الفهرس 
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Abstract
The present work was carried out on 60 quail embryos and chicks to investigate the development of lung air sacs system during different periods of development. The collected materials were obtained from the Poultry production department, Faculty of Agriculture, Assuit University. The specimens were fixed in the proper fixatives and prepared for histological, histochemical and immunohistochemical investigation. Some other specimens were prepared for examination by scanning and transmission electron microscopes. The morphometric studies were performed on stained paraffin sections using Image J software. The main results were summarized as follow:-
- On the 2nd day, the primordium of the quail respiratory system appeared of incubation as a ventral down growth of the foregut endoderm and known as laryngeo-tracheal groove.
On the 3rd ED, the lung buds were firstly appeared as two bilateral poches on either side of the median dorsal mesentery and composed of a single endodermal tube surrounded by splanchnic mesoderm.
On the 4th ED, the primordial lung increased in size. In addition, the secondary bronchi budded from the epithelial lining of the primary bronchus into the surrounding mesenchyme.
On the 5th ED, progressive growth of the prospective lung was observed. The secondary bronchial buds increased in number. The cranial thoracic air sacs were firstly observed as large oval vesicles lying in the post pulmonary septum near the midline at the base of the lungs.
On the 6th ED, the secondary bronchi developed as two main groups; one extended along the dorsolateral aspect of the lung and the other one extended the ventromedial aspect of the lung. This arrangement indicating their development via monopodial branching in which, secondary branches emerged laterally along the length of their parent branches. The primordia of the tertiary bronchi were firstly observed.
On the 7th ED, the lungs assumed a wedge - shape having wide dorsal end medially and narrow ventral one laterally. The lung had three surfaces; vertebral surface, costal surface and septal surface. Telocytes (TCs) were seen extensively in relation to the undifferentiated mesenchymal cells (UMC) and surrounding the developing bronchi and blood vessels. On this day, the caudal thoracic air sacs and the abdominal ones were clearly observed. Cartilaginous plates were added to the wall of the primary bronchus.
On the 8th ED, the number of the small-diameter, smooth-walled parabronchi (PR) was considerably increased. Prismatic shaped lobules were seen with circular parabronchi in the middle. The secondary bronchus was lined with simple cuboidal epithelium.
On the 9th ED, anastomosing of the developing parabronchi was observed forming large diameter ones and the transient open-type endocrine cells in the parabronchial wall cauld be seen.
On the 10th ED, deep costal impressions on the lung tissue were seen. In addition to the development of atrial primordial, the primordia of both cervical and clavicular air sacs were noticed. At this age, the primordia of all air sacs were well established. There were nine primordial air sacs, which included pairs of cranial thoracic, caudal thoracic, abdominal and cervical air sacs and a single clavicular air sac.
On the 11th ED, The atria were well formed, increased in number and depth and the parabronchial diameter was increased. Some of the UMC developed into smooth muscle layer that interrupted by narrow gaps through which developing atria migrated. The epithelium lining the secondary bronchi was a pseudostratified ciliated columnar epithelium consisting mainly of tall columnar ciliated cells, dark brush cells, secretory cells and basal cells.
On the 13th ED, The atria widened and progressively invaded the mesenchymal tissues forming secondary air passages known as infundibula. Appearance of club-shaped like projections (inter atrial septa) into parabronchial lumen were observed. The atrial muscles formed of smooth muscle cells that migrated from the periphery of the parabronchus toward the parabronchial lumen on the top of the inter atrial septa. By this day the air way system of the quail lung became morphologically evident. Presence of the smooth muscle cells in these locations may regulate the air flow through the atria.
On the 15th ED, the lung parenchyma consisted of more or less hexagonal parabronchi that separated by parabronchial septa. The infundibula extended toward the periphery of the parabronchial mantle and gave rise to air capillaries. The cuboid atrial cells (type II pneumocytes) contained lamellar inclusions that consisted of coarse deeply stained structures surrounded by hallow zones. The free alveolar macrophages were first observed in the connective tissue core of the atrial septum and in the interstitium between blood and air capillaries.
On the day of hatching
- The lung parenchyma completely changed, contained enlarged air spaces, reduced mesenchyme and well developed vasculature. The tertiary bronchi were surrounded by sheaths of gas-exchange tissue. The inter parabronchial septum was greatly reduced, and the only persistent ones appeared in areas of pulmonary blood vessels.
- The blood-gas barrier was formed of three components: the epithelium of the air capillary, the pulmonary capillary endothelium and the interstitial space bounded by basement membranes of epithelium and endothelium. We observed some unevenness of the blood gas barrier due to the corrugation of the endothelium lining pulmonary capillaries. These irregularities in thickness may lead to increase the oxygen diffusing capacity.
- Each parabronchus had three types of epithelial cells. 1- Cuboidal shape granular cells which confined to the atria (Pneumocyte type II). 2- Squamous respiratory cells that lined the air capillaries (Pneumocyte type I). 3- The squamous atrial cells that lined the parabronchial lumen above the atrial muscles.
Elastic fibers were demonstrated in the wall of the bronchi, pulmonary blood vessels, pleura, and in the connective tissue core of the inter atrial septa. However, the lung air capillaries lacked these elastic fibers. Therefore, relatively stable air capillaries were formed, leading to a rigid lung with a constant volume during gas exchange.
The PAS and AB stains gave positive reactions in the basal lamina of both primary and secondary bronchi by day 4 and in the developing cartilaginous plates on the 10th day of incubation. By day 13, the PAS and AB stain revealed strong positive reaction in the goblet and secretory cells that located in the primary and secondary bronchi.
VEGF immunoreactivity was observed in the bronchial epithelium, mesenchyme, telocytes, and endothelial cells of the developing blood vessels on the 8th ED. from the 15th ED to one week post hatching this reaction was also detected in type II pneumocytes.. The nestin immunoreactivity wasn’t expressed during the pre-hatching period. However, it was observed in type II pneumocytes and in the endothelial cells of the blood vessels in the one week old chick post hatching.
The morphometric data revealed that the number of parabronchi per unit area was increased significantly from ED 6 to ED 8. However, the parabronchial number was decreased significantly with advancement of age. The average diameter of these parabronchi was increased with the age progressing. Moreover, the total thickness of the blood gas barrier was significantly decreased with advancement of the age.
No new structures developed in the post hatching periods rather, the already presented ones were remodeled, as seen in the thinning of the blood-gas barrier as well as increasing the number of the avian respiratory macrophages.