Developmental events in the lung of the Japanese quail (Coturnix coturnix japonica): Morphological, histochemical and electron-microscopic studies

The lung of birds is the most complex and efficient gas exchanger in the air-breathing vertebrates. A total number of 27 normal Japanese quail embryos during the pre-hatching period were used. The current work aimed to investigate the histomorphological, histochemical and ultrastructural changes of the lung at different stages of development using light and electron microscopy. The results showed that the respiratory primordium was observed on the 2nd embryonic day (ED) as a ventral out-pouching of the primitive foregut into the surrounding mesenchyme. The first evidence of the lung buds appeared on the 3rd ED. On the 4th ED, the buds increased in size. In addition, the secondary bronchi budded from the epithelial lining of the primary bronchus into the surrounding mesenchyme. On the 6th ED, the number of the secondary bronchi increased and began to give rise small tubules (parabronchi). On the 7th ED, telocytes (TCs) were seen extensively in relation to the undifferentiated mesenchymal cells (UMC) and surrounding the developing bronchi and blood vessels. On the 8th ED, the number of the small-diameter, smooth-walled parabronchi (PR) was considerably increased. On the 9th ED, anastomosing of the developing parabronchi was observed forming large diameter ones and the open-type endocrine cells in the parabronchial wall were observed. On the 10th ED, deep costal impressions on the lung tissue were seen, the atrial primordia could be observed and the primordia of all air sac were developed. In conclusion; morphogenesis and differentiation of the developing lung epithelium depend on epithelial-mesenchyme interaction.