More than 99% of the vertebrate ocular lens is comprised of terminally differentiated lens fiber cells.

As occurs in mammalian red blood cells, lens fiber cell differentiation includes the loss of all membranous cytoplasmic organelles.

Unlike red blood cells, however, the lens fiber cell is not destroyed after a limited life span but survives for the life of the organism. The extreme changes in cellular architecture that occur during differentiation require extensive remodeling of the cell cytoskeleton.

Although actin-based thin filaments, microtubules, and vimentin-based intermediate filaments (IFs) are found in the newly differentiated fiber cells, they disappear shortly after differentiation.

The beaded filament is a cytoskeletal structure which is abundant in the fiber cells of the ocular lens. BFSP2 and BFSP1 (MIM.603307) are major components of the beaded filament.

Two lens-specific IF-like proteins, phakinin (BFSP2)(MIM.603212) and filensin (BFSP1), are expressed only after differentiation has begun. Both proteins are found in a structurally unique cytoskeletal element that is referred to as the beaded filament (BF).

References

 Francis PJ, Berry V, Moore AT, Bhattacharya S. Lens biology: development and human cataractogenesis. Trends Genet. 1999 May;15(5):191-6. PMID: #10322486#