Regulation of lens cell growth and polarity by an embryo-specific growth factor and by inhibitors of lens cell proliferation and differentiation

ABSTRACT We used a double-label method, which monitors the rate at which cells enter S-phase of the cell cycle, to identify factors that control the growth of chicken embryo lens epithelial cells in vivo. With this assay, we identified a mitogen for lens epithelial cells in the anterior segment of the embryonic eye. When the anterior chamber was opened briefly, by tearing the cornea or displacing the lens, the growth-promoting activity was lost. None of the purified growth factors tested replaced this growth activity, including EGF, bFGF, PDGF, IGF-1, IGF-2, TGF and mixtures of these factors. However, chicken embryo serum or plasma did cause chicken embryo lens epithelial cells to progress through the cell cycle. The activity in serum was destroyed by heat and protease treatment. It was most active in serum from 10-day embryos, decreased with subsequent development and was undetectable from 2 days after hatching through adulthood. When embryo serum or plasma was mixed with vitreous humor or IGF-1, agents that induce lens fiber cell formation, cell elongation was prevented. In contrast to the mitogenic activity in serum, this inhibitor of differentiation was insensitive to trypsin treatment. We also identified an activity in vitreous humor that inhibited the growth-promoting agent in embryo serum. Plasma proteins readily enter the anterior chamber of the eye of chicken embryos. Therefore, our data imply that an activity in serum enters the anterior chamber of the embryonic eye and controls lens cell division. Furthermore, reciprocal gradients of factors from the vitreous body and the anterior chamber appear to specify lens polarity and assure normal lens morphogenesis. The activities in serum assure that lens epithelial cells divide, but do not differentiate into lens fibers. In a similar manner, factors in vitreous humor specify that cells in the posterior of the lens differentiate into fibers, but do not divide.