Biomedical subjects

R N Han

Publications and source records attributed to R N Han.

At least 19 recordsLinked to original sources

PubMed2000

Posttranscriptional regulation of human leukocyte antigen G during human extravillous cytotrophoblast differentiation.

Human maternal tolerance to a semiallogenic fetus may be maintained, in part, by the unusual expression pattern of antigen-presenting molecules in placental trophoblast cells. Extravillous cytotrophoblast (EVC) cells, which invade the maternal decidua, express high levels of human leukocyte antigen G (HLA-G), a nonclassical, major histocompatibility complex (MHC) class I molecule. HLA-G transcripts have been detected in tumors and other tissues, yet protein accumulation is rare. We show that, within EVC cells themselves, the mRNA is more broadly expressed than the protein. Specifically, accumulation of HLA-G protein was markedly delayed during EVC cell differentiation. To elucidate this mechanism, we performed a comprehensive analysis comparing the expression of HLA-G and proteins essential for MHC class I expression at the cell surface. The transporter for antigen processing proteins TAP1 and TAP2, as well as tapasin and beta(2)-microglobulin, appeared to be coordinately expressed throughout EVC cell columns. Strikingly, they all accumulated well in advance of the HLA-G protein but concurrently with its mRNA. A similar delay in the accumulation of the HLA-G protein was observed in vitro, using cultures of chorionic villi. We conclude that posttranscriptional regulation of HLA-G is fundamental to EVC cell development and is achieved independently of the peptide loading system. This represents a novel mechanism of MHC class I regulation.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed2000

Changes in expression of platelet-derived growth factor and its receptors in the lungs of newborn rats exposed to air or 60% O(2).

PDGF-related gene expression has been well characterized during fetal rat lung development and adult rat lung injury, but not during normal postnatal lung growth or injury. Lung expression of the mRNA for PDGF-A, -B, -alpha R, and -beta R and immunoreactive PDGF-AA, -BB, -alpha R, and -beta R were assessed in rat pups raised in air or 60% O(2) for up to 14 d after birth. Expression of mRNA and immunoreactive ligand did not correlate for pups raised in air. Immunoreactive PDGF-alpha R and -beta R, but not PDGF-AA and -BB, were evident throughout the lung at birth. Both PDGF-AA and -BB were evident in airway epithelium, PDGF-BB in alveolar epithelial cells and PDGF-AA was widely distributed in parenchymal tissue at 4 d. PDGF-alpha R was localized to airway epithelium, and PDGF-beta R to subendothelial perivascular regions and to airway and alveolar epithelium at 4 d. Immunoreactive PDGF ligands all declined after 4 d. Intraperitoneal injection of neutralizing antibodies or truncated soluble receptors to PDGF-BB reduced lung DNA synthesis in air. Exposure to 60% O(2) significantly increased mRNA for PDGF-B, -beta R, and -alpha R, but not PDGF-A, relative to air-exposed lung at various time points after birth. PDGF-A, -B, and -alpha R immunoreactivities in these lungs were reduced and delayed, consistent with a global inhibition of lung growth. Pups exposed to 60% O(2) had a similar distribution of PDGF-beta R to that seen in air, except that at 14 d PDGF-beta R was distributed throughout the lung parenchyma. We conclude that PDGF ligands and receptors are important for normal postnatal lung growth and that their expression is delayed by O(2) exposure.

Animals↗

PubMed1998

Insulin-like growth factor binding proteins in air- and 85% oxygen-exposed adult rat lung.

Expression of insulin-like growth factor (IGF) I and its type I receptor is increased in the adult rat lung exposed to 85% O2. We hypothesized that there would be a parallel up- and downregulation of growth-stimulating and growth-inhibiting IGF binding proteins (IGFBPs), respectively. The normal adult rat lung expresses mRNAs for IGFBP-2, -3, -4, -5, and -6 but not for IGFBP-1. O2 exposure for 6 or 14 days reduced IGFBP-3 and -6 and increased IGFBP-4 mRNA abundance. IGFBP-5 mRNA was reduced at 6 days but increased at 14 days. IGFBP-4 mRNA was localized to perivascular and peribronchial interstitial cells and IGFBP-5 mRNA to airway and alveolar epithelial cells. IGFBP-2, -4, and -5 immunolocalized to airway epithelial cells in normal lung and to perivascular exudates after 6 days in 85% O2. IGFBP-2 was diffusely increased throughout the lung tissue only after a 6-day exposure. IGFBP-5 was reduced after a 6-day exposure but was increased and widely distributed after 14 days. IGFBP-4 increased over airway epithelium and subepithelial cells after 6 days and over perivascular interstitial cells after 14 days of 85% O2. These data are consistent with the predicted changes for IGFBPs on O2 exposure except that the generally growth-inhibitory IGFBP-4 was increased at sites of active cell proliferation.

Air↗

PubMed1996

Insulin-like growth factor-I and type I insulin-like growth factor receptor in 85% O2-exposed rat lung.

The expression of insulin-like growth factor I (IGF-I) and insulin-like growth factor II (IGF-II) was studied in the lungs of adult rats exposed to air or 85% O2, using Northern analysis, in situ hybridization, and immunohistochemistry. Distribution of the type I insulin-like growth factor receptor (IGF-IR) was assessed by immunohistochemistry. IGF-I, but not IGF-II, was localized to airway epithelium, while IGF-IR was localized to perivascular and peribronchial cells, in the lungs of animals breathing air. IGF-II mRNA did not increase with exposure to 85% O2, but IGF-II was localized to sites of perivascular edema and to occasional peribronchial cells. A widespread increase in IGF-I mRNA and peptide was seen after both a 6-day and a 14-day exposure to O2, with maximal expression in the airway and alveolar epithelium, and lesser expression in interstitial cells. After 6 days in 85% O2, increased IGF-IR immunoreactivity was localized to both perivascular and peribronchial cells and to endothelial cells. By 14 days in 85% O2, IGF-IR immunoreactivity was also localized to alveolar epithelial cells. The distribution of IGF-IR immunoreactivity was consistent with a paracrine role for IGF-I in O2-mediated pulmonary hypertension and airway hyperreactivity, by mediating smooth muscle cell hyperplasia, as well as a role in endothelial cell repair and late pneumocyte hyperplasia. The relative insensitivity of IGF-IR immunohistochemistry did not allow us to identify cells with low abundance IGF-IR, and potential cellular targets for IGF-I actions after O2-exposure may be even more extensive than those recognized here.

Animals↗

PubMed1996

Changes in structure, mechanics, and insulin-like growth factor-related gene expression in the lungs of newborn rats exposed to air or 60% oxygen.

Exposure of neonatal rats to > or = 95% O2 for 2 wk, a widely used model of oxidant/antioxidant interactions in neonatal lung injury, results in arrested lung growth without the dysplastic lesions observed in chronic human neonatal lung injury. To determine whether dysplastic lung cell growth would be seen at lesser O2 concentrations, we exposed newborn rats to either 95% O2 for 1 wk followed by 60% O2 for 1 wk, or to 60% O2 for 2 wk. Exposure to 95% O2 for 1 wk profoundly inhibited lung DNA synthesis. Recovery of synthesis did not occur during the 2nd wk in 60% O2, nor were areas of dysplastic growth evident in lung tissue. In contrast, a continuous 2-wk exposure to 60% O2 resulted in a slight increase in lung weight with a significant reduction in lung volume over a range of inflation pressures. Also seen was an overall, but inhomogeneous, reduction in lung cell DNA synthesis. A preliminary analysis of affected cell types suggested that inhibition of DNA synthesis affected endothelial cells more than interstitial cells, whereas DNA synthesis increased in type II pneumocytes. Areas of reduced DNA synthesis were interspersed with patchy areas of parenchymal thickening and active DNA synthesis. These areas of parenchymal thickening, but not other areas, had increased immunoreactive IGF-I and the type I IGF receptor. These data are consistent with a direct effect of O2 on growth factor and growth factor receptor expression in causing dysplastic lung cell growth in chronic neonatal lung injury.

Animals↗

PubMed1995

Altered expression of type I collagen, TGF-beta 1, and related genes in rat lung exposed to 85% O2.

The gene expressions of type I collagen and transforming growth factor-beta 1 (TGF-beta 1) were studied in lung tissue of rats exposed to air or 85% O2 for 14 days. Peak expression of type I collagen mRNA was observed by 14 days of 85% O2 exposure, at the same time as maximal immunoreactive type I collagen, which was most marked surrounding the major airways and vessels. TGF-beta 1 mRNA also significantly increased after 14, but not 4 or 6 days of 85% O2 exposure. TGF-beta 1 immunoreactivity was only detected on day 14 of 85% O2 exposure and was localized primarily to the pulmonary epithelium. As an increase in immunoreactive type I collagen was evident by day 6 of O2 exposure, the gene expressions of interstitial collagenase (MMP-1), stromelysin, and the tissue inhibitor of the metalloproteinases (TIMP) were also examined. Increased mRNA expressions of interstitial collagenase and TIMP preceded those of type I collagen and TGF-beta 1, occurring at 4-6 days of exposure to 85% O2, while there was no significant change in stromelysin mRNA. These findings are compatible with the initial O2-mediated increase in type I collagen deposition being a result of an altered proteinase/antiproteinase balance in the lung, and the subsequent more marked deposition being a response to increased TGF-beta 1 synthesis.

Animals↗

PubMed1995

Basic fibroblast growth factor and growth factor receptor gene expression in 85% O2-exposed rat lung.

Lungs exposed to elevated O2 concentrations suffer an initial loss of type I pneumocytes, followed by a reparative type II pneumocyte hyperplasia. We hypothesized that type II pneumocyte hyperplasia after exposure of young adult rats to 85% O2 in vivo would be temporally related to 1) an increased concentration of intrapulmonary basic fibroblast growth factor (bFGF), a potent stimulator of type II pneumocyte DNA synthesis in vitro, and 2) an upregulation of pneumocyte receptors for bFGF (FGF-R). Increased rat lung bFGF mRNA, relative to air-exposed control animals, was observed at 4 days of exposure, with no increase at days 6 and 14 of exposure. Parallel changes were observed with bFGF receptor (flg) mRNA. Nuclear runoff assays confirmed increased transcription of both bFGF and flg genes in response to 85% O2, whereas increased translation at 6 days of exposure was confirmed by protein immunoanalysis. Immunohistochemistry demonstrated a broad distribution of bFGF throughout the lung, including the alveolar epithelium, which increased after 6 and 14 days of exposure to 85% O2. Our findings are compatible with a role for bFGF in O2-mediated pneumocyte hyperplasia.

Amino Acid Sequence↗

PubMed1994

Differential regulation of platelet-derived growth factor genes in fetal rat lung fibroblasts.

There is increasing evidence to suggest that platelet-derived growth factor (PDGF), or PDGF-like molecules, play a role in fetal lung morphogenesis. The cellular sources of PDGF and its target cells within the fetal lung remain to be defined. In the present study, we investigated the developmental expression of PDGF and its cognate receptor genes in fetal rat lung fibroblasts. Northern analysis revealed that fetal lung fibroblasts express the PDGF A-chain, B-chain, and beta-receptor genes. The cells actively translated these mRNAs into protein as demonstrated by immunocytochemistry and by metabolic labeling with [35S]methionine, followed by immunoprecipitation with specific PDGF-AA and -BB antibodies. Affinity cross-linking with 125I-labeled PDGF-BB demonstrated the presence of PDGF beta-receptors on fetal lung fibroblasts. The development expression of the PDGF genes was examined in fibroblasts derived from the early canalicular (Day 19) to the early saccular stage (Day 21) of lung development (term = 22 days). PDGF A-chain gene expression was at a low but constant level during late gestation. No change in either the transcription rate or stability of the message for this gene was observed with advancing gestation. Despite these mRNA observations, PDGF-AA is the major secreted form in the medium of the fibroblasts. Expression of PDGF B-chain gene was greatest during the early canalicular stage (Day 19) and declined sharply thereafter. The greater expression of PDGF B-chain during the canalicular stage was due to a greater rate of transcription and a greater PDGF B-chain mRNA stability. The PDGF beta-receptor gene was expressed at a lower but constant level in these cells during late gestation. The constant level of PDGF beta-receptor mRNA could be attributed to a balanced increased synthesis of the message coupled to an increased breakdown of the transcript. These data indicate that fetal lung fibroblasts synthesize PDGF-AA, PDGF-BB, and PDGF beta-receptor and that they regulate the developmental expression of these PDGF genes differently.

Animals↗

PubMed1994

Alpha 1-antitrypsin protects neonatal rats from pulmonary vascular and parenchymal effects of oxygen toxicity.

We investigated whether alpha 1-antitrypsin (alpha 1-AT) might protect neonatal rats from the pulmonary parenchymal and vascular effects resulting from hyperoxic exposure. Neonatal rats born into and maintained in hyperoxia (60% fraction of inspired oxygen) or room air were injected with a loading dose of alpha 1-AT (72 mg/kg) followed by 36 mg/kg every 72 h or with vehicle during the first 14 d of life. At the end of the experimental period, we measured body weight, lung compliance, lung volume, alveoli per mm2, and total number of alveoli and assessed right ventricular hypertrophy and vascular changes consisting of medial hypertrophy, muscular extension into peripheral, normally nonmuscular arteries, and number of peripheral arteries relative to alveoli. Our data show that alpha 1-AT treatment prevented the reduced lung compliance observed in the untreated hyperoxia-exposed neonatal rats, as well as the right ventricular hypertrophy and the associated vascular changes of medial hypertrophy of muscular arteries and muscularization of distal arteries. Reduced lung compliance in the hyperoxic but alpha 1-AT-untreated rats was associated with a reduction in lung elastin compared with room-air or alpha 1-AT-treated rats. In room-air rats, alpha 1-AT treatment increased lung compliance but also reduced the number of arteries relative to the number of alveoli, a feature that was not, however, associated with right ventricular hypertrophy. Our data suggest that supplemental alpha 1-AT might restore the imbalance in elastolytic activity induced by hyperoxia and thereby alleviate the toxic effects on lung parenchymal and vascular development.

Animals↗

PubMed1993

Ontogeny of platelet-derived growth factor receptor in fetal rat lung.

There is increasing evidence that platelet-derived growth factor (PDGF) or PDGF-like molecules play a role in fetal lung morphogenesis. Our previous studies demonstrated the presence of PDGF-AA and PDGF-BB homodimers in fetal rat lung. To study further the target cells of PDGF in lung development, immunolocalization studies of PDGF receptors (PDGFR) were conducted on embryonic and fetal rat lung from day 13 to day 21 of gestation (term = 22 days) using two polyclonal PDGFR antibodies, one of which one recognizes both alpha and beta receptors (PDGFR-alpha/beta), while the other is specific for the beta receptor (PDGFR-beta). A similar immunostaining pattern for both antibodies was noted. Immunoreactivity to PDGFR was evident in both epithelial and mesenchymal cells of the embryonic lung bud as early as 13 days gestation. The number of PDGFR immunoreactive cells increased with advancing gestation. Intense immunoreactivity was noted in both epithelial cells and interstitial cells during the saccular stage of lung development. The immunoreactivity to PDGFR was localized to the apical/luminal side of bronchial and distal airway epithelial cells. PDGFR-immunopositive bronchial and vascular smooth muscle cells were detected only during the canalicular and saccular stages of lung development. Immunopositive endothelial cells lining the internal vascular plexuses were observed from days 14-16 of gestation. No PDGFR was detected in endothelial cells of large pulmonary vessels. We conclude that PDGFR are present in airway epithelial cells, interstitial cells, and bronchial and vascular smooth muscle cells and that gestation-dependent up- and down-regulation of PDGFR may play a role in developmental regulation of PDGF bioactivity during lung morphogenesis.

Animals↗

PubMed1992

Platelet-derived growth factor and growth-related genes in rat lung. II. Effect of exposure to 85% O2.

The expression of platelet-derived growth factor (PDGF), its receptor, and related genes was studied in the lung tissue of rats exposed to air or 85% O2. PDGF-B chain mRNA was increased by 6 days and PDGF B-type receptor mRNA was increased by 4 and 6 days of exposure to 85% O2. Despite a continued increase of cell division, both PDGF-B chain and B-type receptor mRNAs had returned to control values by 14 days of exposure to 85% O2. PDGF-A chain mRNA was unaffected by exposure to 85% O2. Nuclear runoff transcription analysis confirmed increased transcription of PDGF-B chain mRNA, whereas Western blot analysis of lung homogenates suggested consequent increased translation of PDGF-B chain mRNA to PDGF-BB at 7 days of exposure to 85% O2. Combined immunocytochemistry and autoradiography localized PDGF-BB to the major site of cell division, the pulmonary interstitium. We speculate that the early pulmonary fibroblast hyperplasia observed following exposure to 85% O2 is mediated by increased PDGF-B chain gene expression and may also be mediated by changes in PDGF B-type receptor gene expression.

Animals↗

PubMed1992

Stimulation of fetal rat lung cell proliferation in vitro by mechanical stretch.

On the basis of in vivo animal studies and on experiments of nature, it has been suggested that fetal breathing movements are essential for normal lung growth in utero. To study this hypothesis in vitro, we examined the effect of mechanical stretch on proliferation of fetal rat lung cells maintained in organotypic culture to provide a three-dimensional matrix. Initial studies demonstrated that stretch-mediated effects on cell division and DNA synthesis in such cultures were influenced by cell inoculation density, fetal calf serum concentration, and by the amplitude, frequency, periodicity, and duration of the applied stretch. After a 48-h exposure to an intermittent stretch pattern (5% elongation, 60 stretches/min for 15 min of each hour), cell number increased 10% (P less than 0.05), cell doubling time was reduced from 71 to 55 h (P less than 0.05), [3H]thymidine incorporation into DNA increased 61% (P less than 0.01), and the [3H]thymidine-labeling index increased 2.8-fold (P less than 0.001) compared with nonstretched controls. This effect did not appear to be mediated by prostaglandins or leukotrienes because the prostaglandin synthase inhibitors ibuprofen (2.5-50 microM) or BW 755C (5 microM), leukotriene biosynthesis inhibitors BW 755C (5 microM) or MK-886 (0.3 microM), and leukotriene D4 receptor antagonist MK-571 (0.3 microM) did not block stretch-mediated effects. We conclude that mechanical forces act directly to stimulate fetal rat lung cell growth and that these results are compatible with a significant role for fetal breathing in normal fetal lung growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Platelet-derived growth factors and growth-related genes in rat lung. III. Immunolocalization during fetal development.

To further study the role of platelet-derived growth factor (PDGF) in fetal lung development, the distribution of the PDGF homodimers PDGF-AA and PDGF-BB was examined by immunohistochemistry in embryonic and fetal rat lung from d 12 to 22 of gestation (term = 22 d). PDGF-AA and PDGF-BB were localized to airway epithelial cells as early as d 12 of gestation, 2 d before their appearance in mesenchymal cells. Both PDGF homodimer immunoreactivities increased until the late pseudoglandular stage of lung development, followed by fluctuations in reactivity during the canalicular stage. Only weak immunoreactivity to either PDGF homodimer was evident during the saccular stage of lung development. Immunodetection by Western blotting revealed that PDGF-AA and PDGF-BB homodimer protein concentrations were high during the embryonic and pseudoglandular stage of lung development and decreased with advancing gestation. We conclude that the presence of PDGF in both developing airway epithelial cells and mesenchymal cells, as well as gestation-dependent changes of PDGF homodimers, is compatible with a role for this growth factor during fetal lung development.

Animals↗

PubMed1992

Expression of basic fibroblast growth factor and receptor: immunolocalization studies in developing rat fetal lung.

To study the role of basic fibroblast growth factor (bFGF) in fetal lung development, the distribution of bFGF peptide and FGF receptor (FGF-R) was examined by immunohistochemistry in embryonic and fetal rat lung [d 12 to term (term = 22 d)]. Throughout development bFGF was localized to airway epithelial cells, their basement membranes, and their extracellular matrix. FGF-R was also detected in airway epithelial cells, especially in the branching areas, and in interstitial cells as early as d 13. The number of FGF-R immunoreactive cells increased during the embryonic and pseudoglandular stages of lung development, followed by fluctuations in reactivity during the canalicular stage. No FGF-R was detected in tissue from the saccular stage of lung development. The presence of bFGF and FGF-R in developing airway epithelium and mesenchyme is compatible with a role for this growth factor during fetal lung development. In the developing lung, bFGF seems to be sequestered and stored in the extracellular matrix, and may be released at times of need. Furthermore, FGF-R up- and down-regulation offers another mechanism by which the growth of specific cell populations may be controlled during fetal lung development.

Amino Acid Sequence↗

PubMed1992

Ontogeny of reactivity to endothelial cell markers during development of the embryonic and fetal rat lung.

The reactivity of endothelial cells to putative endothelial cell-specific markers varies with species, with vessel size and with the organ studied. To determine their value in studies of fetal rat lung, and whether organ immaturity would also influence reactivity, we studied endothelial cell immunoreactivity to antibodies against Factor VIII/von Willebrand factor (VIII/vWF), and binding reactivity to Bandeiraea (Griffonia) simplicifolia 1 lectin (BSL 1) during rat fetal lung development. Using an indirect immunofluorescent technique to detect Factor VIII/von Willebrand factor (VIII/vWF), endothelial cells lining the aortic arches were identified as early as day 11 of gestation (term = 22 days), prior to lung development. Immunoreactivity to VIII/vWF was subsequently localized to intrapulmonary endothelial cells and was not dependent on vessel size. In contrast, binding reactivity of FITC-conjugated BSL 1 was observed to both endothelial cells and to the basement membrane of developing airways, thus limiting its value as endothelial cell marker. During very early lung development solitary angioblasts could not be identified by reactivity to either VIII/vWF antibodies or to BSL 1, and neither marker appears to be of value for studies of early angiogenic events.

Animals↗

PubMed1991

Growth of distal fetal rat lung epithelial cells in a defined serum-free medium.

Fetal rat distal lung epithelial cells, in contrast to adult type II pneumocytes, will divide readily in culture in the presence of 10% (vol:vol) fetal bovine serum. The presence of serum makes purification of uncontaminated cell-derived growth factors difficult and modifies cellular responses to oxidant injury. We report the development of a defined serum-free medium that will support growth of fetal distal lung epithelial cells in primary culture. Initial studies used a low-serum (2%; vol:vol) to determine the effect of basal media, substrata, and various additives. Subsequent studies demonstrated growth on a poly-D-lysine substratum under serum-free culture conditions in Dulbecco's modified minimal essential medium with insulin (50 micrograms/ml), endothelial cell growth supplement (20 micrograms/ml), bovine pituitary extract (100 micrograms/ml), bovine serum albumin (50 micrograms/ml), selenous acid (4 ng/ml), reduced glutathione (500 ng/ml), soybean trypsin inhibitor (100 micrograms/ml), transferrin (5 micrograms/ml), HEPES buffer (2.6 mg/ml), and cholera toxin (5 micrograms/ml). Growth was enhanced by reducing the gas phase oxygen concentration from 21 to 3%. The undefined components of this medium, bovine pituitary extract and endothelial cell growth supplement, could be replaced by platelet-derived growth factor (20 ng/ml) with prostaglandin E1 (25 nM). The response of fetal distal lung epithelial cells to known growth factors differs substantially from that observed with type II pneumocytes from adult lung and is similar in many, though not all, respects to the responses reported for proximal airway cells from adult lung.

Animals↗

PubMed1991

Limited division of low-density adult rat type II pneumocytes in serum-free culture.

A previously reported serum-free medium for the culture of adult rat type II pneumocytes has been further defined by the addition of 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer and 700 U/ml catalase. Both additions significantly enhanced [3H]thymidine incorporation into pneumocyte DNA between days 5 and 6 of culture. In the presence of 100 micrograms/ml bovine pituitary extract, both insulin-like growth factor (IGF) I (20 ng/ml) and II (25 ng/ml) further increased [3H]thymidine incorporation into pneumocyte DNA over that observed with the basal medium. When type II pneumocytes were plated at 2.75 X 10(5) cells/cm2 there was a plating efficiency of 50 +/- 5%, and the increased DNA synthesis in response to IGFs was not associated with an increase in cell number. When the plating number was reduced to 1.6 X 10(5) cells/cm2 the plating efficiency was only 23 +/- 2%. At this low cell density a doubling of cell number was observed in response to IGF-I between days 4 and 9 of culture.

Animals↗

PubMed1991

Spatial and temporal differences in fibroblast behavior in fetal rat lung.

Fibroblast-epithelial interactions were investigated in cells from late-gestation fetal rat lung. Fibroblasts from the pseudoglandular stage of lung development stimulated epithelial cell proliferation, whereas fibroblasts from the saccular stage promoted epithelial cell differentiation. The developmental switch from proliferation to differentiation seemed to be controlled by both cell types. Fibroblast-derived epithelial cell growth-promoting activity, evident in cells from the pseudoglandular period, decreased during development and almost disappeared in cells from the saccular stage. Interestingly, the response of epithelial cells to this growth-promoting activity declined with advancing gestational age as epithelial cells became more responsive to fibroblast-derived differentiation factor(s). Production of differentiation factor(s) by fibroblasts increased during the canalicular stage of lung development. Platelet-derived growth factor (PDGF) and low concentrations of transforming growth factor-beta (TGF-beta) stimulated epithelial cell proliferation. PDGF did not affect differentiation, whereas TGF-beta was inhibitory. Dependent on their proximity to the epithelium, two subpopulations of fibroblasts that differed in their ability to promote epithelial cell proliferation or differentiation were isolated. Fibroblasts in close proximity to the epithelium mainly produced differentiation factors, whereas more distant fibroblasts primarily stimulated proliferation.

Animals↗