Synthesis of actin in the nonciliated bronchiolar epithelial (Clara) cells of perinatal rats demonstrated by electron microscopy and in situ hybridization.

Abstract

Previously, we found abundant filaments consisting of actin and cytokeratin in nonciliated bronchiolar epithelial (Clara) cells by treating the cells with detergent-containing medium. In this study, the presence of the filaments and the expression of actin mRNA in Clara cells of perinatal rats were investigated by electron microscopy and in situ hybridization, respectively. Clara cells in prenatal rat were characterized by the presence of glycogen and the absence of electron-dense granules. Detergent treatment did not clarify the presence of filaments in prenatal cells. After birth, there was a rapid loss of glycogen and electron-dense secretory granules appeared in the cytoplasm of Clara cells. Filaments could be recognized by detergent treatment. By the 3rd postnatal day, some Clara cells showed features similar to the cells in adult rats. To investigate whether these filaments demonstrated by detergent treatment were synthesized during the postnatal period, the localization of actin mRNA was observed using an in situ hybridization technique. Signals showing the presence of actin mRNA were observed in many cells at every stage of development. In the prenatal period, the signals were distributed diffusely in the epithelial cells of the bronchioles, but in the postnatal period, they appeared in Clara cells selectively. These results show that the actin filaments observed in the apical caps of Clara cells are synthesized during the postnatal period when Clara cells are thought to become mature.