DEVELOPMENTAL EXPRESSION OF SP-A AND SP-A MESSENGER-RNA IN THE PROXIMAL AND DISTAL RESPIRATORY EPITHELIUM IN THE HUMAN FETUS AND NEWBORN

We used immunolocalization and in situ hybridization to determine the distribution of SP-A and SP-A mRNA in lungs of human fetuses and norma l newborn infants. Early in the second fetal trimester a few immunosta ined cells were observed in tracheal epithelium, often in mucosal fold s near the origin of submucosal gland ducts. Non-mucous tracheal gland cells were immunostained for SP-A as they became differentiated. Expr ession of SP-A mRNA was similar to that of immunolocalization in the s econd trimester. Immunostained cells and SP-A mRNA also appeared about the same time in gestation in isolated cells of bronchial epithelium and glands. SP-A mRNA was seen in bronchiolar cells and pre-Type II ce lls lining terminal airways of fetuses at 19-20 weeks of gestation. On ly in liveborn infants did cells of bronchioloalveolar portals and mat ure Type II cells contain SP-A mRNA or immunostain for SP-A. In postna tal infants, luminal material was also stained for SP-A. Although some alveolar macrophages contained immunoreactive material, SP-A mRNA was never detected. The abundance of SP-A in tracheal and bronchial gland s and epithelium of conducting airways supports the importance of non- surfactant-associated functions for SP-A and may be related to a role in host defense.