An ultrastructural, morphometric analysis of rabbit fetal lung type II cell differentiation in vivo

Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype

The existing different modes of reproduction in monotremes, marsupials and placentals are the main source for our current understanding of the origin and evolution of the mammalian reproduction. The reproductive strategies and, in particular, the maturity states of the neonates differ remarkably between the three groups. Monotremes, for example, are the only extant mammals that lay eggs and incubate them for the last third of their embryonic development. In contrast, marsupials and placentals are viviparous and rely on intra-uterine development of the neonates via choriovitelline (mainly marsupials) and chorioallantoic (mainly placentals) placentae. The maturity of a newborn is closely linked to the parental care strategy once the neonate is born. The varying developmental degrees of neonates are the main focus of this study. Monotremes and marsupials produce highly altricial and nearly embryonic offspring. Placental mammals always give birth to more developed newborns with the widest range from altricial to precocial. The ability of a newborn to survive and grow in the environment it was born in depends highly on the degree of maturation of vital organs at the time of birth. Here, the anatomy of four neonates of the three major extant mammalian groups is compared. The basis for this study is histological and ultrastructural serial sections of a hatchling of Ornithorhynchus anatinus (Monotremata), and neonates of Monodelphis domestica (Marsupialia), Mesocricetus auratus (altricial Placentalia) and Macroscelides proboscideus (precocial Placentalia). Special attention was given to the developmental stages of the organs skin, lung, liver and kidney, which are considered crucial for the maintenance of vital functions. The state of the organs of newborn monotremes and marsupials are found to be able to support a minimum of vital functions outside the uterus. They are sufficient to survive, but without capacities for additional energetic challenges. The organs of the altricial placental neonate are further developed, able to support the maintenance of vital functions and short-term metabolic increase. The precocial placental newborn shows the most advanced state of organ development, to allow the maintenance of vital functions, stable thermoregulation and high energetic performance. The ancestral condition of a mammalian neonate is interpreted to be similar to the state of organ development found in the newborns of marsupials and monotremes. In comparison, the newborns of altricial and precocial placentals are derived from the ancestral state to a more mature developmental degree associated with advanced organ systems.

Ultrastructural characterization of genetic diffuse lung diseases in infants and children: a cohort study and review

Pediatric diffuse lung diseases are rare disorders with an onset in the neonatal period or in infancy, characterized by chronic respiratory symptoms and diffuse interstitial changes on imaging studies. Genetic disorders of surfactant homeostasis represent the main etiology. Surfactant protein B and ABCA3 deficiencies typically cause neonatal respiratory failure, which is often lethal within a few weeks or months. Although heterozygous ABCA3 mutation carriers are mostly asymptomatic, there is growing evidence that monoallelic mutations may affect surfactant homeostasis. Surfactant protein C mutations are dominant or sporadic disorders leading to a broad spectrum of manifestations from neonatal respiratory distress syndrome to adult pulmonary fibrosis. The authors performed pathology and ultrastructural studies in 12 infants who underwent clinical lung biopsy. One carried a heterozygous SP-B mutation, 3 carried SP-C mutations, and 7 carried ABCA3 mutations (5 biallelic and 2 monoallelic). Optical microscopy made it possible to distinguish between surfactant-related disorders and other forms. One of the ABCA3 monoallelic carriers had morphological features of alveolar capillary dysplasia, a genetic disorder of lung alveolar, and vascular development. One patient showed no surfactant-related anomalies but had pulmonary interstitial glycogenosis, a developmental disorder of unknown origin. Electron microscopy revealed specific lamellar bodies anomalies in all SP-B, SP-C, and ABCA3 deficiency cases. In addition, the authors showed that heterozygous ABCA3 mutation carriers have an intermediate ultrastructural phenotype between homozygous carriers and normal subjects. Lung biopsy is an essential diagnostic procedure in unexplained diffuse lung disorders, and electron microscopy should be performed systematically, since it may reveal specific alterations in genetic disorders of surfactant homeostasis.

Lamellar body formation and dark multivesicular body pockets in fetal and postnatal normal rat alveolar type II cells: ultrastructural study

To investigate the osmiophilic lamellar body (LB) formation in the alveolar type II cells, the authors examined lung tissues from fetal (day 19 of gestation) and postnatal normal rat by electron microscopy. Lamellar body formation was identical in fetal and postnatal rat lungs. Immature dark multivesicular body (MVB) showed incomplete formation of the limiting membrane and contained a cluster of vesicles and an electron-dense granule probably originated from Golgi complex. Dark and light MVBs were seen in alveolar type II cells. However, only dark MVB was involved in the production of LB. First, a drumstick-shaped projection arising from the surface of MVB appeared, which partly trapped cytoplasmic materials such as glycogen granules and ribosomes. The projection was ultimately fused with the MVB surface to form an MVB pocket described here for the first time. In prelamellar body, lamellar structures appeared to provide a communicative MVB pocket and even found in a drumstick-shape. Mature LBs showed disappearance of multivesicles, dense matrix, and MVB pocket. Thus, lamellar body formation might not only need MVB materials, but also glycogen granules and ribosomes in the MVB pocket.

Developmental paradigm for early features of cystic fibrosis

Cystic fibrosis (CF) is a progressive disease in which the lung is perceived to be normal at birth and is injured by recurrent infection. However, there is increasing evidence that the lung is functionally and structurally abnormal prior to the appearance of clinical infection. The cystic fibrosis transmembrane regulator (CFTR) is highly expressed in fetal tissues, and this review examines the role of CFTR in regulatory cascades during lung development. Early changes in the CF lung are examined from a perspective of disrupted fetal development, explaining a number of paradoxes seen with the disease.

Tetrahydrocannabinol (THC) alters synthesis and release of surfactant-related material in isolated fetal rabbit type II cells

Over the years, there has been a great deal of interest in the biological consequences of marijuana use. While evidence indicates that cannabinoids may have therapeutic uses in alleviating certain disease discomfort, there is little recent information on potential health risks, particularly related to the developing fetus. The present study was undertaken to determine the effects of delta 9-tetrahydrocannabinol (THC), the major psychoactive component in marijuana on fetal lung development specifically related to surfactant production. The rationale for the choice of this model lies in the importance of adequate lung development and surfactant production for the successful transition of the fetus to an air-breathing environment. Lung type II cells, the source of pulmonary surfactant, were isolated from fetal rabbit lungs on the 24th gestational day and incubated concurrently with various concentrations of THC and [3H]choline to label disaturated phosphatidylcholine (DSPC) the major surface-active phospholipid of surfactant. Under these conditions THC significantly reduced radiolabelling of DSPC and at the highest concentration (10(-4) M) induced release of DSPC. Pulse-chase studies were also conducted. Cells were prelabelled with [3H]choline, removed to fresh medium with THC (10(-4) M) and incubated for various time periods. Aqueous- and organic-soluble intermediates of DSPC formation were isolated. THC induced a significant increase in radiolabelling of CDPcholine, the rate-limiting conversion in DSPC synthesis. Radiolabelling of total phosphatidylcholine and DSPC was also significantly increased. Assay of CTP: cholinephosphate cytidylyltransferase which enzymatically converts cholinephosphate to CDPcholine showed that THC and phosphatidylglycerol (PG) both induced activation of the enzyme in fetal lung cytosol but not in the membranes. This effect of THC and PG was not additive. THC activated the enzyme only in fetal and not adult rabbit lung. The ability of THC to induce release of surfactant related material was also examined. In cells prelabelled with [3H]choline, THC induced release of [3H]DSPC in both cultured and freshly isolated fetal type II cells. These results suggest THC reduces formation of surfactant DSPC, probably through alterations in membrane dynamics. However, intracellular THC may actually increase formation of DSPC through an effect on the rate-limiting enzyme. THC also increases release of previously formed surfactant-related material.

Role of transcription factors in fetal lung development and surfactant protein gene expression

Branching morphogenesis of the lung and differentiation of specialized cell populations is dependent upon reciprocal interactions between epithelial cells derived from endoderm of embryonic foregut and surrounding mesenchymal cells. These interactions are mediated by elaboration and concerted actions of a variety of growth and differentiation factors binding to specific receptors. Such factors include members of the fibroblast growth factor family, sonic hedgehog, members of the transforming growth factor-beta family, epidermal growth factor, and members of the platelet-derived growth factor family. Hormones that increase cyclic AMP formation, glucocorticoids, and retinoids also play important roles in branching morphogenesis, alveolar development, and cellular differentiation. Expression of the genes encoding these morphogens and their receptors is controlled by a variety of transcription factors that also are highly regulated. Several of these transcription factors serve dual roles as regulators of genes involved in early lung development and in specialized functions of differentiated cells. Targeted null mutations of genes encoding many of these morphogens and transcription factors have provided important insight into their function during lung development. In this chapter, the cellular and molecular mechanisms that control lung development are considered, as well as those that regulate expression of the genes encoding the surfactant proteins.

CFTR modulates lung secretory cell proliferation and differentiation

We have permanently reversed the lethal phenotype in the cystic fibrosis (CF) transmembrane conductance regulator (CFTR)-deficient (knockout) mouse after in utero gene therapy with an adenovirus containing the cftr gene. The gene transfer targeted somatic stem cells in the developing lung and intestine, and these epithelial surfaces demonstrated permanent developmental changes after treatment. The survival statistics from the progeny of heterozygote-heterozygote matings after in utero cftr gene treatment demonstrated an increased mortality in the homozygous normal pups, indicating that overexpression during development was detrimental. The lungs of these pups revealed accelerated secretory cell proliferation and differentiation. The extent of proliferation and differentiation in the secretory cells of the lung parenchyma after in utero transfer of the cftr gene was evaluated with morphometric and biochemical analyses. These studies provide further support of the regulatory role of the cftr gene in the development of the secretory epithelium.

Developmental changes in mitochondrial activity and energy metabolism in fetal and neonatal rat brain

Experiments were undertaken to investigate mitochondrial activity and energy metabolism in the developing rat brain from the late fetal stage to the neonatal stage. Samples of cerebral cortical tissue were obtained from fetuses at 14, 16, 18, and 20 days of gestation, and from pups at 1 h, 1 day and 7 days after birth. Mitochondrial respiration was measured polarographically using homogenates. Fetal and neonatal brains were frozen in situ and fluorometric enzymatic techniques were used for the analysis of ATP, ADP, AMP, and lactate. In the fetal brain, there was a gradual increase in stimulated (+ADP) and uncoupled respiratory rates using glutamate and malate as substrates, from 14 days to 20 days of gestation, together with a moderate increase in ATP concentration and in the sum total of adenine nucleotides, and a significant decrease in lactate. Since non-stimulated (-ADP) respiratory rates did not change with increasing gestational age, the respiratory control ratio appeared to increase over the same period. An increase in mitochondrial activity was more pronounced immediately after birth, together with a marked increase in ATP concentration and in the sum total of adenine nucleotides. The highest rate of mitochondrial respiration was observed in 1-hour-old pups. These results indicate that, in the rat brain, there is maturation of oxidative metabolism in mitochondria that is initiated in late gestation. Acceleration in mitochondrial respiration occurs immediately after birth in order to maintain high-energy phosphate levels, and this may be crucial for the successful outcome of the newborn.

Intraalveolar bubbles and bubble films. I. formation and development during the first 48 hours of extrauterine life in rabbits

BACKGROUND

Aeration of mature lungs at birth depends on formation of intraalveolar bubbles and bubble films (Scarpelli 1978. Pediatr. Res., 12:1070-1076). Bubbles establish immediately structural stability and pulmonary gas exchange. Given that air spaces are cleared in minutes of fetal liquid (the initial substrate for bubble formation), in formation possible beyond this period? If so, is this related to early development of pulmonary function and structure?

METHODS

Mature, spontaneously breathing rabbit pups at 1-10 min and 1, 3, 8, 24, and 48 h after vaginal birth were anesthetized, trachea was occluded at "resting volume" (approximately functional residual capacity), and lungs were rapidly exposed to preserve in vivo intrapulmonary status. The entire lung was examined by stereomicroscopy. Other determinations included resting volume, lung wet weight, volume-pressure curves, histological sections, lung dry weight, tissue hydroxyproline (OH-Pro), and lavage phospholipids (PL). Bubble mobility in situ was tested. Bubbles were released into bathing liquid by incision of peripheral units and monitored over time.

RESULTS

Pup activity and gross appearance of the lungs, together with septal thinning, secondary septal development, clearance of intraluminal liquid, increasing tissue OH-Pro, and PL distribution indicated normal postnatal development. Each aerated unit examined at resting volume (all lobes, all ages) contained intraalveolar bubbles. Transition to free gas exclusively in conducting airways and bubbles/bubble films in peripheral gas exchange units occurred within 1 h. Bubbles appeared to be exclusively within alveoli at 4 h and thereafter. Bubbles persisted and new bubbles were formed during subsequent inflation to maximal volume and deflation to atmospheric pressure (P0). Volume of intact lungs at P0 was maintained by the counterforce of rigid bubble films against tissue retraction. When bubbles were released either at resting volume or at P0, the bubble-free loci became airless. Constant size and stability of released bubbles support preferential incorporation of surfactants into bubble films and constant "near-zero surface tension" (Scarpelli 1978. Pediatr. Res., 12:1070-1076).

CONCLUSIONS

We show the ubiquitous presence of intraalveolar bubbles and bubble films in vivo throughout the first 48 h of postnatal life. Bubble film rigidity sustains aeration and prevents collapse, while low surface tension of the films facilitate liquid removal from the air spaces. Bubbles in situ are stable and, within apparent limits, mobile; after birth they are quickly restricted to the alveolar spaces, leaving airways bubble free.

Morphometric characterisation of the fine structure of human type II pneumocytes

BACKGROUND

Pulmonary type II pneumocytes have been examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and morphometry in numerous mammals. Until now, the fine structure of the human type II pneumocyte has not been studied by means of morphometry.

METHODS

Eleven human donor lungs, which could not be made available for a suitable recipient, were preserved with Euro Collins solution (ECS) according to clinical organ preservation techniques. The lungs were fixed via the airways. Systematic random samples were analyzed by SEM, TEM, and classical stereological methods.

RESULTS

Type II pneumocytes showed normal fine structural characteristics. Morphometry revealed that although inter-individual variation due to some oedematous swelling was present, the cells were in a normal size range as indicated by an estimated mean volume of 763 +/- 64 microns 3. The volume densities were: nucleus 21.9 +/- 2.2%, mitochondria 5.8 +/- 0.9%, lamellar bodies 9.8 +/- 3.6%, and remaining cytoplasmic components 62.4 +/- 2.9% of the cell volume. Since the inter-individual variations in the volume densities referred to the cell may, to variable degrees, reflect the variation in the reference space, the volume densities referred to the constant test point system and the respective volume-to-surface ratios were used for inter-individual comparisons. These parameters indicate that lamellar bodies were independent of cellular swelling, while mitochondria < nucleus < remaining cytoplasmic components increased in size with increasing cell size.

CONCLUSIONS

Two to 7.5 hours of cold ischemia following ECS preservation do not deteriorate the fine structure of type II pneumocytes of human donor lungs. For reliable assessment of fine structural variations, morphometric parameters are required that are independent of variations in cell size.

Ca(+2)-phosphatidylserine-dependent protein kinase C activity in fetal, neonatal and adult rabbit lung and isolated lamellar bodies

Evidence indicates that Ca(+2)-phosphatidylserine-dependent protein kinase C (PKC) is involved in regulation of surfactant secretion. This study was done to examine PKC activity in lung as surfactant synthesis and secretion is initiated, and at birth and to compare these enzyme levels with those in the adult lung. NZW rabbits were used. Fetal and adult lungs were fractionated into subcellular compartments including a lamellar body fraction, which represents intracellular surfactant. The time course for microsomal enzyme activity was compared between 24th gestational day and adult rabbit lung. The reactivity appeared similar in both fractions. PKC specific activity displayed a prominent peak between the 27th and 30th gestational days in total homogenate and lamellar bodies. Specific activity was also high in nuclear, mitochondrial and microsomal fractions the day prior to birth. Adult levels were similar or higher. Total PKC activity was high during late gestation but declined sharply the day prior to birth. A marked increase was present on the first postnatal day. In contrast lamellar bodies displayed a peak in activity between the 27th and 30th gestational days followed by a decline to adult levels. Delipidation of lamellar body fraction indicated that the high enzyme activity in this fraction on the 27th gestational day was not artifactual. The changes observed in PKC in fetal, neonatal and adult lung indicate this enzyme activity changes in lung during the period of onset of surfactant synthesis and secretion during late gestation and may be associated with lamellar bodies, in 27th gestational day fetal lung.

Regulation of mRNA levels for pulmonary surfactant-associated proteins in developing rabbit lung

Gene transcriptional activities and steady-state mRNA levels have been examined for the surfactant-associated proteins SP-A, SP-B and SP-C in developing rabbit lung. It was observed SP-C mRNA levels increase early in gestation, while SP-A and SP-B mRNA levels increase rapidly between 26 and 30 days gestation. Transcriptional activities for all three surfactant apoproteins increase between 26 and 30 days. Studies conducted with fetal lung explants of 26 days gestation demonstrated exposure to low doses of dexamethasone increases SP-A and SP-C mRNA levels, while high doses stimulate transcription, although this only significant for SP-C. Time course studies revealed different temporal patterns and glucocorticoid responses for SP-A and SP-C mRNAs. SP-A and SP-C mRNA production and steady-state levels were reduced after treatment with cycloheximide. In contrast, SP-B gene transcription was selectively stimulated, suggesting involvement of a labile negative regulatory factory. It is concluded that expression of the three surfactant apoproteins is independently regulated. Early in gestation, SP-C mRNA levels may be regulated in vivo through message stabilization. Glucocorticoids can affect SP-A and SP-C mRNA levels in culture at both transcriptional and post-transcriptional levels. The ability of glucocorticoids to influence these processes declines during fetal development.

Late appearance of a type I alveolar epithelial cell marker during fetal rat lung development

Recent studies in fetal lung using immunological and molecular probes have revealed type I and type II cell phenotypic markers in primordial lung epithelial cells prior to the morphogenesis of these cell types. We have recently developed monoclonal antibodies specific for adult type I cells. To evaluate further the temporal appearance of the type I cell phenotype during alveolar epithelial cell ontogeny, we analyzed fetal lung development using one of our monoclonal antibodies (mAb VIII B2). The epitope recognized by mAb VIII B2 first appears in the canalicular stage of fetal lung development, at approx. embryonic day 19 (E19), in occasional, faintly stained tubules. Staining with this type I cell probe becomes more intense and more widespread with increasing gestational age, during which time the pattern of staining changes. Initially, all cells of the distal epithelial tubules are uniformly labelled along their apical and basolateral surfaces. As morphological differentiation of the alveolar epithelium proceeds, type I cell immunoreactivity appears to become restricted to the apical surface of the primitive type I cells in a pattern approaching that seen in the mature lung. We concurrently analyzed developing fetal lung with an antiserum to surfactant apoprotein-A (alpha-SP-A). Consistent with the findings of others, labeling of SP-A was first detectable in scattered cuboidal cells at E18. Careful examination of the double-labeled specimens suggested that some cells were reactive with both the VIII B2 and SP-A antibodies, particularly at E20. Confocal microscopic analysis of such sections from E20 lung confirmed this impression. Three populations of cells were detected: cells labeled only with alpha-SP-A, cells labeled only with mAb VIII B2, and a smaller subset of cells labeled by both.(ABSTRACT TRUNCATED AT 250 WORDS)

Exposure of rabbit fetal lung to glucocorticoids in vitro does not enhance transcription of the gene encoding pulmonary surfactant-associated protein-B (SP-B)

We have investigated the ontogeny and hormonal regulation of both synthesis rates and cellular accumulation of the mRNA for surfactant-associated protein B (SP-B) in rabbit fetal lung. The developmental pattern for SP-B mRNA synthesis increased as a function of gestational age and paralleled that for SP-B mRNA levels except on days 22-26 of gestation where relatively higher levels of gene transcription were observed. Time-course studies with explants from 26- and 30-day fetal lung maintained in culture revealed a gradual increase in mRNA levels and a much smaller increase in gene transcription relative to adult values. Within 48 h of exposure of 26-day explants to dexamethasone at 10(-8) M there was a rapid increase in SP-B mRNA levels to 7-fold adult levels. A similar overall although somewhat slower and attenuated pattern was observed with 30-day explants. Dexamethasone at 10(-8) M had no effect on SP-B gene transcription with explants of either gestational age. We conclude that the major effect of dexamethasone treatment in vitro on SP-B mRNA levels appears to be post-transcriptional and there are small but distinct differences in the effects of glucocorticoids on SP-B mRNA levels with explant cultures from early and late stages of fetal lung maturation.

Localization of surfactant-associated proteins SP-A and SP-B mRNA in rabbit fetal lung tissue by in situ hybridization

Pulmonary surfactant is a lipoprotein substance, comprised of approximately 80% phospholipid and approximately 10% protein, that lowers surface tension at the air-alveolar aqueous interface. Surfactant is synthesized and secreted by alveolar type II epithelial cells where it is stored intracellularly in lamellar bodies. In the present study, we used the technique of in situ hybridization to localize the mRNA for two surfactant-associated proteins, SP-A and SP-B, in developing rabbit fetal lung tissue. We found that SP-A mRNA was first localized in rabbit fetal lung alveolar type II cells on day 26 of gestation, the time at which lamellar bodies are first observed within fetal lung type II cells. On day 28 of gestation, a very small amount of SP-A mRNA was also detectable in the epithelial cells of some bronchioles. In neonatal and adult rabbit lung tissue, SP-A mRNA was primarily restricted to alveolar type II cells; however, the epithelial cells of some bronchioles contained small amounts of SP-A mRNA. SP-B mRNA was first detected in cuboidal epithelial cells in the prealveolar region of the rabbit fetal lung tissue on day 24 of gestation, i.e., at least 2 days before the appearance of SP-A mRNA and lamellar bodies within differentiated alveolar type II cells. SP-B mRNA was detected in most bronchiolar epithelial cells of the rabbit fetal lung tissue at day 28 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphogenetic and functional activity of type II cells in early fetal rhesus monkey lungs. A comparison between primates and rodents

To evaluate further the role of type II alveolar epithelial cells in primate lung development, lungs of fetal (46 to 155 days gestational age [DGA]), postnatal, and adult rhesus monkeys were investigated with antibodies against surfactant protein A (SP-A), Alcian blue (AB) staining, and periodic acid-Schiff (PAS) staining with/without alpha-amylase pre-treatment. In adult and postnatal lungs, type II cells (cuboid shape; large, roundish nucleus) displayed a unique cytoplasmic staining for SP-A. In prenatal lungs, a low-columnar to cuboid type of cell with a large, roundish nucleus was first detectable by 62 DGA. It was the only cell type to line the distalmost tubules or buds of the prospective respiratory tract. It exhibited (initially partial) cytoplasmic staining for SP-A. AB and PAS stainings showed the presence of acid glycoconjugates and large apical and/or basal glycogen fields. After 95 DGA, the lining of the distal respiratory tract additionally displayed flatter cells with immunoreactivity for SP-A and non-reactive zones. Columnar epithelium (pseudostratified or simple) never stained for SP-A. We conclude that morphologically identifiable type II cells first appear in fetal rhesus monkey lungs by 62 DGA (pseudoglandular period). The cells may already synthesize surfactant and extracellular matrix components. They generate type I cells, and thus the entire pulmonary acinus lining. These conclusions for the rhesus monkey fully agree with our earlier conclusions for another primate, the human, and for rodents. However, as presently shown, primates differ greatly from rodents with respect to the timing of type II cell differentiation (at 29-38% versus 73-75% of gestation or at 22-25% versus 48-49% of prenatal lung development).

Localization of surfactant-associated protein C (SP-C) mRNA in fetal rabbit lung tissue by in situ hybridization

Surfactant is a lipoprotein substance that is synthesized and secreted by alveolar type II epithelial cells and acts to reduce surface tension at the air-alveolar interface. SP-C is a 5,000-D molecular weight, hydrophobic, surfactant-associated protein. In the present study, we used a ribonuclease protection assay to show that SP-C mRNA is induced in rabbit fetal lung tissue early in development, increases in relative concentration as development proceeds, and is present in maximal concentration at term (31 days of gestation). We also used the technique of in situ hybridization to localize SP-C mRNA in fetal, neonatal, and adult rabbit lung tissue. SP-C mRNA was present in all of the epithelial cells of the prealveolar region of day 19 gestational age rabbit fetal lung tissue, i.e., about 7 days before the appearance of differentiated alveolar type II cells in the fetal lung tissue. By day 27 of gestation, SP-C mRNA was restricted to epithelial cells with the morphologic characteristics of alveolar type II cells. SP-C mRNA was not detected in bronchiolar epithelium at any stage of lung development. The intensity of SP-C mRNA hybridization in the prealveolar and alveolar type II epithelial cells increased as a function of gestational age and was maximal at term. The pattern of SP-C mRNA localization in neonatal and adult rabbit lung tissue was consistent with the restriction of SP-C gene expression to differentiated alveolar type II cells. Our data are suggestive that SP-C may serve some as yet unknown function early in lung development because it is present in fetal lung prealveolar epithelial cells much earlier in gestation than are differentiated, surfactant-producing alveolar type II cells.

Glucocorticoid effects on rabbit fetal lung maturation in vivo: an ultrastructural morphometric study

Maternal administration of glucocorticoids is known to stimulate fetal lung maturation. In the present study, we used microscopy and stereology to evaluate the morphological effects of maternal glucocorticoid treatment on rabbit fetal lung tissue. Betamethasone was administered to pregnant rabbits on days 25 and 26 of gestation at a dose of 0.2 mg/kg body weight. The animals were sacrificed on day 27 of gestation. Glucocorticoid treatment significantly increased the presumptive airspace in the fetal lung tissue but did not alter the relative proportion of epithelium, connective tissue, or vasculature in the tissue. In addition, glucocorticoid treatment significantly increased the proportion of type II cells in the prealveolar epithelium, increased the rate of phosphatidylcholine synthesis, and increased the content of the major surfactant-associated protein, SP-A, in the fetal lung tissue. We could detect no effect of betamethasone on lamellar body cross-sectional area, numerical density, or volume density within fetal lung type II cells. Glucocorticoid treatment of the pregnant doe caused a decrease in the volume density of intracellular glycogen and an increase in the volume density of mitochondria in fetal lung type II cells. Betamethasone treatment did not alter the distance between fetal lung epithelial cells and subadjacent connective tissue cells. However, glucocorticoid treatment increased the number of connective tissue foot processes that pierced the epithelial basal lamina. Thus, glucocorticoid treatment of the pregnant doe results in structural changes in the fetal lung tissue, an acceleration of some aspects of type II cell differentiation, and a concomitant increase in epithelial-mesenchymal interactions.