Fine structure of myogenesis and elastogenesis in the developing rat lung

Development of visceral smooth muscle

The development of the smooth musculature of viscera has attracted the interest of only relatively few investigators, and thus the field appears somewhat underexplored. The major emphasis on histochemical evidence--at the expense of ultrastructural and functional studies--may have limited the progress in this area. Mature tissue is formed through the differentiation of precursors into muscle cells and through the organization of these cells into a complex tissue where distribution and orientation of muscle cells, deployment of abundant extracellular materials and addition of other cellular elements (interstitial cells, fibroblasts, nerves, blood vessels) are characteristic and specific features. The precursor cells are found at sites where a muscle develops, and they derive predominantly from the mesoderm, but also from the neuroectoderm and from the endoderm. The process starts at different times in different organs. The earliest stages of differentiation are characterized by the precursor cells aggregating and becoming elongated; their longitudinal axis lies in a position similar to the one they will have in the mature muscle. Both the cytological and the histochemical differentiation follow distinct patterns in various muscles, with characteristic temporal sequences in the appearance of key features. This process must impart distinct functional properties to a muscle cell at each stage of its development. However, the chronological correspondence between ultrastructural and histochemical development is poorly understood. Histochemical studies have detected gradients of maturation of the muscle cells, for example, across the thickness of the gizzard musculature and along the length of the small intestine; ultrastructural studies have not yet confirmed the existence of these gradients. Muscle growth is accounted for by muscle cell enlargement (without nucleus duplication) and an increase in muscle cell number by mitosis of pre-existing differentiated muscle cells. De-differentiation and division of muscle cells, migration of muscle cells and late development of muscle cell precursors have all also been considered as possible mechanisms for muscle growth. Several authors have described the presence of precursor cells within developing smooth muscles, and they have described late differentiation of some muscle cells or waves of differentiation that would give rise to phenotypic heterogeneity of the mature muscle cell population. In contrast, other studies, mainly by electron microscopy, have suggested that, within large visceral muscles, the muscle cells differentiate synchronously. There are interesting data on the influence of adjacent tissues on the development of a smooth muscle, but the interplay of these and other factors has not been fully investigated. Smooth muscles contract from early in their development, hence mechanical factors are likely to influence development: on the one hand, passive stresses imposed on the muscle by other tissues, such as adjacent muscles or the contents of the viscera and, on the other hand, active forces generated by the muscle itself. The very attraction of visceral smooth muscles in the study of cellular morphogenesis--an attraction that has not yet been highlighted or exploited in scientific studies, either descriptively or experimentally--is that, onto a single type of cell, a large range of factors interact, such as the genetic expression, chemical influences (from other muscles, endocrine glands, nerves, other intramuscular cells) and mechanical factors.

Alveogenesis failure in PDGF-A-deficient mice is coupled to lack of distal spreading of alveolar smooth muscle cell progenitors during lung development

PDGF-A(−/−) mice lack lung alveolar smooth muscle cells (SMC), exhibit reduced deposition of elastin fibres in the lung parenchyma, and develop lung emphysema due to complete failure of alveogenesis. We have mapped the expression of PDGF-A, PDGF receptor-alpha, tropoelastin, smooth muscle alpha-actin and desmin in developing lungs from wild type and PDGF-A(−/−) mice of pre- and postnatal ages in order to get insight into the mechanisms of PDGF-A-induced alveolar SMC formation and elastin deposition. PDGF-A was expressed by developing lung epithelium. Clusters of PDGF-Ralpha-positive (PDGF-Ralpha+) mesenchymal cells occurred at the distal epithelial branches until embryonic day (E) 15.5. Between E16.5 and E17.5, PDGF-Ralpha+ cells multiplied and spread to acquire positions as solitary cells in the terminal sac walls, where they remained until the onset of alveogenesis. In PDGF-A(−/−) lungs PDGF-Ralpha+ cells failed to multiply and spread and instead remained in prospective bronchiolar walls. Three phases of tropoelastin expression were seen in the developing lung, each phase characterized by a distinct pattern of expression. The third phase, tropoelastin expression by developing alveolar SMC in conjunction with alveogenesis, was specifically and completely absent in PDGF-A(−/−) lungs. We propose that lung PDGF-Ralpha+ cells are progenitors of the tropoelastin-positive alveolar SMC. We also propose that postnatal alveogenesis failure in PDGF-A(−/−) mice is due to a prenatal block in the distal spreading of PDGF-Ralpha+ cells along the tubular lung epithelium during the canalicular stage of lung development.

Development of the human fetal visceral pleura. An ultrastructural study

The visceral pleura of human fetuses aged from 9 to 36 weeks of gestation was studied by means of transmission electron microscopy. The main components of the visceral pleura (mesothelium, basal lamina and submesothelial connective tissue layer) are formed in the fetal period. They develop asynchronously in different pleural areas, and do not reach maturity. Fetal differentiation of the lung pleura can be divided in two stages--early (until 17 gestation week) and late stage--up to birth. The high mesothelial cells appear later than the flat cells, but the first type predominates in the final covering layer during the period investigated. The most significant developmental phenomena of the mesothelium involve membranous differentiation (the microvillous covering, vesicular system and intercellular contacts). The different transport and secretory potentials of the mesothelial cells during the various prenatal periods are discussed. The mode of development of the basal lamina suggests its mesothelial origin. The elastic membrane appears during the late stage of fetal life. The components of the submesothelial connective tissue layer (fibroblasts, collagen and elastic fibres, blood and lymph vessels) undergo several phases of differentiation.

Three-dimensional organization of the elastic fibres in the rat lung

BACKGROUND

The elastic framework of the distal lung has been studied by light microscopy (LM) and transmission electron microscopy (TEM). The preservation of the elastic fibres, for the three-dimensional observation in their relative positions, is difficult because they lack support when the normal methods of tissue processing are used. The goal of the present study was to understand the three-dimensional ultrastructure and organization of the elastic fibres of the lung preserved in their relative positions.

METHODS

A combination of intravascular resin injection and formic acid digestion was used. The resin cast of the microvasculature acted as a scaffold to preserve the in vivo arrangement of the elastic fibres that are, otherwise, easily collapsible. Scanning electron microscopy (SEM) samples were further processed for TEM in order to confirm that the fibres were indeed components of the elastic system.

RESULTS

SEM demonstrated a fine framework of elastic fibres, representing remnants of the alveolar walls, with the casted capillaries interwoven with the network of elastin. Each individual elastic fibre is composed of a small bundle of discrete fibrils. Some of these fibrils emerge from the fibre and join other fibres, producing an anastomosing appearance. Several elastic fibres link the walls of the intrapulmonary conducting airways, the vessels walls and the alveolar network, thus establishing an interrelated and interlaced framework.

CONCLUSIONS

The method we have applied to visualize the elastic fibres of the lung is a unique approach to define the spatial organization of the pulmonary elastic fibres. We have demonstrated here the close relationship between the elastic fibres and the capillaries of the septal alveoli. The arrangement of the interwoven network of elastin and its relationship with the capillaries offers the structural setting for the distending capacity of the alveolar wall.

Developmental expression of fibrillin genes suggests heterogeneity of extracellular microfibrils

Extracellular microfibrils, alone or in association with elastin, confer critical biomechanical properties on a variety of connective tissues. Little is known about the composition of the microfibrils or the factors responsible for their spatial organization into tissue-specific macroaggregates. Recent work has revealed the existence of two structurally related microfibrillar components, termed fibrillin-1 and fibrillin-2. The functional relationships between these glycoproteins and between them and other components of the microfibrils and elastic fibers are obscure. As a first step toward elucidating these important points, we compared the expression pattern of the fibrillin genes during mammalian embryogenesis. The results revealed that the two genes are differentially expressed, in terms of both developmental stages and tissue distribution. In the majority of cases, fibrillin-2 transcripts appear earlier and accumulate for a shorter period of time than fibrillin-1 transcripts. Synthesis of fibrillin-1 correlates with late morphogenesis and the appearance of well-defined organ structures; fibrillin-2 synthesis, on the other hand, coincides with early morphogenesis and, in particular, with the beginning of elastogenesis. The findings lend indirect support to our original hypothesis stating that fibrillins contribute to the compositional and functional heterogeneity of the microfibrils. The available evidence is also consistent with the notion that the fibrillins might have distinct, but related roles in microfibril physiology. Accordingly, we propose that fibrillin-1 provides mostly force-bearing structural support, whereas fibrillin-2 predominantly regulates the early process of elastic fiber assembly.

Morphometric analysis of fetal rat lung development

Applying the zone concept described previously, we quantitatively analyzed fetal rat lung development. The zone concept allowed us to coherently define reference spaces in the developing lung, a prerequisite for morphometric analysis. The peripheral zone I corresponds to a zone of growth of yet undifferentiated tissues; zone II arises from zone I and represents a region of structural and cellular differentiation; zones III and IV comprise the conducting airways and vessels. Lungs of fetal rats aged 17-23 days and 20 hours postnatal were fixed with OsO4 and glutaraldehyde and processed for light and electron microscopic morphometry implemented by point and intersection counting. Fetal lung volume grew in proportion to body weight. Zone II being the largest compartment, its volume changes largely determined lung growth rates. Zone II increased in mass owing to differentiation processes at the interface to zone I where the proximal portions of zone I were continuously shifted into zone II by differentiation. New tissue was generated within zone I. Due to these combined processes zone I changed little in volume until it disappeared at the end of the canalicular stage. The presence in the pseudoglandular stage of half of the parenchymal epithelial mass available around birth indicated that parenchymal development started earlier than assumed so far. While the endothelial surface increased most at birth, the epithelial surface grew by more than 600% at day 21, reflecting the onset of canalisation. The study confirmed the usefulness of the zone concept for morphometry and provided some new insights into lung development.

Spontaneous contractility of human fetal airway smooth muscle

Physical forces such as fetal breathing and fluid secretion may influence lung development, perhaps by their ability to distend the fetal lung. We found that airway smooth muscle cells in first-trimester human lung tissue and cultured lung tissue explants spontaneously contract. The contractions caused visible movement of intraluminal fluid and distended the distal ends of the epithelial tubules, suggesting that they produced significant changes in intraluminal pressure. Smooth muscle contractility and responses to pharmacologic manipulations were recorded with video microscopy. The interval between contractions ranged from 8 to 135 s (mean +/- SEM, 54 +/- 5 s; n = 20). Smooth muscle contractility was not inhibited by tetrodotoxin or atropine, implying a myogenic rather than neurogenic origin. Because cholinergic nerves modulate adult airway smooth muscle contractility, we asked whether fetal airway smooth muscle cells were regulated by cholinergic agents. The cholinergic agonists acetylcholine and carbachol both increased fetal airway smooth muscle contractility. Contractions were inhibited by the calcium channel blockers CdCl2 and nifedipine, suggesting that an influx of extracellular Ca2+ accompanied airway smooth muscle contractions. Isoproterenol dilated the contractile regions of the epithelial tubules and stopped contractions. Lemakalim, an activator of smooth muscle ATP-sensitive K+ channels, also arrested contractions and relaxed fetal airway smooth muscle. In conclusion, human fetal airway smooth muscle contacts spontaneously and exhibits pharmacologic responsiveness similar to adult airway smooth muscle. We speculate that fetal airway smooth muscle contractions, possibly exerting effects through phasic changes in intraluminal pressure, may be an important physical force contributing to lung development.

Pulmonary elastic fibers in normal human development and in pathological conditions

Normal human pulmonary elastic fiber development and development in some pathological conditions were examined using elastic stains by light microscopy, electron microscopy, and immunohistochemistry. In normal development elastic fibers, composed mainly of microfibrils, first appeared around primitive bronchioles at 10 weeks of gestation. As they matured, their appearance became more amorphous, and they extended into the peripheral alveolar walls. Development of elastic fibers was retarded in the hypoplastic lungs of the oligohydramnios syndrome, diaphragmatic hernia, and hydrops fetalis. Elastic development was also retarded in congenital pulmonary lymphangiectasia and in focal areas of lungs with pulmonary dysplasia. Distribution of well-developed elastic fibers was found around the dilated bronchioles and alveoli in cases of congenital cystic adenomatoid malformation and extralobar pulmonary sequestration. Elastic fibers were distributed irregularly and unevenly in the lungs of bronchopulmonary dysplasia and ventilated cases of Wilson Mikity syndrome. In addition, four very immature infants who had progressively deteriorating respiratory function showed an almost total lack of elastic fibers in their alveolar walls.

Histologic, morphometric, and immunocytochemical analysis of myometrial development in rats and mice: I. Normal development

Myometrial development from the prenatal to adult period was examined in rats and mice 1) by histologic and immunocytochemical methods with anti-actin, -vimentin, and -laminin to assess cytodifferentiation of smooth muscle and fibroblastic cells; and 2) by morphometric procedures to assess quantitatively the expression of cellular orientation in the emerging inner circular myometrial layer. Uterine mesenchymal cells initially were uniformly vimentin-positive, undifferentiated, and randomly oriented during the late fetal period. By the early neonatal period, three mesenchymal layers became recognizable histologically, the middle one of which (prospective circular myometrium) developed distinct circular orientation and differentiated into a layer composed of actin-positive smooth muscle cells. The cells of the inner mesenchymal layer initially exhibited radial orientation. By 10 days postpartum, the outer longitudinal mesenchymal layer differentiated into bundles of smooth muscle cells representing the longitudinal myometrium. The inner mesenchymal layer remained vimentin-positive and differentiated into the randomly ordered endometrial stroma. The cells of the middle and outer mesenchymal layers that were destined to form myometrium initially expressed vimentin throughout and then coexpressed vimentin and actin, but with time vimentin staining disappeared in the maturing smooth muscle cells as they expressed actin.

Smooth muscle isoactin and elastin in fetal bovine lung

The formation of elastic fiber network in the lung is developmentally regulated. In this study we first demonstrated that tropoelastin mRNA per unit total RNA in the fetal bovine lung increased from 110 to 250 days of gestation (270 day term) as measured by Northern blot analysis. To examine the extent that smooth muscle (SM) type cells contribute to this gestational increase in elastin phenotype, we utilized a dual immunofluorescent staining technique on lung sections with anti-elastin polyclonal and anti-SM isoactin monoclonal antibodies. Elastin staining was always found to localize in proximity to SM isoactin-positive cells at various stages of prenatal lung parenchymal development. Minimal, if any, elastin was seen at interstitial fibroblasts, which were negative for the SM isoactin staining. Distribution of SM (type) cells and elastic fiber together along the airways became sparse and discontinuous distally, and it seemed that formation of air sacs was between the discontinuous elastic fibers. We speculate that smooth muscle (type) cells may be the major elastogenic cells of distal airways and may play an important role in alveolar formation.

Differentiation of interstitial cells and stromal proteins in the secondary septum of early postnatal rat: effect of maternal chronic exposure to whole cigarette smoke

The intention of this investigation was to ascertain the effect of maternal exposure to cigarette smoke on the early postnatal morphogenesis of pulmonary interstitium in offspring. Female rats were chronically exposed to whole cigarette smoke. Offspring of these and control animals were sacrificed at postnatal day 15, and their tissues were prepared for quantitative and qualitative analyses. Results indicate a diminished quantitative representation of parenchymal tissue (P less than 0.01) and a slower pace of secondary septal growth (P less than 0.07) in the experimental lung. Furthermore, a greater cellular volume density (P less than 0.0002) and, inversely, a lesser quantitative representation of extracellular matrix (P less than 0.0002) was ascertained for the experimental septal interstitium. There was proportionately less of elastin substances (P less than 0.009), collagen together with basal laminae (P less than 0.0008), and nonfibrillar, amorphous matrix (P less than 0.02) in the experimental extracellular stroma. Fibrillar collagen and nonfibrillar matrix were represented quantitatively 6.3 times more in the experimental extracellular interstitium than elastin, whereas that ratio for the control tissue was only 4.2. Most experimental interstitial cells (80%) contained numerous lipid globules, which, in contrast, were only occasionally present in control cells (7.3%). Experimental cells, consequently, possessed a larger cross-sectional diameter and a smaller nucleus-to-cytoplasm volume ratio than control cells. These divergent developmental patterns are possibly suggestive of a delayed differentiation of interstitial cells and a modified production to degradation balance of stromal proteins in offspring of animals chronically exposed to whole cigarette smoke.

Morphogenesis of the respiratory bronchiole in rhesus monkey lungs

The epithelium of the respiratory bronchiole in the adult rhesus monkey consists of two populations: a pseudostratified epithelium with basal, mucous goblet, and ciliated cells located near the pulmonary artery (PA); and a simple cuboidal epithelium composed only of nonciliated bronchiolar epithelial (or Clara) cells in areas away from the PA. This study describes the pattern of differentiation of these two epithelial populations, and their relationship to the PA and to the time of appearance of alveoli in the respiratory bronchiole of the rhesus monkey during the period of 90-125 days gestational age (DGA). These events were related to changes in the adjacent parenchyma. Dissected airways of infusion-fixed, critical-point-dried lungs were evaluated by scanning microscopy followed by light microscopy of the same airways. At 54% of gestation (90 DGA), the distal airway was lined by a mixture of ciliated and nonciliated cells. By 67% of gestation (110 DGA), the ciliated cells were confined to the epithelium over the PA. The underlying connective tissue initially was cellular containing few fibers but was fibrous by 76% of gestation (125 DGA). Alveolarization began near the most distal cartilage at 57% of gestation (95 DGA), the same period at which secondary septation occurred in the distal acinus. Thus, alveolarization occurred simultaneously in two centers: 1) the proximal centriacinar region in the vicinity of the most distal cartilage and 2) the distal lung parenchyma. The duration of centriacinar alveolarization was short, approximately 5 days.

The effect of age on lung structure in male BALB/cNNia inbred mice

Morphometric examination using light, scanning electron, and transmission electron microscopy was performed on the lungs from 32 inbred male BALB/ cNNia mice between 38 days and 28 months of age. Between 38 days and 9 months of age the changes were primarily those of growth. Alveolar multiplication and total elastic-fiber length appeared complete by 38 days of age. The major increase in the number of interalveolar pores occurred by 68 days, but there was a significant further increase from 68 days to 9 months of age. At 9 months, approximately 10% of the alveolar wall was formed by pores. Alveolar ducts, the cylindrical core of air central to alveolar mouths, increased more in diameter than length. Between 9 and 28 months the changes were attributed to aging and were different from those reported in humans and other species. Lung volume, alveolar surface area, and total volume of alveolar wall increased with age; there was no change in mean linear intercept and volume proportion of alveolar and alveolar duct air. Total area of pores increased with age, but their number and area fraction of the alveolar wall did not change. No transmission electron microscopic changes were seen in the alveolar walls. We speculate that the morphometric differences between our animals and those studied in other reports may reflect the fact that our animals were specific-pathogen-free animals and kept under protected circumstances.

The development of alveolar septa in fetal sheep lung. An ultrastructural and immunohistochemical study

The morphogenesis of pulmonary alveolar septa in the sheep was studied by light microscopy, transmission electron microscopy and light microscopic immunohistochemistry for the detection of elastin. The primordia of alveolar septa developed in the glandular stage in areas subjacent to the epithelium, and formed alveolar septa by protruding into the glandular lumina. In their earliest stage, the primordia consisted of groups of fibroblasts, which were associated with elastic fibers and unit collagen fibrils and were surrounded by epithelial basement membrane and by more immature fibroblasts. The fibroblasts in the primordia subsequently became myofibroblasts or smooth muscle cells. In the alveolar zone of the glands, elastic fibers were exclusively found in the primordia of alveolar septa in early developing lung. In early developing lung, wavy, thickened epithelial basement membranes were found in the regions of the glands, which eventually underwent considerable expansion of their surface areas, especially in the primordia of alveolar septa and the bifurcations in the alveolar zones. Areas of fusion of the basement membranes of capillary endothelial cells and epithelial cells in the alveolar zone were found after the formation of the primordia of alveolar septa was accomplished. These areas of fusion were not found in the primordia themselves, but in regions between the primordia. Epithelial cell flattening and differentiation occurred after the formation of the primordia of alveolar septa, and flattening was first observed in the areas of the primordia and the bifurcations of the alveolar zones.

Multiple pulmonary (hamartomatous?) leiomyomas. Light and electron microscopic study

The light and electron microscopical features of the lung tumors in a case of multiple pulmonary leiomyomas are described. The differential diagnosis of leiomyomatous tumors of the lung is discussed. They have to be differentiated from lymphangio-leiomyomatosis of the lungs. In the literature, multiple pulmonary leiomyomas are generally considered to be metastases from low grade uterine leiomyosarcoma or to be hamartomatous lung tumors. This is suggested by the glandular structures both within the tumor and on the surface. However, our ultrastructural observations showed these epithelia to have features of granular pneumocytes (type II), in particular they contain lamellar bodies and possess microvilli on their surface. Their formation is considered to be a secondary reaction of alveolar lining cells to tumor growth. A possible origin of multiple pulmonary leiomyomas from the contractile system of the lung acini (contractile interstitial cells) is discussed.

Ultrastructural changes in hamster lung four hours to twenty-four days after exposure to elastase

A single endotracheal instillation of elastase initiates a series of changes in animal lungs that results in a condition resembling human panlobular emphysema. An ultrastructural examination of this series of changes was conducted on the lungs of male golden hamsters exposed to 3H-methylated pancreatic elastase and sacrificed at intervals between 4 hour and 24 days after exposure to enzyme. Lung tissue between 4 and 48 hours showed evidence of hemorrhage and progressive degradation of elastic fibers. Very little indication of epithelial cell damage accompanied these changes. Four days after exposure to elastase, synthesis of new elastic fibers began with the appearance of small clumps of microfibrils in close association with interstitial cells, fibroblasts, and smooth muscle cells. There was also evidence of alterations in alveolar type II cells at this time. Small fibrillar elastic fibers continued to be present in the lung through twenty-four days and may represent a slow repair process or may indicate a structural difference in elastic fibers synthesized after exposure to elastase. Evidence of the continued degradation of elastic fiber could be found up to 16 days after exposure to elastase, revealing that repair processes were occurring in some areas of the lung while destructive process still predominated in other areas.

Ultrastructure of developing alveoli. I. The role of the interstitial fibroblast

We examined the ultrastructural features of postnatal alveolar septal formation in rats from birth to 28 days of age. At birth, the rat lung consists of large saccules with thick walls and cellular interstitium. Interstitial cells have large oval nuclei with scant cytoplasm containing few organelles and scattered lipid droplets. These cells appear to be poorly differentiated mesenchymal cells not engaged in active protein synthesis or secretion. Between 5 and 15 days of age, saccule walls thin and many new alveolar septa form. Two types of interstitial fibroblasts are present: one which appears at the tips of newly formed septa has the characteristics of a myofibroblast and appears to be engaged in synthesis and secretion of elastin; the other fibroblast appears at the base of new septa, is filled with lipid and contains few other cytoplasmic organelles. After 15 days of age, alveolar walls become thinner, few new septa form and interstitial fibroblasts begin to resemble the dormant type of fibroblasts seen at birth. Thus, the process of postnatal alveolarization of lung parenchyma involves differentiation of the interstitial fibroblast and elastogenesis. The factors which control this process, the precise role of elastogenesis in alveolar septal formation, the origin and fate of the lipid filled fibroblast and the ultimate fate of the myofibroblast remain to be determined.

The ultrastructure of pulmonary hamartoma

Ultrastructural and histochemical examination of chondromatous pulmonary hamartomas revealed the epithelial component to be comprised elements similar to those lining the distal bronchioles and the alveoli of adult lung. The stromal cells nearest the epithelium include a population resembling mature fibroblasts and a population of glycogen-containing, primitive appearing cells. More deeply situated stromal cells showed features of chrondroid differentiation.

Pulmonary lymphangiomyomatosis: three new cases studied with electron microscopy

Three cases of pulmonary lymphangiomyomatosis are described, with emphasis on the ultrastructural changes. The clinicopathologic features corresponded to those previously described. Each patient was a female in the reproductive years; breathlessness and recurrent pneumothoraces were the predominant clinical features. Histologically, the lungs showed a focal interstitial infiltrate of short, spindle-shaped mononuclear cells compatible with primitive smooth muscle, which was associated with irregular emphysema and hemosiderosis. Electron microscopy confirmed the smooth muscle nature of the pulmonary infiltrate and showed the presence of cells intermediate between smooth muscle and fibroblasts. Abnormalities were also noted in the pulmonary connective tissue that are possibly related to the fragility of the lung in this condition.

In vitro behavior of human fetal lung maintained in organ culture. Light and electron microscopic studies

Human fetal lung obtained from 9-26 = weeks = old embryos were maintained in organ culture for 2-5 weeks. The in vitro survival and changes are clearly age dependent. The best survival was obtained with lung tissue from the early glandular period. With these young embryos tubular dilation was frequent during the 1st week. The relatively short duration of culture permitted only a fragmentary study of differentiation of the human lung in vitro but, with the exception of tubular dilations, most of the in vitro changes were also found during lung differentiation in vivo = monostratification of epithelium, bronchiolar development, decrease of glycogen, appearance of myelinlike figures, fibroblastic and myoblastic transformation of mesenchymal cells.

Monoamine oxidase activity in the fetal lung and liver

Monoamine oxidase (MAO) in lung and liver is important in the degradation of circulating 5-hydroxytryptamine. These sites of MAO activity have been investigated histochemically in the human fetus of 12 to 18 weeks gestation. Enzyme activity could be demonstrated in the liver by both tryptamine and adrenaline oxidation. In the lung, MAO activity was present only when adrenaline was used to substrate. It may be, therefore, that in the premature baby the capacity of MAO to metabolize 5-hydroxytryptamine is not fully developed, which could lead to deleterious effects on pulmonary function.

Evolution of mesenchymal cells in fetal rat lung

The evolution of connective tissue cells in the developing fetal rat lung is studied under the electron microscope from the 15th until the 21st day of gestation and is compared to the evolution of epithelial cells. Three successive types of stem cells ("mesocytoblasts") are present during the first stages of lung development studied (15 to 18 days of gestation). These stem cells appear to be able to differentiate into fibroblasts or into smooth muscle cells, according to their localization along the broncho-alveolar tubule. Myoblasts are situated near the bronchial epithelium, whereas fibroblasts occur under the alveolar epithelium. Epithelo-mesenchymal interactions are assumed to play a role in this differentiation process. Synthesis of both, collagen and elastic fibers and of cytoplasmic filaments by fibroblasts as well as by myoblasts reveal the multiple potentialities of the mesenchymal stem cell and suggest a common origin. The early fibroblast in characterized by long cytoplasmic processes which contain numerous cytofilaments, and by the presence of collagen fibers in the vicinity of the cell. Later on, (20 days of gestation) the mature fibroblast of the lung mesenchyme shows areas of RER, glycogen and lipidic vacuoles in its cytoplasm. Cytofilaments are numerous within very long cytoplasmic processes and elastic and collagen fibers are very frequent beside the cytoplasmic membrane. The earliest fibroblast differentiation occurs under the epithelium of primitive respiratory bronchioles, which indicate the limit between the bronchial and the alveolar territories. Later on, differentiating fibroblasts are found throughout the whole alveolar walls. Connective tissue cells other than mesenchymal stem cells, fibroblasts or myoblasts are observed during lung development. Vacuolar cells, similar to Hofbauer cells, transiently appear on the 16th day of gestation. On the 20th and the 21st day macrophage-like cells are present in the septal space of the alveolar wall. The absence of intermediate stages of differentiation and parallel evolution of blood cells suggest that those connective tissue cells are differentiated elsewhere and have then migrated from blood into lung mesenchyme. No cell death has been observed in the developing lung.