الفهرس 
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Abstract
Clara cells are non-ciliated, non-mucous secretory epithelial cells lining the pulmonary airways. The purpose of this study was to assess the following changes in Clara cells in the conducting airways of the albino rat: 1) The spatial and temporal pattern of development and distribution of Clara cells in the normal airway epithelium. 2) The role of Clara cells in the cell renewal in the steady state. 3) The cellular morphology and ultrastructural peculiarities of Clara cells. Twenty timed pregnant albino rats were used in this study. Five fetuses and neonates were used for each age group of the study. The normal distribution and the f .. mctional maturation of Clara cells during their differentiation were detected immunohistochemically using the CC10 protein antibody. Clara cell contribution to the proliferation was detected immunohistochemically by using the proliferation marker Ki-67 antibody. The nature of Clara cell secretions was detennined histochemically by the P AS/ AB. Scanning electron microscopy marked the surface changes during maturation regarding the mechanisms of secretion. Transmission electron microscopy characterizes the morpho-logic changes that accompany the functional differentiation of Clara cell.
The important finding in this study included; defining the time point ( age) at which Clara cell protein is first manufactured by cells in the developing rat lung, timing of Clara cell phenotypical maturation, and demonstarion of the normal distribution of Clara cells in the bronchial rree. The expression of CC 1 0, was detected in the apex of distal
onciliated airway cells by the 19 DGA. Clara cells were gradually . creased from the terminal to the respiratory bronchioles at all studied ges. The intensity of the reaction varied in cells. The strongest labeling <brSummary and Conclusion
was in the apical region of the cells. Labeling was increased by 7 DPN to the same intensity and distribution of adult. The Clara cell population was virtually restricted to the bronchioles. The lining epithelium was formed of single layer, at all studied ages, but showed gradual increase in height during maturation.
The present study demonstrated that Clara cells In adult rats contribute substantially to cell renewal in normal conducting airway epithelium either in the terminal or the respiratory bronchioles. Clara cells were extensively positive for Ki-67 either in the terminal or in the respiratory bronchioles prenatally and early postnatally, this decreased by the age of21 DPN, and were present in few number at the adult age.
Clara cells were Alcian blue-positive indicating that they contained acid mucopolysaccharide. During prenatal life, Clara cells appeared to have PAS positive granules within a bluish alcian blue background. This was interpreted by the transmission electron microscopic study which revealed that Clara cells contained much of glycogen granules in the cytoplasm at 19 DGA, decreased by 1 day postnatally and then disappeared that could explain why Clara cells after these ages appeared with a bluish cytoplasm without red granules in sections stained with PAS/ Alcian blue.
The morphometric study demonstrated an age- dependent pattern in CC 1 0 expression which restricted to the bronchioles. There was a variation in the number of cells labeled at the different airway levels, which gradually increased. By 7 DPN they reached a mature phenotypic distribution in the terminal and in the respiratory bronchioles (59.2%, 79.7% respectively) which resembled those of the adult in the terminal and in the respiratory bronchioles (60.4%, 80.1% respectively). The contribution of Clara cells to the proliferation compartment is higher in