Developmental expression of crystallin genes: In situ hybridization reveals a differential localization of specific mRNAs

The time and place of the accumulation of αA-, βB1- and γ-crystallin RNA in the developing rat lens have been studied by in situ hybridization. αA- and γ-crystallin RNA were first detected in the lens vesicle, while βB1-crystallin RNA could be seen only after elongation of the primary fiber cells. Both βB1- and γ-crystallin RNA were confined to the fiber cells of fetal lenses, while αA-crystallin mRNA could also be detected in the epithelial cells. A quantification of the hybridization pattern obtained in the differentiation zone of the newborn rat lens showed that αA-crystallin RNA is concentrated in the cortical zone. αB-crystallin mRNA has the same distribution pattern. βB1-crystallin RNA was relatively poorly detectable by in situ hybridization in both fetal and newborn rat lenses. The grain densities obtained with this probe increased from the periphery of the lens toward the interior, indicating that βB1-crystallin RNA accumulated during differentiation of the secondary fiber cells. A similar accumulation pattern was obtained for γ-crystallin mRNA, but, unexpectedly, this RNA could also be detected in the elongating epithelial cells. Our results show that γ-crystallin RNA starts to accumulate as soon as visible elongation of epithelial cells occurs, during differentiation of the primary as well as the secondary fiber cells.