SUMMARY & CONCLUSIONThe present study is performed to describe the histological and ultrastructural modifications in the lungs of rabbits at different ages. Twenty healthy male New Zealand White rabbits from four different age groups (between 27th day of gestation and 5 months) were used in the present work. Groups A, B, C, and D according to their ages:Group (A): Included five animals aged 27th day of gestation.Group (B): Included five animals aged one day old.Group (C): Included five animals aged six weeks.Group (D): Included five animals aged five months.The animals were anaesthetized and thoracotomy was performed, then the lungs were carefully dissected out and small specimens were fixed in formol saline for 24 hours for light microscope examination. Sections (five mm thick) were obtained, stained with H & E to study the general structure of the lung, Mallory trichrome stain to identify the collagen fibers and Orcein stain to identify the elastic fibers. Morphometric study was done to measure the alveolar perimeter, alveolar surface area, thickness of primary septa and length of secondary septa. Minute specimens were double fixed in gluteraldehyde and osmium tetraoxide and were processed to obtain semithin sections for toulidine blue stain and ultrathin sections for TEM.The results of premature lung revealed that, the lung had a saccular structure with little alveolar formation. The intersacular septa showed little branching. A capillary network was present on both sides of a thick layer of interstitium containing interstitial cells. These cells were arranged in many layers with ill defined shape and irregular nuclei. Their cytoplasm contained lipid droplets. The blood-air barrier was formed of flat extensions of endothelial and epithelial cytoplasm, separated by fused basal laminae. The saccules are lined by type I and type II pneumocytes. Pneumocytes type II were large round cells, had indented nucleus with prominent nucleoli. They had short apical microvilli and lamellar bodies.At birth, the saccules were transformed into alveolar sacs by the outgrowth of many small ridges or crests from the smooth primary septa. These secondary septa subdivided the primitive air spaces and show a zigzag pattern. The capillary networks were still present on both sides. The interstitium became less cellular with reduction of the layers of interstitial cells and increase in the intersititial matrix. The matrix contained increased amounts of collagen and elastic fibers. Pneumocytes type II contains their prominent lamellar bodies.At six weeks of age, the lung reached its adult structure. The smooth walled-saccules presented in the premature became elongated and gave rise to alveolar ducts that ended in alveoli. There were marked thinning of the septa containing single capillaries. The interstitial cells appeared to be less numerous and smaller. They had fusiform nuclei and intracytoplasmic lipid droplets. Within the interstitium, there were increase in the amounts of collagen and elastic fibers. Air blood barrier was very thin. The inclusion bodies within the type II cells were mostly vacuolated. Alveolar macrophages could have two nuclei with prominent nucleoli. Their cytoplasm was highly vacuolated containing lysosomes.In adult rabbit, the lung showed the same features of six weeks old rabbit, but with more thinning of interalveolar septa and blood-air barrier.الملخص العربيالهدف من البحث: