The foetal development of the rabbit lung: a cytologic, cytochemical and histoenzymatic study

The developmental peculiarities of the rabbit lung were analyzed in foetuses of 14 and 23 days, and in newborns having respired 30 min. and 48 hrs. Cytochemical, histoenzymatic and quantiative cytologic methods were used. The parallel evolution of epithelial and mesenchymal cells was quantified using conventional fields. The development of air spaces was morphometrically appreciated. Acid and neutral mucopolysaccarides, nucleic acids, and enzymic activities (AcPh-ase, AlkPh-ase, ATP-ase, AMP-ase, SDH, MDH, LDG, G1-6-ph-DH, proline-oxydase, hydroxyproline-2-epimerase, unspecific esterase, TwE-ase, beta-gal-ase and beta-gluc-ase, alanyl- and leucineaminopeptidase) were investigated. This complex analysis showed that in a first phase the development mainly involved the epithelial cells, while the proliferation of mesenchymal ones remained constant. In a second phase, the epithelial cell increase became slower, and the mesenchymal cells were decreasing. At the same time the air spaces were continuously increasing. During this process, neutral mucopolysaccharides were synthesized in epithelial cells and in cartilaginous nodules, and sometimes in mesenchymal cells. The RNA was continuously increasing both in epithelial and mesenchymal cells. The high enzymic activities in the 14-day foetuses appeared to be limited to AcPh-ase, AlkPh-ase, and SDH in both epithelial and mesenchymal cells, the LDH in epithelial and the ATP-ase and AMP-ase mainly in mesenchymal cells. At the same time, the G1-6-ph-DH obviously marked the epithelial cell differentiation. In the other foetal and newborn lungs, the enzymic activities appeared to be more various by limitation of AcPh-ase to epithelial elements and of AlkPh-ase to mesenchymal and vascular ones, by activation of proline-oxydase and especially of hydroxyproline-2-epimerase in pleura and peribronchovascularly, by intensification of the unspecific esterase: the other enzymes active in the 14-day foetuses were now weaker. The activity of beta-gal-ase, beta-gluc-ase, and of peptidases was missing during the entire development of the foetal rabbit lung. The corroboration of these data suggested the relation between the differentiation of enzymic activities and the development of foetal rabbit lung, the strong relations between AcPh-ase activity and the epithelial elements, and of AlkPh-ase and ATP-ase with the mesodermo-mesenchymal ones, the marking of epithelial cell differentiation by the G1-6-ph-DH activity, the presence of SDH in the basal corpuscles of differentiating cili, the increase of enzymes making inactive the hydroxyproline in zones in which connective tissue is developing, the low differentiation of hydrolases (related to the absence of air and blood transport of products) and the lack of peptidase activity corresponding to the reduced pulmonary degradation of proteins (as in adult lungs).