Cell division, cell elongation and distribution of α-, β- and γ-crystallins in the rat lens

The vertebrate lens comprises cells from only one cell lineage and these exist in two distinct forms. Fibre cells make up the bulk of the lens and a monolayer of epithelial cells covers the anterior surface of the fibres. A quantitative analysis of cell division and cell specialization (elongation) in the lenses of 1-day-old and 6-to 7-week-old rats showed that the rat lens can be separated into two compartments; a proliferation compartment in the epithelium of the anterior lens and an elongation compartment in the posterior lens. In 6-to 7-week-old rats the proliferation compartment is divided into three sub-compartments which appear to be related to the anatomy of the anterior eye. Lens epithelial cells lying in the vicinity of the ciliary body have an average cell cycle time of 7·2 days, epithelial cells which lie below the iris have an average cell cycle time of 32·3 days, and cells in the central epi-thelium divide most slowly. The epithelium is bathed for the most part by the aqueous humour and the posterior part of the lens is surrounded by the vitreous humour. From this analysis as well as evidence from other laboratories it is suggested that a proliferation factor(s) may be present in the aqueous and that an elongation factor(s) which comes from the retina may be present in the vitreous. There are three major groups of lens specific proteins, α-, β-and γ-crystallins. These proteins were separated by gel chromatography and antibodies raised against them in rabbits. Alpha-, β-and γ-crystallins were localized in sections of the 1-day-old rat lens by the indirect immunofluorescence method. Epithelial cells in the proliferation compartment contained only α-crystallin but all three groups of crystallins were detected in cells of the elongation compartment. β-Crystallin was detected first in elongating cells that had stopped dividing and γ-crystallin was detected later in the elongation process. Thus, synthesis of α-and γ-crystallins may normally depend on the elongation factor(s) from the retina. Also, since there is a close relationship between cessation of cell division and detection of β-crystallin, another factor governing the synthesis of β-crystallin may be whether the cells have ceased dividing. In this way the expression of crystallin genes may be co-ordinated as the lens grows and cells make the transition from proliferation to elongation compartments.