Ultrastructural analysis of the human lens fiber cell remodeling zone and the initiation of cellular compaction

Abstract

The purpose is to determine the nature of the cellular rearrangements occurring through the remodeling zone (RZ) in human donor lenses, identified previously by confocal microscopy to be about 100 µm from the capsule. Human donor lenses were fixed with 10% formalin followed by 4% paraformaldehyde prior to processing for transmission electron microscopy. Of 27 fixed lenses, ages 22, 55 and 92 years were examined in detail. Overview electron micrographs confirmed the loss of cellular organization present in the outer cortex (80 µm thick) as the cells transitioned into the RZ. The transition occurred within a few cell layers and fiber cells in the RZ completely lost their classical hexagonal cross-sectional appearance. Cell interfaces became unusually interdigitated and irregular even though the radial cell columns were retained. Gap junctions appeared to be unaffected. After the RZ (40 µm thick), the cells were still irregular but more recognizable as fiber cells with typical interdigitations and the appearance of undulating membranes. Cell thickness was irregular after the RZ with some cells compacted, while others were not, up to the zone of full compaction in the adult nucleus. Similar dramatic cellular changes were observed within the RZ for each lens regardless of age. Because the cytoskeleton controls cell shape, dramatic cellular rearrangements that occur in the RZ most likely are due to alterations in the associations of crystallins to the lens-specific cytoskeletal beaded intermediate filaments. It is also likely that cytoskeletal attachments to membranes are altered to allow undulating membranes to develop.

Role of Aquaporin 0 in lens biomechanics

“…The cortex changes shape to accommodate while the lens nucleus does not. Normally, fiber cells undergo compaction in the IC and nucleus [34,35] to adjust the refractive index gradient to avoid spherical aberration while focusing [14,16]. Compaction, is probably achieved through dehydration which causes the fiber cells to become stiffer in a gradient-dependent manner, possibly due to gradual loss of water from the cytosol and extracellular spaces of IC fibers, and due to the absence of cytoskeletal proteins [14,16,33–36].…”

“…Compaction, is probably achieved through dehydration which causes the fiber cells to become stiffer in a gradient-dependent manner, possibly due to gradual loss of water from the cytosol and extracellular spaces of IC fibers, and due to the absence of cytoskeletal proteins [14,16,33–36]. In addition, CTCA is favored by the formation of large patches of AQP0 thin junctions [14,16,35–37]. …”

“…In a biomechanical sense, stiffness at the IC and nucleus makes the inner core less flexible which probably serves as a scaffold for allowing increased flexibility for the OC during accommodation. Flexibility at the OC could also be due to high water content in the fiber cells, presence of cytoskeletal proteins in the cytosol, fewer and smaller patches of AQP0 thin junctions (~11–13 nm) and greater numbers of thick junctions formed by N-cadherin (~20–40 nm) [14,35,36]. Therefore, AQP0-aided differential biomechanics at the cortex and nucleus of the lens could be playing an important role in accommodation.…”

Optical properties of the lens: an explanation for the zones of discontinuity

“…7A but is less clear than C1a. This zone (C1b) is thought to represent a loss of transparency of the fibre cells that were previously in C1a (Michael and Bron, 2011) and has been shown subsequently, from microscopic analysis, to contain a high number of cell interface inter-digitations and irregularities that could contribute to scatter (Costello et al, 2013). This region, termed the remodelling zone (RZ), contains cells that are undergoing structural rearrangement and has been postulated to be a region in which fibre cell differentiation changes abruptly (Lim et al, 2009).…”

Autophagy in the lens

“…During fiber cell differentiation, the plasma membrane is extensively remodeled to form interlocking membrane domains between neighboring cells such as ball-and-sockets and protrusions (Bassnett, Shi et al 2011). These structures can appear as autophagosome-like double-membrane structures if they are cut perpendicular to the protrusion axis (Costello, Mohamed et al 2013). Thus, it is recommended that detection M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT of autophagy in the lens be performed in combination with other methods such as immunohistochemistry of autophagosome markers (e.g., LC3) using antibodies, the specificity of which should be confirmed in the lens using autophagy-deficient animals (Morishita, Eguchi et al 2013).…”