The effect of elevated intraocular oxygen on organelle degradation in the embryonic chicken lens

Abstract

SUMMARY In the vertebrate lens, nuclei and other cytoplasmic organelles are degraded in fiber cells situated in the center of the tissue. This is believed to ensure the transparency of the tissue. The mechanism that triggers this process is unknown. We hypothesized that standing gradients of oxygen generated within the tissue may serve as a spatial cue for organelle degradation. To examine this possibility, we incubated fertilized chicken eggs under hyperoxic (50% O2) or normoxic (21% O2)conditions. Hyperoxic treatment was initiated on the seventh day of embryonic development (E7), five days before organelle degradation normally commences in the lens core. Hyperoxia was maintained until E17. Under normoxic conditions,the partial pressure of oxygen (PO) within the vitreous compartment was low. Direct measurement of PO using an optode oxygen sensor indicated values of 1.3 kPa and 0.4 kPa for the mid- and anterior vitreous, respectively. Similarly, treatment with pimonidazole, a bio-reductive hypoxia marker, led to the formation of immuno-positive protein adducts within the lens, suggesting that the embryonic lens is chronically hypoxic in situ. Following hyperoxic treatment, vitreous PO significantly increased, although pimonidazole staining in the lens was not markedly affected. Confocal microscopy of slices prepared from hyperoxic lenses revealed a significant increase in the size of the lens relative to age-matched normoxic controls. By E13, an organelle-free zone(OFZ) was present in the center of normoxic and hyperoxic lenses. However, in hyperoxic lenses, the OFZ was consistently smaller, and the distance from the lens surface to the border of the OFZ significantly larger, than in normoxic controls. These observations suggest that hyperoxia delays organelle breakdown and are consistent with a model in which hypoxia in the deep cortical layers of the normal lens serves as a trigger for the organelle loss process.

Lens fibre cell differentiation and organelle loss: many paths lead to clarity

“…These studies demonstrated that TNF-related molecules and their signalling components are expressed in the developing lens and therefore support the suggestion that TNFrelated signalling is potentially involved in regulating organelle loss in lens fibre cells and/or lens fibre cell differentiation. The results therefore suggest that organelle loss could be a process activated by TNFamediated cell signalling cascades, although the involvement of a passive process (such as a reduction in oxygen tension at a specific point within the lens fibre cell mass) cannot be ruled out [62].…”

Differentiation State-Specific Mitochondrial Dynamic Regulatory Networks Are Revealed by Global Transcriptional Analysis of the Developing Chicken Lens

“…These regions are diagrammed in Figure 1. The elimination of organelles from the FC region of the chicken lens begins at E12 and a transparent organelle-free zone (OFZ) is apparent by E15 (Bassnett and McNulty 2003; Costello et al 2013; Basu et al 2014b). The E13 stage of embryonic chicken lens development (E13) was chosen because this represents an early stage of OFZ formation when molecules required for removal of organelles would be expected to be up-regulated in the fiber cell zones.…”

Role of the Executioner Caspases during Lens Development

“…Conversely, an increase in D signifies that organelle degradation is delayed (32). Organelle degradation commences in the mouse lens during the late stages of embryonic development, and by P2 (the age examined here) the OFZ is a well defined region in the center of the tissue.…”