The regulated expression of mRNA for Clara cell protein in the developing airways of the rat, as revealed by tissue in situ hybridization

Tissue in situ hybridization has been used on sections of developing rat lung to follow the cellular sites of mRNA expression for a protein identified only in bronchiolar Clara cells. The mRNA for this Clara cell protein (CCP) was first detected on gestational day 16 in only one of the two types of tubules existing in the lung at this developmental stage. During the next 2 days CCP mRNA expression increased uniformly only in the epithelium lining the respiratory tubules. By gestational day 19, CCP mRNA expression became limited to secretory epithelial cells lining the bronchi, and terminal bronchioles. By neonatal day 1, an intense hybridization signal was observed along all of the conducting airways, but it was irregular due to the fact that expression of the CCP gene was limited to the secretory epithelial cells. In adult rats, CCP mRNA was expressed not only in secretory cells of the intrapulmonary airways at all anatomical levels, but also in secretory epithelial cells lining the trachea and its glands, as well as in specific alveolar cells thought to be type II pneumocytes. These findings demonstrate that the regulation of the CCP gene during lung development is a complicated process and that the expression of CCP mRNA does not parallel exactly the sequential development of the airways.