Retinoic Acid: A Key Regulator of Lung Development

Retinoic acid (RA) is a key molecular player in embryogenesis and adult tissue homeostasis. In embryo development, RA plays a crucial role in the formation of different organ systems, namely, the respiratory system. During lung development, there is a spatiotemporal regulation of RA levels that assures the formation of a fully functional organ. RA signaling influences lung specification, branching morphogenesis, and alveolarization by regulating the expression of particular target genes. Moreover, cooperation with other developmental pathways is essential to shape lung organogenesis. This review focuses on the events regulated by retinoic acid during lung developmental phases and pulmonary vascular development; also, it aims to provide a snapshot of RA interplay with other well-known regulators of lung development.

Deferoxamine Improves Alveolar and Pulmonary Vascular Development by Upregulating Hypoxia-inducible Factor-1α in a Rat Model of Bronchopulmonary Dysplasia

Fetal lung development normally occurs in a hypoxic environment. Hypoxia-inducible factor (HIF)-1α is robustly induced under hypoxia and transactivates many genes that are essential for fetal development. Most preterm infants are prematurely exposed to hyperoxia, which can halt hypoxia-driven lung maturation. We were to investigate whether the HIF-1α inducer, deferoxamine (DFX) can improve alveolarization in a rat model of bronchopulmonary dysplasia (BPD). A rat model of BPD was produced by intra-amniotic lipopolysaccharide (LPS) administration and postnatal hyperoxia (85% for 7 days), and DFX (150 mg/kg/d) or vehicle was administered to rat pups intraperitoneally for 14 days. On day 14, the rat pups were sacrificed and their lungs were removed and examined. A parallel in vitro study was performed with a human small airway epithelial cell line to test whether DFX induces the expression of HIF-1α and its target genes. Alveolarization and pulmonary vascular development were impaired in rats with BPD. However, DFX significantly ameliorated these effects. Immunohistochemical analysis showed that HIF-1α was significantly upregulated in the lungs of BPD rats treated with DFX. DFX was also found to induce HIF-1α in human small airway epithelial cells and to promote the expression of HIF-1α target genes. Our data suggest that DFX induces and activates HIF-1α, thereby improving alveolarization and vascular distribution in the lungs of rats with BPD.

[Evaluation of the development of pulmonary vessels with pulmonary venous index in congenital heart disease with decreased pulmonary blood flow]

OBJECTIVE

The development status of pulmonary artery is one of the most important criteria for decision-making strategy and predicting postoperative outcome in congenital heart disease with decreased pulmonary blood flow. Currently, Nakata index and McGoon index have been used as morphologic index in evaluating the development status of pulmonary artery. Those indices have some shortcoming. It was recently found that pulmonary veins index is a more precise morphological indicator of pulmonary blood flow and development status of pulmonary vessels. This study aimed to explore an index of evaluating pulmonary blood stream and the development of pulmonary vessels, as a criterion for surgical decision-making strategy.

METHODS

The diameters of left and right pulmonary arteries and pulmonary veins were measured on DSA films in 74 patients with congenital heart disease with decreased pulmonary blood flow, The correlative analysis was done between Nakata index, McGoon index, pulmonary vein index (PVI) and postoperative outcome which were the length of stay in ICU, duration of mechanical ventilation and dose of inotropic drugs.

RESULTS

Excellent correlations between the size of pulmonary veins and pulmonary arteries were found, the correlation between left pulmonary veins and distal portion of left pulmonary artery was 0.73, between left pulmonary veins and proximal portion of left pulmonary artery was 0.72, right pulmonary veins and distal portion of right pulmonary artery was 0.67, and right pulmonary veins and proximal portion of right pulmonary artery was 0.71. The length of stay in ICU, duration of mechanical ventilation and dose of inotropic drugs correlated well with PVI (r = -0.51, -0.478, and -0.693). Compared with Nakata index and McGoon index, PVI was a better criterion for evaluating the developmental status of the whole pulmonary vessels. In the right ventricular outlet reconstruction patients, the McGoon index for patients with low cardiac output syndrome (LCOS) was 1.36 +/- 0.51, and 1.97 +/- 0.58 for patients without LCOS (t = 2.347, P < 0.05), the Nakata index for patients with LCOS was 164 +/- 106 mm(2)/m(2) and 269 +/- 124 mm(2)/m(2) for patients without LCOS (t = 2.218, P < 0.05), the PVI for patients with LCOS was 152 +/- 77 mm(2)/m(2) and 273 +/- 125 mm(2)/m(2) for patients without LCOS (t = 2.936, P < 0.01), pulmonary vessel index of patients with LCOS was less than that of those without LCOS. When PVI was < or = 180 mm(2)/m(2), postoperative hemodynamics was unstable, the frequency of low cardiac output syndrome and mortality significantly increased.

CONCLUSIONS

The development of pulmonary arteries and pulmonary veins correlated with each other. PVI is a precise morphological indicator of pulmonary blood flow and development of pulmonary vessels. It is a helpful indicator to decide surgical strategy.

MRI and echocardiographic assessment of the diastolic dysfunction of normal aging: altered LV pressure decline or load?

Changes in diastolic indexes during normal aging, including reduced early filling velocity ( E), lengthened E deceleration time (DT), augmented late filling ( A), and prolonged isovolumic relaxation time (IVRT), have been attributed to slower left ventricular (LV) pressure (LVP) decay. Indeed, this constellation of findings is often referred to as the “abnormal relaxation” pattern. However, LV filling is determined by the atrioventricular pressure gradient, which depends on both LVP decline and left atrial (LA) pressure (LAP). To assess the relative influence of LVP decline and LAP, we studied 122 normal subjects aged 21–92 yr by Doppler echocardiography and MRI. LVP decline was assessed by color M-mode ( Vp) and the LV untwisting rate. Early diastolic LAP was evaluated using pulmonary vein flow systolic fraction, pulmonary vein flow diastolic DT, color M-mode ( E/ Vp), and tissue Doppler ( E/ Em). Linear regression showed the expected reduction of E, increase in A, and prolongation of IVRT and DT with advancing age. There was no relation of age to parameters reflecting the rate of LVP decline. However, older age was associated with reduced E/ Vp ( P = 0.008) and increased pulmonary vein systolic fraction ( P < 0.001), pulmonary vein DT ( P = 0.0026), and E/ Em ( P < 0.0001), all suggesting reduced early LAP. Therefore, reduced early filling in older adults may be more closely related to a reduced early diastolic LAP than to slower LVP decline. This effect also explains the prolonged IVRT. We postulate that changes in LA active or passive properties may contribute to development of the abnormal relaxation pattern during the aging process.

[Histogenesis and structural organization of the walls of rat venae cavae and pulmonary veins]

Using light and electron microscopic methods, the histogenesis and structural organization of the walls of rat venae cavae and pulmonary veins were studied in prenatal and postnatal periods of development. The special attention was paid to the appearance of the striated myocytes in the walls of these vessels during the process of ontogenesis. The time of initial divergent development of myoblastic differon was established, the stages of differentiation of striated myoblasts and the peculiarities of intercellular junctions were characterized, as well as the innervation and vascularization of the walls of venae cavae and pulmonary veins.

Development of the human pulmonary vein and its incorporation in the morphologically left atrium

OBJECTIVE

Using a newly acquired archive of previously prepared material, we sought to re-examine the origin of the pulmonary vein in the human heart, aiming to determine whether it originates from the systemic venous sinus ("sinus venosus"), or appears as a new structure draining to the left atrium. In addition, we examined the temporal sequence of incorporation of the initially solitary pulmonary vein to the stage at which four venous orifices opened to the left atrium.

METHODS

We studied 26 normal human embryos, ranging from 3.8 mm to 112 mm crown-rump length, and representing the period from the 12th Carnegie stage to 15 weeks of gestation.

RESULTS

The pulmonary vein canalised as a solitary vessel within the mediastinal tissues so as to connect the intraparenchymal pulmonary venous networks to the heart, using the regressing dorsal mesocardium as its portal of cardiac entry. The vein was always distinct from the tributaries of the embryonic systemic venous sinus. The orifice of the solitary vein became committed to the left atrium by growth of the vestibular spine. During development, a marked disparity was seen between the temporal and morphological patterns of incorporation of the left-sided and right-sided veins into the left atrium. The pattern of the primary bifurcation was asymmetrical, a much longer tributary being formed on the left than on the right. Contact between the atrial wall and the venous tributary on the left initially produced a shelf, which became effaced with incorporation of the two left-sided veins into the atrium.

CONCLUSIONS

The initial process of formation of the human pulmonary vein is very similar to that seen in animal models. The walls of the initially solitary vein in humans become incorporated by a morphologically asymmetric process so that four pulmonary veins eventually drain independently into the left atrium. Failure of incorporation on the left side may provide the substrate for congenital division of the left atrium.

Cardiac and smooth muscle cell contribution to the formation of the murine pulmonary veins

Previous studies have demonstrated that the primordial pulmonary veins originate as an outgrowth of the atrial cells and anastomosis with the pulmonary venous plexus. As a consequence of this embryologic origin the tunica media of these vessels is composed of cardiac cells that express atrial specific markers (Lyons et al. [1990] J Cell Biol 111:2427-2436; Jones et al. [1994] Dev Dyn 200:117-128). We used transgenic mice for the cardiac troponin I (cTNI) gene and smooth muscle (SM) myosin heavy chain as differentiation markers, to analyze how cardiac and SM cells contribute to the formation and structural remodeling of the pulmonary veins during development. We show here that the tunica media of the adult mouse pulmonary veins contains an outer layer of cardiac cells and an intermediate SM cell compartment lining down on the inner endothelium. This structural organization is well expressed in the intrapulmonary veins from the beginning of vasculogenesis, with cardiac cells accumulating over preexisting roots of endothelial and SM cells and extending to the third bifurcation of the pulmonary branches without reaching the more distal tips of the vessels. On the other hand, SM cells, which are widely distributed in the intrapulmonary veins from the embryonic stage E16, accumulate also in the extrapulmonary branches and reach the posterior wall of the left atrium, including the orifices of the pulmonary veins. This event takes place around birth when the pulmonary blood flow starts to function properly. A model for the development of the pulmonary veins is presented, based upon our analysis.

Pulmonary venous flow from fetal to neonatal period

There are few reports about the effect of fetal or transitional circulation on the pulmonary venous flow. The purposes of this study were to investigate flow patterns of the pulmonary vein serially from fetal to neonatal period and to determine the relationship between pulmonary venous flow and other parameters from aortic and mitral valve. Pulmonary venous flow velocity was analyzed in 21 normal term human fetuses. Postnatal follow-up studies were performed at 1, 6, 24 h, 3 days, 1 week and 1 month. In each time point, pulsed Doppler echocardiography was used to interrogate right upper pulmonary vein, mitral and aortic valve. The measured parameters of pulmonary vein were heart rate, velocity time integral (VTI), and velocities at systolic peak (S), at diastolic peak (D), at nadir between S and D (O), and at nadir between D and the next S (X). E/A ratio and VTI were measured for mitral valve and peak systolic velocity and VTI for aortic valve. Pulmonary venous flow in fetus was phasic and continuous with low velocity. One hour after birth, without a change of flow pattern, all velocities increased dramatically. These high velocities showed a significant decrease during 24 h after birth. Three days after birth, the velocity decreased slightly and flow pattern changed from continuous to interrupted pattern with or without atrial reversal. No Doppler parameters from aortic or mitral valve showed any correlation with parameters from pulmonary vein. In conclusion, the flow pattern of the pulmonary vein in fetus may result from low pulmonary flow and decreased capacitance of the pulmonary venous system. Sudden increase in the pulmonary flow after birth is likely to be responsible for the highest velocities recorded immediately after birth. Left to right shunt through the ductus arteriosus may also contribute to the flow pattern observed in the first several days, as do changes in reservoir function of the pulmonary vein.

Newborn intrapulmonary veins are more reactive than arteries in normal and hypertensive piglets

The reactivity of pulmonary veins during adaptation from pre- to postnatal life is not well characterized. With an in vitro organ bath technique, the responses to the contractile and relaxant agonists U-46619 (10(-10) to 3 x 10(-6) M) and acetylcholine (10(-9) to 10(-4) M) were compared in adjacent conduit pulmonary vein and artery rings from 66 piglets aged 1 wk preterm to 14 days of postnatal life and from adult tissue. Five additional piglets were made hypertensive by exposure to chronic hypoxia for 3 days after birth. Both arteries and veins showed smaller contractile and relaxant responses before birth than after. By 5 min after birth, the contraction by arteries and relaxation by veins had increased (P < 0.05). By 3 days of age, arterial relaxation increased, but in all animals, venous relaxation exceeded that in arteries (P < 0.05). Veins contracted more than arteries in animals aged 3-14 days. Neonatal hypoxia diminished the responses to both agonists in the veins (P < 0.05), whereas the response in the arteries remained similar to that in the normal newborn. We speculate that veins may be more important in postnatal adaptation than previously suggested.

Phosphodiesterase activity in intrapulmonary arteries and veins of perinatal lambs

The transition from fetal to newborn life is marked by a reduction in pulmonary vascular tone mediated by the intracellular second messengers, cGMP and cAMP. We have compared the rates of phosphodiesterase (PDE)-catalyzed hydrolysis of cGMP and cAMP in intrapulmonary vessels of fetal (146 +/- 2 days gestation) and newborn (3-7-day-old) lambs, each n = 6. Lung vessels of second to sixth generations were dissected and cytosol was prepared by differential centrifugation. PDE activity in cytosol was determined by radiometric assay of the hydrolysis of exogenous nucleotides at 30 degrees C for 10 min. Rates of hydrolysis (pmol/min/mg protein) of cGMP were 225 +/- 38 in fetal arteries and different from 151 +/- 7 in veins. In newborn vessels, the rates were 155 +/- 49 and 63 +/- 13 in arteries and veins, respectively. Rates of cAMP hydrolysis by the fetus were 80 +/- 11 in arteries and 45 +/- 16 veins. In newborn lambs the rates were 69 +/- 10 in arteries and different from 18 +/- 4 in veins. Inhibition of PDE activity by zaprinast, a cGMP-specific PDE inhibitor, and rolipram, a cAMP-specific PDE inhibitor, was more in veins of fetal and newborn lambs. Our data show that rates of hydrolysis of the cyclic nucleotides were faster in fetal vessels than in the newborn. We speculate that this would result in a greater accumulation of the cyclic nucleotides in newborn vessels, particularly the veins, and therefore endow the veins with less vascular tone.

Ultrastructural localization of nitric oxide synthase and endothelin in rat pulmonary artery and vein during postnatal development and ageing

The ultrastructural distribution of nitric oxide synthase (neuronal isoform, type I) and endothelin immunoreactivity was examined in the developing and ageing male Wistar rat pulmonary artery and vein. This study demonstrates that from birth to old age (24 months) nitric oxide synthase and endothelin are localized within subpopulations of endothelial cells in the pulmonary vasculature. In the pulmonary artery and vein of newborn rats, and pulmonary vein of 24-month-old rats, positive labelling for nitric oxide synthase was also observed in the vascular smooth muscle. During development and ageing there were ultrastructural and immunocytochemical alterations in the intima of the pulmonary artery and vein. In older animals, damaged endothelial cells were seen alongside healthy-appearing cells, rich in cytoplasmic vesicles and endoplasmic reticulum. In contrast to damaged cells, the healthy-appearing endothelial cells displayed positive cytoplasmic labelling for nitric oxide synthase or endothelin. These immunopositive cells also appeared in the altered regions of the vessels where substantial enlargement of subendothelial extracellular matrix and the presence of various forms of degenerating macrophages and large bundles of collagen fibres were evident. Damage to the pulmonary artery was particularly evident at the ages of 12 and 24 months; various forms of macrophages, some of which displayed positive labelling for nitric oxide synthase and endothelin, were present in the altered intimal subendothelial zone. In conclusion, this study on the pulmonary vasculature suggests that endothelin and NOS in endothelial cells play a role in the local control of vascular tone throughout the lifespan of rats, even in older animals when there is intimal thickening and some endothelial damage. NOS and endothelin was also seen in smooth muscle and macrophages at certain stages in postnatal development and ageing.

In vitro responses of ovine intrapulmonary arteries and veins to endothelin-1

We determined responses of third-generation intrapulmonary arteries and veins of fetal, newborn, and adult sheep to endothelin-1 (ET) and the role of endothelium and cyclooxygenase metabolites in ET effects in adult sheep lung vessels. Presence of endothelium in vessel rings was confirmed by response to endothelium-dependent vasodilators, acetylcholine or bradykinin. Vessel tension induced by ET was expressed as a percentage of tension induced by 100 mM KCl. We found that arteries and veins contracted to 10(-9) to 10(-6) M ET in a dose-dependent manner. Veins exhibited greater sensitivity to ET than arteries in all age groups. Arteries and veins of adult sheep lungs were more sensitive to ET than those of fetal and newborn lambs. In adult sheep lung vessels, pretreatment with indomethacin (5 x 10(-6) M) and SQ 29548, a thromboxane A2-prostaglandin H2 receptor antagonist (10(-5) M), significantly attenuated venous contraction to ET; arterial contraction was unaffected. Denuding vessels of endothelium did not affect responses to ET. We conclude that, in ovine lungs, veins are more sensitive to ET than arteries and that developmental differences in pulmonary vascular responses to ET exist.

Tropoelastin gene expression in the rat pulmonary vasculature: a developmental study

The elastic laminae in a vessel provide resilience to its wall. In perinatal and adult rats, we used in situ hybridization to localize the mRNA for tropoelastin (TE) in endothelial cells, medial smooth muscle cells, and adventitial fibroblasts of pulmonary arteries and veins to determine the contribution of these cells to laminae formation. We found that 1) all three cell types are elastogenic but for each the ontogenic pattern is different, 2) signal in the artery is strongest in the late fetal lung, 3) postnatally TE expression decreases first in the outer medial smooth muscle cells, and 4) the pattern of expression in arteries differs from that in veins. In the d 19 fetus, the signal for TE mRNA was higher in arteries than in veins. In the immediate postnatal period, the arterial signal declined, whereas the signal in veins increased. By postnatal d 21, the arterial TE signal per cell had significantly decreased to an intensity lower than that in veins. In the adult rat lung, no TE mRNA was detected by in situ hybridization. The reciprocal alterations in TE expression in pulmonary arteries and veins may suggest a response to the postnatal change in pulmonary blood pressure. We speculate that because all three cell types are potentially elastogenic they may all play a role in the remodeling that occurs after vascular injury.

Development of the pulmonary vasculature in newborn lambs: structure-function relationships

Our objectives were 1) to describe the quantitative light microscopy and ultrastructure of newborn lamb lungs and 2) to correlate hemodynamic changes during normoxia and hypoxia with the morphology. By light microscopy, we measured the percent muscle thickness (%MT) and peripheral muscularization of pulmonary arteries and veins from 25 lambs aged less than 24 h, 2-4 days, 2 wk, and 1 mo. At the same ages, lungs were isolated and perfused in situ and, after cyclooxygenase blockade with indomethacin, total, arterial (delta Pa), middle (delta Pm), and venous pressure gradients at inspired O2 fractions of 0.28 (mild hyperoxia) and 0.04 (hypoxia) were determined with inflow-outflow occlusion. During mild hyperoxia, delta Pa and delta Pm fell significantly between 2-4 days and 2 wk, whereas during hypoxia, only delta Pm fell. The %MT of all arteries (less than 50 to greater than 1,000 microns diam) decreased, and peripheral muscularization of less than 100-microns-diam arteries fell between less than 4 days and greater than 2 wk. Our data suggest that 1) the %MT of arteries determines normoxic pulmonary vascular resistance, because only arterial and middle segment resistance fell, 2) peripheral muscularization is a major determinant of hypoxic pulmonary vasoconstriction, because we observed a fall with age in peripheral muscularization of less than 100-micron-diam arteries and in delta Pm with hypoxia, and 3) the arterial limit of the middle segment defined by inflow-outflow occlusion lies in 100- to 1,000-microns-diam arteries.

Pattern of connective tissue development in swine pulmonary vasculature by immunolocalization

Light microscopic immunolocalization studies were carried out on lung tissue from eight newborn and four adult pigs using antibodies to six extracellular matrix components. Antibodies to fibronectin, collagen type IV, and laminin localized on the same structures in adult and newborn lungs. By contrast, antibodies to the interstitial collagens (types I, III, and V) were less extensively localized in the newborn than in the adult, particularly those to type I because the fibres on which they localized in the newborn were thin and sparse. At all ages, antibodies to collagen types III and V co-localized on type I fibres and also on thin individual fibres which formed networks, more dense in the adult than in the newborn. At all ages, anti-type III collagen antibodies also localized on smooth muscle cells. In the adult, but not in the newborn, anti-type I and type V collagen antibodies localized on the connective tissue around the smooth muscle cells in the pulmonary arterial media and vein wall. The dominance of collagen type III suggests greater plasticity in the newborn pulmonary vasculature, which helps explain the recently described rapid changes in arterial wall structure which constitute adaptation to extrauterine life. The postnatal increase in collagen type I helps explain the documented postnatal increase in structural stiffness of the pulmonary arteries.

Oxygen-induced alterations in lung vascular development in the newborn rat

Newborn rats were exposed to air or hyperoxic conditions for the first 6 days of life. Resulting effects on the pulmonary vascular bed were determined by analysis of barium angiograms, scanning electron microscopy of methylmethacrylate corrosion casts and whole lung, morphometric estimations of pulmonary arteries/area and capillary number/area, and arterial blood gas measurements. Similar studies were also performed on the lungs of animals allowed to recover in air for 1 and 2 wk. Although the general pattern of the pulmonary arterial bed by barium angiograms appeared similar, diminished branching or underfilling of the distal arterial segments was more frequently encountered in hyperoxic-exposed animals. Morphometric examination and corrosion casts revealed differences in vascular pattern and density between hyperoxia and air-exposed animals. The number of capillaries/mm2 of lung tissue was less in hyperoxic-exposed pups than controls after 6 days of exposure to hyperoxia but markedly increased to slightly above control levels by 2 wk of air recovery. The number of 20--50 micrometers size vessels/mm2 followed a similar pattern of change. Corrosion casts of lung exposed to 6 days of hyperoxia revealed less microvascular density compared to air controls, but after 1 wk recovery in air, hyperoxic-exposed animal had a more extensive network of microvessels. Maximum PaO2 attained by animals in the various groups closely resembled the patterns of change in microvessel density. These findings support the thesis that a major alteration of lung vascular growth and development occurs subsequent to exposure of the newborn to hyperoxia.

Postnatal growth and development of the lung

The lung is qualitatively different at birth, compared to adult life, in many species. In the neonatal period, the peripheral part of the acinus is formed by primary saccules, and it is from the walls of these that alveoli develop postnatally in rats, mice, and rabbits. Alveolar development starts in utero in man and probably in several other species, including sheep, cat, and, probably, mammals with relatively advanced physical development at birth. A sequence of events is well established in rats and mice in the postnatal period. A phase of expansion first occurs, during which lung growth lags behind the increase in body weight and lungs enlarge primarily by expansion, but new tissue is laid down in the saccular wall. This is succeeded by a phase of tissue proliferation during which the saccule is subdivided by numerous secondary crests that develop in the saccular wall and result in the formation of alveoli. During this stage, the rate of lung growth exceeds the rate of body growth and there is a rapid increase in alveolar surface area. Elastic tissue plays a key role in the development of alveoli, and elastic fibers lie in the free margins of the secondary crests and around the mouths of the alveoli. At approximately 2 weeks of age, the rate of cellular multiplication and formation of alveoli is markedly diminished and the lung grows mainly by cellular enlargement, dilatation, and modification of pre-existing structures...

Fetal and childhood development of the intrapulmonary veins in man--branching pattern and structure

The drainage pattern and the structure of the pulmonary veins have been described quantitatively in a series of fetal and child lungs after injection of the veins with a radio-opaque medium. The drainage pattern of the pre-acinar region is complete by half-way through gestation and corresponds with the growth of the pulmonary arteries. There is new growth of veins within the acinus during childhood. Four types of veins arising from five peripheral sites have been described. The change in dimensions with age have also been measured. A measurable muscle layer could not be found in the walls of the veins before birth though some muscle cells were present from 28 weeks of gestation. The thickness of the muscle coat in any vein was similar at birth and in a 3- and 10-year-old child and in all cases was less than in a pulmonary artery of the same size. The muscle cells were of smaller diameter in the veins than in the arteries. Along any venous pathway between a non-muscular and muscular structure was a region where part of the wall was muscular: these veins were termed partially muscular. Their distribution by size was similar at all ages.