The development and differentiation of the parabronchial unit in quail (Coturnix coturnix)

The present study has been inspired by the conflicting data in the relevant literature concerning the embryogenesis of cell types of the parabronchial epithelium and the formation, discharge and distribution of trilaminar sub stance and lamellar bodies. Lung tissue from embryonic, newly hatched, imma ture and mature quail was subjected to standard processing for light and tr ansmission electron microscopy. The parabronchial rudiments form shallow pr imitive atria on embryonic day 13. The precursors of granular cells differe ntiate with lamellar bodies in their cytoplasm. The residual population of non-granular epithelial cells is the common source for the differentiation of primitive squamous atrial and respiratory cells, the potential producers of trilaminar substance. The primitive squamous atrial cells sprout as bra nching infundibular canaliculi into the mesenchyme on embryonic day 14. The infundibular epithelium differentiates into the squamous respiratory cells that constitute with blood capillaries the blood-air barrier. Not until th e time of hatching could the trilaminar substance be visualized being produ ced by squamous atrial and respiratory cells. In the late prehatching and e arly posthatching period the granular cells intensely escalate the producti on and discharge of lamellar bodies. The lamellar bodies form, together wit h sheets of trilaminar substance, mixed multilayered masses in atria. They disappear fast in the successive posthatching period. The formation of tril aminar substance in squamous atrial and respiratory cells is governed by th e agranular endoplasmic reticulum, the cisternae of which take part in the formation of trilaminar units. The gas exchange tissue is predominantly rep resented by infundibula in immature quail. The posthatching growth of the g as exchange tissue of immature to mature quail occurs via intense multiplic ation of air and blood capillaries.