figure 3.14 illustrates the stages of lens regeneration in the newt eye. Two days after lentectomy, there is an increase in the synthesis of ribosomal RNA by the amplification of rRNA genes and their increased rate of transcription (Reese et al., 1969; Collins, 1972). Dedifferentiation and proliferation begin by four days (Yamada and Roesel, 1969, 1971; Reyer, 1971). Expression of the c-myc proto-oncogene is enhanced in dedifferentiating PECs and proliferative activity is increased (Agata et al., 1993). The cells take on an irregular shape and eliminate melanosomes, which are phagocy-tized by macrophages invading the iris epithelium (Reyer, 1982, 1990a, b). The numbers of mitochondria, free ribosomes, and microfilaments increase (Reyer, 1977). Proteoglycans are lost from the surface of the dedifferentiating cells (Zalik and Scott, 1972, 1973).
Eguchi (1988) identified a specific cell surface glycoprotein, 2NI-36, that disappears from the surface of dorsal but not ventral, iris cells. Ventral iris treated with antibody to this antigen and implanted into a lentecto-mized eye can regenerate a lens. These observations suggest that the disappearance or inhibition of 2NI-36 is necessary and sufficient to trigger dedifferentiation of pigmented epithelial cells. 2NI-36 is also found in many other tissues of the newt and has been postulated to exert a general stabilizing effect on differentiation (Imokawa et al., 1992; Imokawa and Eguchi, 1992).
With continued proliferation, the dedifferentiated dorsal iris cells differentiate into a- and P-crystallin-expressing lens epithelial cells that form a small lens vesicle at the edge of the dorsal iris (Yamada and McDevitt, 1974). The lens epithelial cells facing the retina subsequently withdraw from the cell cycle, express y-crystallins and differentiate into lens fibers (Takata et al., 1966; Eguchi, 1967). Newts (Cynops pyr-rhogaster) transgenic for the EGFP gene under the regulation of the pB1-crystallin promoter express EGFP as the lens epithelial cells begin to differentiate (Ueda et al., 2005). The new lens enlarges to normal size by the continuing division of lens epithelial cells and their differentiation into lens fibers.
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