Pre-and post-partum growth of feline lung : light, scanning and transmission electron microscopic and morphometric studies

Development of the lung in prenatal, postnatal, and adult cats was observed. After fixation by intratracheal instillation of 3% glutaraldehyde in phosphate buffer at a pressure of 20 cm H₂O, tissue samples were processed for light, SEM and TEM. The epithelial lining of the tracheobronchial tree (TBT) was pseudostratified ciliated with goblet cells. Ciliated, goblet, microvillus-rich, intermediate, nonciliated bronchiolar (NBC), and basal cells were present in growing cats. The serous cell was observed only in the adult. Epithelial thickness increased significantly with age (r_{2=0.518, P<,0.0025) but not linearly (r²=0.353, P<0.0196) with body weight. However, regressing with body weight, the epithelial thickness was correlated when all ages were included (r²=0.644, P<0.0001). The submucosa between and over cartilage was correlated with age (r²=0.655, P<0.0003; 0.534, P<0.0020). Submucosal glands were present in the bronchi of all ages. However, in the adult, they extended into the medium bronchioles. Bronchial-associated lymphoid tissue extended into the terminal bronchiole (TB) and adventitia of pulmonary vessels only in the adult. The adult ciliated cell was covered with microvilli which often branched near or away from the cell's surface. Ciliated cells undergoing ciliogenesis were observed in TBT of growing cats and adult bronchi. The goblet cell was covered with microvilli in the early stages of their secretory cycle, but lacked microvilli late in the cycle. NBCs lining the (TBs) had apical protrusions covered with microvilli, except in prenatal kittens. NBCs contained abundant mitochondria and a and b glycogen inclusions. Serous cells were observed near the basal cell in the trachea and near the luminal surface of adult bronchi. Lung volume and weight increased proportionately with the body weight and age. Surface density increased from 648 cm²/cm³prenatal to 1218 cm²cm³ in 21-day-old kittens. The harmonic mean thickness reduced significantly (from 0.54μm to 0.31μm) with age. Prenatal saccules were surrounded by thick primary septa, lined by type 11 pneumocytes (TII), divided by secondary septa with double capillaries. Eosinophils were observed in the septum, intraepithelium, and in alveolar space of growing cats. Saccular division was not completed by 21 days of age. Elastic fibers accumulated at the tip of the septum and play a role in alveolar formation. Microvilli of TII increased with age. TIl were located at the base of the secondary septum, giving stability to the septum. Type I pneumocytes formed by transformation from TIl pneumocytes. Pores of Kohn were observed only in the adult cats.}