PURPOSE. To describe a previously uncharacterized structural specialization in the mouse lens fiber cell and to delineate its emergence relative to lens development and fiber cell differentiation. METHODS. Lens fixation efficiency was explored using 14C-formaldehyde and autoradiography. Lens fiber cell architecture was examined by scanning electron microscopy and by DiI labeling of methacrylate sections in lenses ranging from 2 weeks to 8 months. RESULTS. Scanning electron microscopy identified an elaborate structural specialization that emerges late in fiber cell differentiation, largely after the cell has lost its nucleus. These elaborations project from the short side of the cell, are regularly spaced throughout the central region of the cell and are aligned with similar structures in adjacent cells. The structures are not found in fiber cells of lenses younger than two weeks of age, nor in the fiber cells that initially differentiate before that time. CONCLUSIONS. Fiber cells that arise later than 2 weeks of age undergo a structural differentiation program that is different from that of cells that arise earlier in development. This program includes the assembly of a series of regularly spaced, complex, lateral projections from the fiber cell that align themselves with similar structures in adjacent cells. Most if not all of the structural specialization occurs in cells that have lost their nuclei and organelles, suggesting that this component of fiber cell differentiation may not require ongoing transcription/translation.
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