation of distal lung epithelium (LgE). Tracheae and distal lung tips from day 13 rat fetuses were separated into purified epithelial and mesenchymal components, then recombined as homotypic (LgM + LgE or TrM + TrE) or heterotypic (LgM + TrE or TrM + LgE) recombinants and cultured for 5 days; unseparated lung tips and tracheae served as controls. Control lung tips, LgM + LgE, and LgM + TrE recombinants all branched in an identical pattern. Epithelial cells, including those from the induced TrE, contained abundant glycogen deposits and lamellar bodies, and expressed surfactant protein C (SP-C) mRNA. Trachea controls, and both TrM + TrE, and TrM + LgE recombinants did not branch, but instead formed cysts. The epithelium contained ciliated and mucous secretory cells; importantly, no cells containing lamellar bodies were observed, nor was SP-C mRNA detected. Mucin immunostaining showed copious production of mucous in both LgE and TrE when recombined with TrM. These results demonstrate that epithelial differentiation in the recombinants appears to be wholly dependent on the type of mesenchyme used, and that the entire respiratory epithelium has significant plasticity in eventual phenotype at this stage in development.     

A study of patients with Nelson''s syndrome

Tooth development is regulated by a reciprocal series of epithelial-mesenchymal interactions. Bmp4 has been identified as a candidate signalling molecule in these interactions, initially as an epithelial signal and then later at the bud stage as a mesenchymal signal (Vainio et al. [1993] Cell 75:45-58). A target gene for Bmp4 signalling is the homeobox gene Msx-1, identified by the ability of recombinant Bmp4 protein to induce expression in mesenchyme. There is, however, no evidence that Bmp4 is the endogenous inducer of Msx-1 expression. Msx-1 and Bmp-4 show dynamic, interactive patterns of expression in oral epithelium and ectomesenchyme during the early stages of tooth development. In this study, we compare the temporal and spatial expression of these two genes to determine whether the changing expression patterns of these genes are consistent with interactions between the two molecules. We show that changes in Bmp-4 expression precede changes in Msx-1 expression. At embryonic day (E)10.5-E11.0, expression patterns are consistent with BMP4 from the epithelium, inducing or maintaining Msx-1 in underlying mesenchyme. At E11.5, Bmp-4 expression shifts from epithelium to mesenchyme and is rapidly followed by localised up-regulation of Msx-1 expression at the sites of Bmp-4 expression. Using cultured explants of developing mandibles, we confirm that exogenous BMP4 is capable of replacing the endogenous source in epithelium and inducing Msx-1 gene expression in mesenchyme. By using noggin, a BMP inhibitor, we show that endogenous Msx-1 expression can be inhibited at E10.5 and E11.5, providing the first evidence that endogenous Bmp-4 from the epithelium is responsible for regulating the early spatial expression of Msx-1. We also show that the mesenchymal shift in Bmp-4 is responsible for up-regulating Msx-1 specifically at the sites of future tooth formation. Thus, we establish that a reciprocal series of interactions act to restrict expression of both genes to future sites of tooth formation, creating a positive feedback loop that maintains expression of both genes in tooth mesenchymal cells.     

Proteoglycans are required for maintenance of Wnt-11 expression in the ureter tips

Development of the metanephric kidney requires the concerted interaction of two tissues, the epithelium of the ureteric duct and the metanephric mesenchyme. Signals from the ureter induce the metanephric mesenchyme to condense and proliferate around the ureter tip, reciprocal signals from the mesenchyme induce the ureter tip to grow and to branch. Wnt genes encode secreted glycoproteins, which are candidate mediators of these signaling events. We have identified three Wnt genes with specific, non-overlapping expression patterns in the metanephric kidney, Wnt-4, Wnt-7b and Wnt-11. Wnt-4 is expressed in the condensing mesenchyme and the comma- and S-shaped bodies. Wnt-7b is expressed in the collecting duct epithelium from 13.5 days post coitum onward. Wnt-1l is first expressed in the nephric duct adjacent to the metanephric blastema prior to the outgrowth of the ureteric bud. Wnt-l1 expression in Danforth's short-tail mice suggests that signaling from the mesenchyme may regulate Wnt-ll activation. During metanephric development, Wnt-11 expression is confined to the tips of the branching ureter. Maintenance of this expression is independent of Wnt-4 signaling and mature mesenchymal elements in the kidney. Moreover, Wnt-ll expression is maintained in recombinants between ureter and lung mesenchyme suggesting that branching morphogenesis and maintenance of Wnt-ll expression are independent of metanephric mesenchyme-specific factors. Interference with proteoglycan synthesis leads to loss of Wnt-ll expression in the ureter tip. We suggest that Wnt-11 acts as an autocrine factor within the ureter epithelium and that its expression is regulated at least in part by proteoglycans.     

Nocturnal oxygen and hypercapnic ventilatory response in patients with congestive heart failure

Apoptosis has been shown to be involved in several processes during embryogenesis, but the ontogeny of apoptosis during lung development ahs not been studied. The goals of the current study were to determine if apoptosis occurs during lung development, and to determine the ontogeny of the changes in apoptosis that occur. We studied the ontogeny of apoptosis in vivo using lungs from 14-18-d gestation fetal rats, newborn rats, and 1-d-, 2-d-, 5-d-, and 10-d-old rat pups. Apoptosis was assessed by electron microscopy and the terminal deoxyribonucleotidyl transferase dUTP nick end-labeling assay. We compared the in vivo results with explants of 14-d gestation fetal rat lung placed in culture for 1-4 d because the biochemical development of the lung in organ culture has been shown to closely parallel the development of the lung in vivo. We found apoptosis of mesenchymal cells at the periphery of distal lung buds in early fetal lung (14-16-d gestation). Apoptosis of both mesenchyme and epithelium was present in later fetal lung (18-d gestation). There were no qualitative differences in apoptosis between in vivo fetal lung and explant cultures of fetal lung. There was a 14-fold increase in apoptosis at birth and in the first postnatal day of life (9-12% of cells) compared with fetal lung (0.6-1% of cells). This was followed by a rapid decline in the percentage of apoptotic cells to fetal levels at postnatal d 2-10. We conclude that apoptosis occurs in a spatially, temporally, and cell-specific manner during lung development. The number of cells undergoing apoptosis increases dramatically in the first day after birth.     

Immunocytochemical localization of transforming growth factor alpha, epidermal growth factor and epidermal growth factor receptor in the cat endometrium and placenta

Hoxb-5 is one of the few homeobox genes strongly expressed in the developing mouse lung. To explore the hypothesis that Hoxb-5 acts to regulate epithelial cell fate and branching morphogenesis in the developing lung, we studied the temporal, spatial, and cell-specific expression of Hoxb-5 from gestational day (d) 13.5 to postnatal day (P) 2. Immunocytochemistry demonstrated regional localization of Hoxb-5 protein to developing conducting airways and surrounding mesenchyme. The cellular expression pattern changed from diffusely positive nuclei of mesenchymal cells on d13.5 to become more localized to nuclei of subepithelial fibroblasts and some adjacent columnar and cuboidal epithelial cells on d14.5. After d14.5, Hoxb-5 protein expression continued to decrease in mesenchymal cells distal from developing airways, but persisted in fibroblasts underlying conducting airways. Hoxb-5 protein expression persisted in nuclei of columnar and cuboidal epithelial cells on d16.5 and d17.5, with expression in low cuboidal epithelial cells as well from d17.5 to P2. Western blot analysis showed temporal and quantitative changes in Hoxb-5 protein expression with peak expression on d14.5-15.5. We conclude that Hoxb-5 protein is developmentally regulated in a temporal, spatial, and cell-specific manner throughout the pseudoglandular, canalicular, and terminal saccular periods of lung development in the mouse. This localization and expression pattern suggests that Hoxb-5 may influence branching morphogenesis, cell-cell communication, cell fate, and differentiation of conducting airway epithelia.     

Alterations in the developmental properties of stroma during the development of the urogenital ridge into ductus deferens and uterus in embryonic and neonatal mice

The ability of mesenchyme from the urogenital ridge to participate in the development of male and female urogenital organs was studied by preparing homo- and heterotypic recombinants of epithelium and stroma from ductus deferens and uterus of neonatal mice. The recombinants as well as intact 14-day old embryonic urogenital ridges (URG) were grown for two to eight weeks in adult male and female hosts. The development of ductus deferens and seminal vesicle from female UGR's grafted into male hosts and uterus from male UGR's grafted into female hosts demonstrate that the mesenchyme of embryonic UGR's is capable of participating in the development of either male or female urogenital structures. During normal development of the female urogenital tract, the bisexual morphogenetic potentiality of UGR mesenchyme persists postnatally as demonstrated by the ability of uterine stroma to support normal differentiation of epithelium of ductus deferens in male hosts. Conversely, during the normal development of the male urogenital tract, the morphogenetic potentiality of mesenchyme of the urogenital ridge is restricted as stroma from the ductus deferens appears incapable of participating in uterine morphogenesis in male or female hosts. These differences in the developmental properties of UGR stroma may be attributed to differences in hormonal conditions in male and female fetuses.     

Instructive induction of prostate growth and differentiation by a defined urogenital sinus mesenchyme

Instructive influences of fetal mesenchyme were examined in heterotypic tissue recombinants consisting of urogenital sinus mesenchyme (UGM) from male and female rats and distal ductal tips from adult rat prostate. Tissues were grown under the renal capsule of male hosts for periods up to 28 days. Resultant growths exhibited typical prostate histology. Expression of lobe-specific proteins for the ventral (prostatic steroid binding protein [PSBP]) lateral (seminal vesicle secretion II [SVS II]), and dorsal prostate (secretory transglutaminase [TGase]) were examined by immunocytochemistry. Male or female UGM combined with terminal segments of the ventral or dorsal prostate and immunolabeled with antibodies to lobe-specific proteins demonstrated expression of all three secretory products. The pattern of staining was consistent with a compound inductive response from the UGM. Unique to this study was our ability to use a defined mesenchymal tissue (female ventral mesenchymal pad [VMP]). This tissue is specifically associated with ductal branching morphogenesis and cytodifferentiation of the ventral prostate. Distal ductal tips from the dorsal lobe of the adult male prostate when recombined with female VMP and grown in vivo exhibited transformation of secretory phenotype, and the epithelium expressed mRNAs for PSBP. Immunocytochemistry of serial sections did not demonstrate labeling for TGase in the new epithelial growth. Ultrastructural analysis of the heterotypic recombinants indicated that the epithelium had similar characteristics to those of normal ventral prostate. Early stages of the mesenchymal-epithelial interactions resulted in dedifferentiation of the adult epithelium to solid cords of stratified cells. These findings illustrate the potent instructive capacity of a defined fetal UGM to influence development and cytodifferentiation of adult prostate epithelium.     

Abrogation of Smad3 and Smad2 or of Smad4 gene expression positively regulates murine embryonic lung branching morphogenesis in culture

Smad genes are recently identified intracellular effectors for receptor signaling in the BMP/activin/TGF-beta pathway. Since TGF-beta ligands are known to inhibit embryonic lung branching morphogenesis, we tested the hypothesis that Smad genes negatively regulate lung organogenesis. Antisense oligodeoxynucleotides were designed to attenuate Smad3 and Smad2 gene expression in embryonic (E11) mouse lungs over 4 days in culture. Endogenous Smad3 and Smad2 mRNA levels were suppressed by 97 and 91%, respectively, in cultured embryonic lungs when antisense oligodeoxynucleotide (40 microM) to Smad was added, compared to scrambled and sense sequence controls. The corresponding Smad3 and Smad2 protein amounts were also decreased respectively by 86 and 90% in lungs treated with Smad3 and Smad2 antisense oligodeoxynucleotide. Phenotypically, Smad antisense oligodeoxynucleotides resulted in a concentration-dependent increase in lung branching: embryonic lung branching was stimulated by up to 53% in culture with 40 microM antisense oligodeoxynucleotide, whereas both scrambled and sense controls showed no stimulatory effect. Thus, inhibition of endogenous Smad3 and Smad2 gene expression resulted in stimulation of embryonic lung branching similar to that caused by inhibition of TGF-beta type II receptor expression and signaling (J. Zhao et al., 1996, Dev. Biol. 180, 242-257). Abrogation of Smad4 (DPC4), the downstream mediator of Smad3 and Smad2 proteins, with antisense oligodeoxynucleotide, also resulted in increased branching morphogenesis. Furthermore, while TGF-beta alone inhibited lung branching morphogenesis in culture, addition of exogenous TGF-beta 1 could not overcome the stimulatory effect on lung branching of Smad antisense oligodeoxynucleotide treatment. By immunohistochemistry, Smad proteins were localized mainly to the epithelial cells lining the branching distal airways, indicating that Smad genes could regulate lung morphogenesis through mesoderm-endoderm interaction. Our results demonstrate, for the first time, that abrogation of Smad2 and Smad3 or of Smad4 gene expression stimulated early mouse embryonic lung branching morphogenesis in culture, possibly through reversing the negative influence of endogenous TGF-beta signaling upon lung branching morphogenesis.     

Gene expression, polarising activity and skeletal patterning in reaggregated hind limb mesenchyme

The developing chick limb has two major signalling centres; the apical ectodermal ridge maintains expression of several important genes and outgrowth of the limb, and the polarising region specifies the pattern of skeletal elements along the anteroposterior axis. We have used reaggregated leg grafts (mesenchyme dissociated into single cells, placed in an ectodermal jacket and grafted to a host) to study patterning in a system where the developmental axes are severely disrupted. Reaggregates from different regions of leg mesenchyme developed correspondingly different digits, giving a system in which skeletal phenotype could be compared with the expression of genes thought to be important in patterning. We found that posterior third and whole leg reaggregates gave rise to different digits, yet expressed the same combination of HoxD, Bmp-2 and shh genes throughout their development. Anterior thirds initially only express the 3' end of the HoxD cluster but activate the more 5' members of the cluster sequentially over a period of 48 hours, a period during which Bmp-2 is activated but no shh or Fgf-4 expression could be detected. Our results suggest that there are two independent mechanisms for activating the HoxD complex, one polarising region-dependent and one independent, and that shh expression may not be necessary to maintain outgrowth and patterning once a ridge has been established.     

Stromal development in the ventral prostate, anterior prostate and seminal vesicle of the rat

The prostate and seminal vesicle (SV) are androgen-dependent secretory glands of the male genital tract. They produce the bulk of the seminal secretions. The object of the present study was to examine and document the ontogeny of stromal maturation in the rat anterior and ventral prostate and SV. These organs have a loosely organized cellular mesenchyme during fetal development. During prostatic development the mesenchyme condensed to form smooth muscle sheaths immediately surrounding the epithelium, with looser connective tissue between individual ducts. In the SV, a loose connective tissue layer called the lamina propria lies between the epithelium and developing muscle. Smooth muscle alpha-actin, myosin, desmin, laminin, vinculin, vimentin and androgen receptor (AR) expression were examined by immunocytochemical methods during the pre- and postnatal developmental periods. The first marker to be detected was vimentin, which was initially found throughout the mesenchyme. During development vimentin became mostly restricted to the interductal tissue of the prostate and the lamina propria of the SV. Smooth muscle markers were expressed in an orderly sequence in a proximal to distal manner along prostatic ducts, from the urethra towards the tips. Expression of alpha-actin was followed by vinculin, myosin, desmin, and laminin. These markers became localized to the developing smooth muscle sheaths and were not expressed in the interductal tissue of the prostate or the lamina propria of the SV. Organ culture experiments demonstrated that androgens were required for the differentiation of smooth muscle sheaths. Castration of adult rats demonstrated that androgens were required to maintain smooth muscle differentiation. In castrates, the stroma was relatively thicker but less dense than in intact animals. Following castration, expression of the smooth muscle markers was lost sequentially in the reverse order of their expression during development. In long-term castrates alpha-actin, vimentin and a small amount of vinculin were detected. AR were first detected in the urogenital sinus mesenchyme immediately surrounding the epithelium at 16 days of gestation. As development progressed expression of AR became more widespread, and postnatally was found throughout the mesenchyme. As maturation of smooth muscle occurred, stromal expression of AR became localized to the muscular sheath immediately surrounding the epithelium. In the prostate the interductal connective tissue displayed very low levels of AR expression. In the SV, AR were also observed in the lamina propria. In summary, stromal differentiation and dedifferentiation in the rat prostate and SV were found to be androgen-dependent processes with ordered sequential ontogenic expression of specific markers.     

The cell adhesion molecule L1 is developmentally regulated in the renal epithelium and is involved in kidney branching morphogenesis

We immunopurified a surface antigen specific for the collecting duct (CD) epithelium. Microsequencing of three polypeptides identified the antigen as the neuronal cell adhesion molecule L1, a member of the immunoglobulin superfamily. The kidney isoform showed a deletion of exon 3. L1 was expressed in the mesonephric duct and the metanephros throughout CD development. In the adult CD examined by electron microscopy, L1 was not expressed on intercalated cells but was restricted to CD principal cells and to the papilla tall cells. By contrast, L1 appeared late in the distal portion of the elongating nephron in the mesenchymally derived epithelium and decreased during postnatal development. Immunoblot analysis showed that expression, proteolytic cleavage, and the glycosylation pattern of L1 protein were regulated during renal development. L1 was not detected in epithelia of other organs developing by branching morphogenesis. Addition of anti-L1 antibody to kidney or lung organotypic cultures induced dysmorphogenesis of the ureteric bud epithelium but not of the lung. These results suggest a functional role for L1 in CD development in vitro. We further postulate that L1 may be involved in the guidance of developing distal tubule and in generation and maintenance of specialized cell phenotypes in CD.     

The limb field mesoderm determines initial limb bud anteroposterior asymmetry and budding independent of sonic hedgehog or apical ectodermal gene expressions

We have analyzed the pattern of expression of several genes implicated in limb initiation and outgrowth using limbless chicken embryos. We demonstrate that the expressions of the apical ridge associated genes, Fgf-8, Fgf-4, Bmp-2 and Bmp-4, are undetectable in limbless limb bud ectoderm; however, FGF2 protein is present in the limb bud ectoderm. Shh expression is undetectable in limbless limb bud mesoderm. Nevertheless, limbless limb bud mesoderm shows polarization manifested by the asymmetric expression of Hoxd-11, -12 and -13, Wnt-5a and Bmp-4 genes. The posterior limbless limb bud mesoderm, although not actually expressing Shh, is competent to express it if supplied with exogenous FGF or transplanted to a normal apical ridge environment, providing further evidence of mesodermal asymmetry. Exogenous FGF applied to limbless limb buds permits further growth and determination of recognizable skeletal elements, without the development of an apical ridge. However, the cells competent to express Shh do so at reduced levels; nevertheless, Bmp-2 is then rapidly expressed in the posterior limbless mesoderm. limbless limb buds appear as bi-dorsal structures, as the entire limb bud ectoderm expresses Wnt-7a, a marker for dorsal limb bud ectoderm; the ectoderm fails to express En-1, a marker of ventral ectoderm. As expected, C-Lmx1, which is downstream of Wnt-7a, is expressed in the entire limbless limb bud mesoderm. We conclude that anteroposterior polarity is established in the initial limb bud prior to Shh expression, apical ridge gene expression or dorsal-ventral asymmetry. We propose that the initial pattern of gene expressions in the emergent limb bud is established by axial influences on the limb field. These permit the bud to emerge with asymmetric gene expression before Shh and the apical ridge appear. We report that expression of Fgf-8 by the limb ectoderm is not required for the initiation of the limb bud. The gene expressions in the pre-ridge limb bud mesoderm, as in the limb bud itself, are unstable without stimulation from the apical ridge and the polarizing region (Shh) after budding is initiated. We propose that the defect in limbless limb buds is the lack of a dorsal-ventral interface in the limb bud ectoderm where the apical ridge induction signal would be received and an apical ridge formed. These observations provide evidence for the hypothesis that the dorsal-ventral ectoderm interface is a precondition for apical ridge formation.     

FGFR-3 and FGFR-4 function cooperatively to direct alveogenesis in the murine lung

Mammalian lungs begin as an outpocket of the foregut, and depend on multiple stages of branching morphogenesis and alveogenesis to reach their final form. An examination of fgf receptor gene expression indicated that all four receptors (fgfr-1 to fgfr-4) are expressed in postnatal lungs at varying levels. We show that mice homozygous for a targeted mutation of fgfr-4 exhibited no overt abnormalities in the lungs or any other organ. However, mice doubly homozygous for disruptions of the fgfr-3 and fgfr-4 genes display novel phenotypes not present in either single mutant, which include pronounced dwarfism and lung abnormalities. Lungs of fgfr-3(-/-)fgfr-4(-/- )animals, which are normal at birth, are completely blocked in alveogenesis and do not form secondary septae to delimit alveoli. Consequently, air spaces in the lung are expanded and no alveoli can be seen. The mutant lungs failed to downregulate postnatal elastin deposition despite their normal levels of surfactant expression and cell proliferation. These data revealed a cooperative function of FGFR-3 and FGFR-4 to promote the formation of alveoli during postnatal lung development.     

Site specificity of surfactant protein expression in airways of baboons during gestation

BACKGROUND: There are disparate reports concerning the presence of surfactant proteins in the airways of lung. The recent finding of SP-A in tracheobronchial epithelium and submucosal glands in lungs from second trimester humans has renewed interest in potential new functions of surfactant in lung biology. METHODS: In situ hybridization studies were done to determine the distribution of SP-A, SP-B, and SP-C in baboon lung specimens from 60, 90, 120, 140, 160, and 180 (term) days of gestation and adults. Lungs from gestation controls were obtained at the time of hysterotomy and adult lungs at necropsy. Riboprobes used for in situ hybridization contained the entire coding regions for human SP-A, SP-B, and SP-C. RESULTS: At 60 days, SP-C mRNA expression was evident in focal portions of primitive tubular epithelium but not bronchi. This distal pattern of SP-C mRNA expression persisted and was present in some epithelial cells of respiratory bronchioles at term. At 90 days, SP-A mRNA expression was present in the epithelium of trachea and large bronchi. SP-B mRNA expression was found in small bronchi, bronchioles, and distal tubular epithelium at 120 days of gestation. SP-A mRNA bronchiolar localization became evident at 140 days of gestation and alveolar type 2 cellular expression at 160 days of gestation. Abrupt transitions of surfactant protein expression were identified (e.g., SP-A mRNA-positive cells in the epithelium of large bronchi with adjoining SP-B mRNA expression in small bronchi and bronchioles). CONCLUSIONS: Findings in the baboon indicate that there are well-delineated sites of surfactant protein mRNA expression in bronchial and bronchiolar epithelia. mRNA expressions of SP-A and SP-B are present in both bronchial and bronchiolar epithelium but at different sites, whereas SP-C expression is seen in loci of epithelial cells in respiratory bronchioles.