Developmental patterns of crystallin expression during lens fiber differentiation in amphibians.

Data on activation of crystallin synthesis during lens fiber (LF) formation in amphibians are summarized to point out the questions particularly interesting in the context of lens cell lineage-specific expression programming under different developmental conditions. LFs are known to differentiate throughout life along the same pathway that includes at least five compartments. Using the amphibian eye lens as a model, we have studied how crystallins are expressed in the course of: (1) embryonic LF formation, (2) LF differentiation in adults, and (3) LF transdifferentiation from other (non-lens) eye tissues. Our experiments showed that synthesis of crystallins during morphologically similar LF differentiation in embryonic and adult amphibian lens has different spatial-temporal patterns (i.e., is apparently activated according to different programs). Certain results obtained in our studies suggest the absence of any direct relationship between the capacity of adult newt iris cells to transdifferentiate into LFs and crystallin synthesis (<> to such transdifferentiation) in them. Crystallins appear at the advanced stages of iris transdifferentiation into the lens and dynamics of their synthesis in the <> lens resembles that in the embryonic lens, although a new lens rudiment develops from the adult iris epithelium. Data on alternative patterns of the crystallin gene activation are summarized and compared with recent observations on spatial-temporal expression of Pax genes, which play an essential role in lens cell commitment and crystallin synthesis. On this basis, it is suggested that ontogenetic and tissue- or cell-specific changes in Pax gene expression may result in altered programs for activation of crystallin genes in embryonic, adult, and regenerating lens.