Expression of Tcf/Lef and sFrp and localization of beta-catenin in the developing mouse lung.

Recent evidence that Wnts and other genes in the Wnt signaling pathway are expressed in embryonic and adult mouse lung suggests that this pathway is important for cell fate decisions and differentiation of lung cell types. We therefore examined the expression and protein distribution of several Wnt pathway components during prenatal mouse lung development using whole-mount in situ hybridization and immunohistochemistry. Between embryonic days 10.5 and 17.5 (E10.5-E17.5), beta-catenin was localized in the cytoplasm, and often also the nucleus, of the undifferentiated primordial epithelium (PE), differentiating alveolar epithelium (AE; present from E14.5 onward), and adjacent mesenchyme. Tcf1, Lef1, Tcf3, Tcf4, sFrp1, sFrp2 and sFrp4 were also expressed in the PE, AE, and adjacent mesenchyme in specific spatio-temporal patterns.

Functional overexpression of wild-type p53 correlates with alveolar cell differentiation in the developing human lung.

At 15 weeks after conception (a.c.), the human pulmonary acinus is lined by distal low-columnar and more proximal cuboidal cells that are successive stages in alveolar type II cell differentiation (pseudoglandular period of lung development). From 16 weeks a.c. onward, there are also 'flatter' cells that are intermediate stages in the differentiation of cuboidal type II cells into squamous type I cells (canalicular period). We investigated the role of wild-type p53 protein and the proliferation marker Ki-67 in the differentiation of type II and type I cells in these two periods. Serial sections from fetal lungs (n = 30) were immunoincubated with antibodies against p53 and Ki-67. The presence of prospective type II and type I cells was confirmed using immunohistochemistry for surfactant protein SP-A as a differentiation marker and light and electron microscopy. The p53 and Ki-67 positive nuclei were quantified per alveolar cell phenotype (i.e., low-columnar; cuboidal; flatter). The occurrence of cell apoptosis was studied using propidium iodide (PI) and 4',6'-diamino-2-phenylindol dihydrochloride (DAPI) staining. The combined increase in p53 expression and decrease in Ki-67 expression during alveolar epithelial cell differentiation suggests that wild-type p53 protein plays a role in the differentiation of alveolar type II and type I cells in the human lung, and that this function is mediated through cell cycle arrest. The rare incidence of apoptotic nuclei in alveolar type II cells, together with their absence in alveolar type I cells, supports the view that p53 is involved in the differentiation, rather than the death, of alveolar epithelial cells.

The alveolar type II cell is a pluripotential stem cell in the genesis of human adenocarcinomas and squamous cell carcinomas.

Studies in a canine bronchogenic carcinoma model indicate that alveolar type II cells may differentiate from carcinogen-exposed epithelium of larger bronchi and generate adenocarcinomas with bronchioloalveolar and other growth patterns. In this study, we investigated whether type II cells are one of the major proliferating cells (= stem cells) in the genesis of two major subsets of bronchogenic carcinoma in humans. Adenocarcinomas (17 bronchioloalveolar; 3 papillary; and 10 other) and squamous cell carcinomas (n = 27) as well as (pre)neoplastic lesions in adjacent bronchi and bronchioles were examined for the presence of type II cell markers and cellular proliferation markers (PCNA; Ki-67) using light and electron microscopy and immunohistochemistry. Distinctive features of type II cells, which do not depend upon the degree of cell maturity, are the approximately cuboid shape, large and roundish nucleus, cytoplasmic staining for surfactant protein A (SP-A), and presence of multilamellar bodies or their precursory forms. Cells with this phenotype were found in early progressive (i.e., dysplastic, in situ, microinvasive) lesions in conducting airways and in all the carcinomas investigated, although with a much greater abundance among glandular lesions compared to squamous lesions. The most consistent sites of type II cells were the basal and adjacent epithelial layers. Nuclear PCNA (Ki-67) expression usually predominated in the same region. None of the lesions displayed specific Clara cell features. Our findings strongly suggest that the type II cell is a pluripotential stem cell in human lung carcinogenesis. Based on our findings in humans and dogs, we postulate that type II tumor stem cells may originate from one of two sources: (1) normal bronchial epithelium (by an oncofetal mechanism of differentiation); and (2) normal alveolar type II cells.

Stereological methods: a new approach in the assessment of pulmonary emphysema.

In order to develop a reliable and sensitive method for studying the development and progression of pulmonary emphysema, we compared stereological indices with the usual index for grade of emphysema, i.e., the mean linear intercept (Lm), in elastase-induced emphysema in mice. The Lm and stereological indices, including volumes of total lung tissue (V(lt)), airspaces (V(air)), and surface area of alveolar walls (S(alv)), were determined in 5-microns, H&E-stained, paraffin-embedded lung sections from elastase- (n = 7) or saline-treated (n = 8) mice. The indices were measured by point counting, using Cavalieri's principle (V(lt)) and V(air)) or by counting intersections of alveolar walls with test lines of a known length (S(alv) and Lm). Elastase treatment resulted in a significant increase of Lm and of V(air), both indicating airspace enlargement, and in a significant decrease of V(lt) and S(alv), indicating destruction of alveolar walls. Between each of the stereological indices and the Lm, significant correlations were found when all lungs were included, but not when the emphysematous lungs were considered separately. We conclude that stereological methods can be powerful morphometric tools for studying pulmonary emphysema development and progression, since they give information not only about the grade of airspace enlargement but also about the grade of destruction of alveolar walls. Based on this unique property, stereological methods also allow a distinction between pulmonary emphysema and unrelated conditions with dilatation of airspaces only.

Alveolar stem cells in canine bronchial carcinogenesis.

Alveolar type II cells are not present in normal epithelium of canine segmental bronchi but after carcinogen exposure they do occur in intra-epithelial lesions with all degrees of atypia and in invasive lesions with different glandular growth patterns. Immunohistochemistry for proliferation markers (PCNA; Ki-67) strongly suggest that such novel type II cells are pluripotential stem cells in canine bronchial carcinogenesis. Very likely, bronchial carcinogenesis is subject to an oncofetal mechanism of differentiation: bronchial epithelial retrodifferentiation followed by novel differentiation of alveolar tumor stem cells.

Inhibition of T3-receptor binding by Nitrofen.

Lung development is controlled by various hormones, including thyroid hormone. The herbicide 2,4-dichlorophenyl-p-nitrophenyl ether (Nitrofen) induces lung hypoplasia in fetal rats, when administered to the mother during gestation. Nitrofen might be teratogenic by an anti-thyroid activity. The present study shows that Nitrofen decreases the binding of T3 to the alpha 1 and beta 1 form of the thyroid hormone receptor in a non-competitive way. Consequently, rat lung hypoplasia might result from the decreased binding of T3 to its receptor, via exposure to Nitrofen during fetal development.

Alveolar epithelial composition and architecture of the late fetal pulmonary acinus: an immunocytochemical and morphometric study in a rat model of pulmonary hypoplasia and congenital diaphragmatic hernia.

The aim of the present study was to compare the architecture and alveolar epithelial cell composition of the pulmonary acinus in hypoplastic and normal fetal rat lungs. For this purpose, a rat model of pulmonary hypoplasia in association with congenital diaphragmatic hernia (CDH) induced by Nitrofen (100 mg on day 10 of pregnancy) was studied. Sections (5 microns) from lungs of control and Nitrofen-exposed fetal Sprague Dawley rats with or without CDH aged 18-22 days (vaginal plug on day 1, birth on day 23) were stained with hematoxylin and eosin. To identify developing alveolar epithelial cells, sections were incubated with anti-surfactant protein A (SP-A; rabbit anti-mouse) or preimmunization serum (indirect immunofluorescence). On days 18 and 19, control lungs and exposed lungs from fetuses with and without CDH looked similar (pseudoglandular stage of lung development). The prospective pulmonary acinus consisted of acinar tubules with small round lumens, lined by cuboid, fluorescent type II cells. Morphometric analysis on day 19 showed significantly smaller lung volumes and lung tissue volumes after Nitrofen exposure. On day 20 (canalicular stage), some tubules were slightly dilated and lined by cuboid and thinner fluorescent cells; these dilated tubules were less numerous in lungs from exposed fetuses with CDH. On days 21 and 22 (saccular stage), the saccular lining consisted of cuboid to thin fluorescent cells in exposed lungs from fetuses with and without CDH, and fluorescent (low) cuboid cells interspersed with dark zones (type I cell areas) in control lungs. In the exposed lungs from fetuses with CDH, the lumens of all airspaces were frequently slit-like, and the septa were thicker. These phenomena gave the lungs a primitive, compact aspect. Morphometric analysis on day 22 showed smaller lung volumes and lung tissue volumes, smaller airspace/tissue ratios, smaller epithelial surface areas, and more type II cells per surface area in Nitrofen-exposed lungs than in normal control lungs. The results suggest that Nitrofen-exposed, and thus hypoplastic, fetal rat lungs are retarded with respect to the differentiation of cuboid type II cells into squamous type I cells whether or not CDH is present, and with respect to the development of the future airspaces between days 20 and 22 if CDH is present.

In favour of an oncofoetal concept of bronchogenic carcinoma development.

Our recent studies in a heterotopic model of non-small cell lung cancer in dogs (subcutaneous bronchial autografts treated with 3-methylcholanthrene) have provided evidence that alveolar type II cells may newly arise during initial phases of bronchial carcino-genesis. In the light of these novel findings, which are in agreement with our observations in human non-small cell lung cancer, and in view of present insights into embryonic lung differentiation, we discuss evidence that favours a new, oncofoetal concept of bronchogenic carcinoma development. According to this concept, the primary cells of origin for these tumors are undifferentiated primordial-like cells that derive from bronchial epithelial cells present in major bronchi or their divisions by retrodifferentiation. Such primordial-like cells of origin undergo novel differentiation into the potential (alveolar, bronchial or primordial) tumor stem cells, which occupy the dividing cellular layers of the (pre)neoplastic lesions and constitute the actively dividing and invading part of the neoplasm. Examples of tumors that may originate from alveolar tumor stem cells are carcinomas of the bronchiolo-alveolar, papillary, acinar, and adenoid-cystic types. Squamous cell carcinomas could possibly belong to this group as well, but much more evidence is required to reach conclusions regarding this type of cancer. We suggest that epithelial retrodifferentiation followed by novel differentiation (oncofoetal mechanism) is fundamental in bronchial carcinogenesis.

Ultrastructural features of alveolar epithelial cells in the late fetal pulmonary acinus: a comparison between normal and hypoplastic lungs using a rat model of pulmonary hypoplasia and congenital diaphragmatic hernia.

The aim of this study was to describe and compare the ultrastructural features and functional maturity of alveolar epithelial cells in hypoplastic and normal fetal rat lungs. Pulmonary hypoplasia in association with congenital diaphragmatic hernia was induced in fetuses by administration of 2,4-dichlorophenyl-p-nitrophenylether (Nitrofen) to pregnant Sprague Dawley rats (100 mg on day 10 of gestation). Lung tissue of Nitrofen-exposed and control fetal rats aged 19-22 days (vaginal plug day 1, birth day 23) was embedded in Epon. Semithin (1 micron) toluidine blue-stained sections were examined by light microscopy; ultrathin sections (approximately 80 nm) were studied via transmission electron microscopy. In bronchoalveolar lavage fluid from control and Nitrofen-exposed fetuses (day 22), phospholipid fractions and surfactant protein A content were measured semiquantitatively. On day 19 both control and Nitrofen-exposed lungs contained only cuboid alveolar epithelial cells; from day 20 there were cuboid, low cuboid, and thinner epithelial cells. The (low) cuboid cells contained large glycogen fields, some precursory stages of multilamellar bodies (MLBs), and just a few mature MLBs on day 19 and 20; smaller glycogen fields, more precursory stages, and more mature MLBs on day 21; and little or no glycogen but many precursory stages and mature MLBs on day 22. The thinner cells contained little or no glycogen and a few precursory stages of MLBs on days 20-22; very thin cells on day 22 contained neither glycogen nor any precursory stages of MLBs. MLBs and tubular myelin were seen in the lumens of future air spaces from day 20 onward. Nitrofen-exposed lungs differed from control lungs in that inclusion bodies (IBs) were less numerous in (low) cuboid alveolar cells on days 19 and 20, and more glycogen was seen on day 22. In addition intra- and extracellular "MLBs" in exposed lungs more often had an unusual appearance, i.e., a confluent structure and higher electron density. However, despite morphologic differences, there was no clear difference in phospholipid composition and SP-A content per mol phospholipid in bronchoalveolar lavage fluid. We conclude that morphologically hypoplastic lungs are less mature near term, without an apparent effect on surfactant composition.

Clara cell differentiation in the mouse: ultrastructural morphology and cytochemistry for surfactant protein A and Clara cell 10 kD protein.

The morphologic and functional differentiation of the nonciliated columnar (Clara) cell, one of two secretory cell types in distalmost bronchioles in mammals, was studied in the mouse. Lungs from embryos (16-19 days of developmental age, dDA; birth on day 19), postnatal animals (5-20 days postnatally dPN), and adult animals were investigated by transmission electron microscopy, using standard staining procedures and immunogold (GAR-Au10) labeling for SP-A and Clara cell 10 kD antigen (CCA). At 16 dDa, all the columnar epithelial cells lining prospective distalmost bronchioles lacked distinctive features. By 17 dDa, some cells displayed a few cilia or apical dense granules. At 18 dDa, many nonciliated columnar cells had apical protrusions, as are seen in adult Clara cells. Apical concentrations of glycogen observed in nonciliated columnar cells perinatally were absent by 5 dPN, whereas apical dense granules became more abundant. Profiles of smooth and rough endoplasmic reticulum (ER), which had been randomly distributed, exhibited a selective, adult distribution at 20 dPN (apical vs. basal cytoplasmic domains). Labeling for SP-A and CCA, which was almost absent between 17 and 19 dDa, reached adult levels at the same time. The two proteins differed in distribution. SP-A predominated in adluminal cytoplasmic areas, where it was found over dense granules, vesicles, and multivesicular bodies; it was also present in bronchiolar lumens and intercellular spaces but not in rough ER or Golgi apparatus. In contrast, CCA showed a more uniform distribution; it was present over the same structures as SP-A and in the synthetic organelles. Ciliated columnar cells were virtually devoid of SP-A and CCA. We conclude that mouse Clara cells acquire a mature phenotype by 20 dPN. They are likely to be involved in recycling and/or degradation of SP-A that is internalized from airway lumens through their apical or lateral cell borders; furthermore, they synthesize the Clara cell 10 kD protein. These two Clara cell functions (first detectable late prenatally) reach mature levels by 20 dPN.

Protective effect of pulmonary surfactant on elastase-induced emphysema in mice.

The aim of this study was to obtain some evidence of a protective role for pulmonary surfactant in the pathogenesis of emphysema. Firstly, we developed a quick and easy method to treat mice with a series of intratracheal instillations. Subsequently, three groups of mice were treated as follows: two groups received intratracheal instillations with pancreatic elastase (1.8 mg.kg-1 BW) followed after 3, 48 and 96 h in one group (El/Surf group) by intratracheal administration of surfactant (100 mg phospholipid.kg-1 BW), and in the other group by instillations with saline (El/s group). The third group of control mice was treated with saline followed by three doses of surfactant (s/Surf group). After eight weeks, the mice were killed and emphysema was measured by calculating the mean linear intercepts (Lm) of airspaces. The Lm values in the different groups were statistically tested for differences by the Mann-Whitney test. Instillation of pancreatic elastase (El/s group) resulted in an evenly distributed increase in Lm compared with the control group. Administration of surfactant in elastase-treated mice (El/Surf group) resulted in a statistically significant inhibition of airspace enlargement. Although the Lm in the El/Surf group was still higher than in the control group, analysis of histograms of Lm values per field of examination revealed that the Lm distribution in the former group was similar to that of the s/Surf group. The El/s group, on the contrary, showed the presence of many fields with enlarged air spaces. Repeated instillations with saline and/or surfactant had no effect on the Lm.(ABSTRACT TRUNCATED AT 250 WORDS)

Derivation of tumorigenic and non-tumorigenic mouse alveolar type-II cell lines from fetal type-II cells after a combined in vivo/in vitro carcinogen treatment.

Alveolar type-II cells were isolated from the lungs of fetuses (day 18 of gestation) of the A/WySnAf (A/Sn) mouse strain, which were treated in utero at day 15 with the directly-acting carcinogen N-ethyl-N-nitrosourea (ENU). The isolated type-II cells were again treated with ENU during their initial growth in vitro. After a prolonged culture period, 5 cell lines were obtained, which were identified as type-II cell lines. Differences between cell lines were found with respect to contact-inhibited growth, cell doubling time and ability to grow in a serum-free medium. Two out of the 5 cell lines produced highly invasive type-II cell carcinomas after s.c. injection of 5 x 10(6) cells into nude mice. Thus, both tumorigenic and non-tumorigenic mouse alveolar type-II cell lines were derived after this combined in vivo and in vitro carcinogen treatment of fetal mouse alveolar type-II cells. This offers the possibility of studying in vitro the factors thought to influence lung tumorigenesis in vivo. In addition, our findings strongly suggest that alveolar type-II cells are the progenitor cells of malignant mouse lung tumors.

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).

Lack of type II cells and emphysema in human lungs.

Ten surgically removed human lungs or lobes were studied, to assess the relationship between the abundance of type II alveolar epithelial cells and the degree of emphysema. Type II cell abundance (total number as well as percentage of the total parenchymal cell population) was determined in sections of randomly selected tissue samples of these lungs or lobes by using a type II cell specific antibody specific anti-lavage serum (SALS-Hu), which recognizes surfactant-associated proteins. In these tissue samples we also determined the degree of emphysema with the aid of a number of morphometric parameters, destructive index (DI), mean linear intercept (Lm in mm), and the number of normal alveolar attachments on (pre)terminal bronchioles (normal AA.mm.1). We subsequently calculated the Spearman rank correlation coefficients (rs) between the abundance of type II cells and parameters for emphysema. We found a significant negative correlation between the percentage of type II cells and DI at tissue sample level (rs = 0.55; p = 0.02). We also calculated correlation coefficients between the abundance of type II cells and the degree of small airways disease in (pre)terminal and respiratory bronchioles (SADscore), lung function, age and smoking habits. The results suggest a role for type II cells in the pathogenesis of emphysema.

The proximal border of the human respiratory unit, as shown by scanning and transmission electron microscopy and light microscopical cytochemistry.

The epithelial cell types present in respiratory (= distal alveolarized) and terminal (= distal nonalveolarized) bronchioles in adult human lung were characterized with scanning and transmission electron microscopy (SEM, TEM) and light microscopic cytochemistry, using specific antibodies against surfactant protein SP-A and mucins, and Alcian blue/periodic acid-Schiff (AB/PAS) staining. In the respiratory bronchiole, two epithelial cell populations share the same basal lamina: one pseudostratified columnar with ciliated, secretory, and basal cells and the other predominantly simple cuboid with some interspersed flat (type I) cells. The columnar secretory cells show the ultrastructure of mucous cells. Light microscopically, they react with mucin antibodies and contain primarily periodate-reactive acid mucins. The mucous cells are the distal secretory cells described by Clara (1937). The cuboid cells are identified as type II (precursor) cells based on ultrastructural criteria for embryonic type II cells (Ten Have-Opbroek et al., 1988a, 1990a), including a cuboid cell shape, a large and roundish nucleus, rough and smooth endoplasmic reticulum (ER), osmiophilic multivesicular bodies, and dense bodies. These dense bodies in turn frequently exhibit--like those in embryonic type II cells--internal vesicles or lamellae, variability in size and shape, a specific relationship to ER and a widespread cytoplasmic distribution. Finally, the cuboid cells show a cytoplasmic staining pattern for SP-A. The terminal bronchiole is lined by the columnar cell population. In the respiratory bronchiole, the columnar (bronchial) and cuboid (alveolar) cell populations occupy distinctly different zones (pulmonary artery zone versus remaining wall). The alveolar part of the respiratory bronchiole (called alveolar tubule) defines the proximal border of a true respiratory unit.

Bronchiolo-alveolar regions in adenocarcinoma arising from canine segmental bronchus.

Adenocarcinomas induced in canine bronchial segments placed subcutaneously have bronchiolo-alveolar regions. Immunocytochemistry and routine staining of adjacent sections strongly suggests that the lining of these regions consists of type II cells. These regions may thus represent true prospective alveolar regions, as also seen in embryonic lungs. This first observation of bronchoalveolar cancer arising from a major bronchus indicates that the carcinogen-induced neoplastic progression in bronchial epithelium may lead to type II cell differentiation and type II cell tumor development. The preservation of cell properties in serial nude mouse transplants suggests that it is a stable type II cell population.

Fetal mouse alveolar type II cells in culture express several type II cell characteristics found in vivo, together with major histocompatibility antigens.

Alveolar type II cells were isolated from fetal mouse lung by differential adherence and obtained in monolayer culture. Cultures display a high degree of purity as shown by histochemical and immunocytochemical staining procedures. Seventy-five percent of cells stained positive with specific anti-lavage serum mouse (SALS-M), an antiserum specific for (pre)alveolar type II cells of the mouse, and osmiophilic bodies were present in 82% of cells. These and other characteristics of type II cells in culture correspond to those of alveolar type II cells in fetal mouse lung. The pattern of reactivity of these cells with various anti-cytokeratin antibodies is described, and we show that, in contrast to rat type II cells, they do not exhibit alkaline phosphatase activity. Identity of the type II cell cultures was shown by their specific phospholipid composition and surfactant protein A (SP-A) content. The fetal alveolar type II cells in culture were found to synthesize and express class I but not class II major histocompatibility complex (MHC) antigens. The possibility to culture fetal alveolar type II cells of the mouse and the availability of genetically well-defined inbred and transgenic mouse strains opens ways to study the genetics of type II cell differentiation and function. Also, the in vitro availability of alveolar type II cells, the progenitor cells of mouse lung tumors, will enable us to study in vitro several of the processes involved in lung tumorigenesis in the mouse.

Expression of the major surfactant-associated protein, SP-A, in type II cells of human lung before 20 weeks of gestation.

In a previous paper (Otto-Verberne et al., Anat. Embryol. 178, 29-39 (1988) we reported that the type II alveolar epithelial cell can be identified in fetal human lung on the basis of morphological and immunological characteristics from 10 to 12 weeks after conception (a.c.) onward. For immunological recognition we used a lung-specific antibody, called SALS-Hu (specific anti-lavage serum, rabbit antihuman). The present immunoblotting experiments, after one-and two-dimensional electrophoresis, showed that SALS-Hu-reactive proteins in lavage fractions obtained from alveolar proteinosis patients exhibited molecular masses of mainly 29, 31 to 36, and 62 to 66 kDa. All SALS-Hu-reactive proteins migrated in the same acidic isoelectric point range (pI 4.4-5.1) and were almost undetectable when we used SALS-Hu preabsorbed with recombinant surfactant-associated protein A. We concluded that SALS-Hu recognizes exclusively isoforms of the major surfactant-associated protein, SP-A. In vitro translation assays in which we used mRNA isolated from adult human lung confirmed that SALS-Hu recognized the 29 to 31 kDa SP-A precursor proteins. These SALS-Hu-immunoreactive precursors for SP-A were already detectable (though in much lower amounts) in human fetuses aged 17 to 18 weeks, indicating that mRNA coding for SP-A is present at that time. We concluded that the cytoplasmic staining of fetal (from 10-12 weeks a.c. onward) and adult human type II cells by SALS-Hu is due to the presence of SP-A.