Abstract
The eye lens is avascular, deriving nutrients from the aqueous and vitreous humours. It is, however, unclear which mechanisms mediate the transfer of solutes between these humours and the lens' fibre cells (FCs). In this review, we integrate the published data with the previously unpublished ultrastructural, dye loading and magnetic resonance imaging results. The picture emerging is that solute transfer between the humours and the fibre mass is determined by four processes: (i) paracellular transport of ions, water and small molecules along the intercellular spaces between epithelial and FCs, driven by Na(+)-leak conductance; (ii) membrane transport of such solutes from the intercellular spaces into the fibre cytoplasm by specific carriers and transporters; (iii) gap-junctional coupling mediating solute flux between superficial and deeper fibres, Na(+)/K(+)-ATPase-driven efflux of waste products in the equator, and electrical coupling of fibres; and (iv) transcellular transfer via caveoli and coated vesicles for the uptake of macromolecules and cholesterol. There is evidence that the Na(+)-driven influx of solutes occurs via paracellular and membrane transport and the Na(+)/K(+)-ATPase-driven efflux of waste products via gap junctions. This micro-circulation is likely restricted to the superficial cortex and nearly absent beyond the zone of organelle loss, forming a solute exchange barrier in the lens.
Highlights
Unlike the majority of tissues, the post-natal lens has no direct blood supply
The picture emerging is that solute transfer between the humours and the fibre mass is determined by four processes: (i) paracellular transport of ions, water and small molecules along the intercellular spaces between epithelial and fibre cells (FCs), driven by Naþ-leak conductance; (ii) membrane transport of such solutes from the intercellular spaces into the fibre cytoplasm by specific carriers and transporters; (iii) gap-junctional coupling mediating solute flux between superficial and deeper fibres, Naþ/Kþ-ATPase-driven efflux of waste products in the equator, and electrical coupling of fibres; and (iv) transcellular transfer via caveoli and coated vesicles for the uptake of macromolecules and cholesterol
SUMMARY AND DISCUSSION The overall picture emerging from the re-assessment of the available physiological and morphological literature, combined with our preliminary magnetic resonance imaging (MRI) and dye-coupling studies, is that the solute transfer from the aqueous and vitreous humours towards the lens fibres is governed by four mechanisms
Summary
Unlike the majority of tissues, the post-natal lens has no direct blood supply. Instead, it obtains its nutrients mainly from the aqueous humour, which bathes the anterior face of the lens. The posterior and anterior sutures are formed by the convergence of the basal and apical plasma membranes of FCs from neighbouring lens sectors, respectively, which means that the suture lines are specific meeting points of membranes which most likely lack GJs and TJs [57] This concurs with a recent suggestion [53,59] that the sutures are in some way involved in solute entry. By reason of location of connexins, GJs do not seem to be a major factor in the uptake of solutes via the apical membranes of the LECs. The general absence of GJs at the EFI further challenges the view that solute transfer from the aqueous to the FCs by transcellular epithelial transport and direct transfer to the underlying fibres by GJ-coupling is an important route of solute transfer into (or out of ) the lens. We propose that this function of GJs is largely limited to either ECs or FCs and for inter-cellular communication within these cell populations, and only very occasionally between these two cell populations in specific lens regions such as the equatorial region
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