Purchase this article with an account.
Joseph M. Corless; How Do Cone Outer Segments (COSs) Maintain Their Conical Shapes?. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5428.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To develop a model for explaining how the average right conical geometry (basal diameter=φ, conical angle=α) of COSs in amphibian/lower vertebrates persists with repeated cycles of addition and shedding of lamellae/discs. In Xenopus laevis maintained on a 12h light:12 h dark cycle, COSs contain n=170 lamellae on average at light onset; 12h later, n=300 (Eckmiller, 1990; 1997). Thus, 130 lamellae are added over a 12h period, or 1 lamella per 5.54 min, with φ~ 4.2 µm and α~9.5°.
Using these data, an integrated model of COS membrane dynamics has been developed. Assumptions. 1) New lamellae form exclusively via a basal evagination (BE) mechanism. 2) Formation of a BE is coupled to the apical displacement of all previously formed lamellae by 1 lamella repeat period. 3) Apical displacement of lamellae is coupled to a process that systematically extracts lamellar membrane area via the saddle points (SPs), with intermediate transfer to the axonemal plasmalemma (PL). 4) New lamellar components derive from two sources: extracted and recycled components from older lamellae via the PL, and additional components from the inner segment (CIS) via the cilium. 5) The driving force for membrane recycling is linked to the mechanism that elaborates a new lamella. Extension of the BE from the cilium produces a local stretching of the membrane bilayer, increasing the cross-sectional area per lipid molecule and exposing hydrophobic domains to aqueous regions. This entropic driving force is propagated throughout the continuous COS bilayer system, and is relaxed by the influx of lipids (and other components) from this membrane phase. 6) The removal of membranous components from older lamellae is limited by the rate at which the open margin lattice (OML) is disassembled near SPs. This rate appears to be constant along the length of the COS.
1) BE composition: at light onset, ~72% of the BE is derived from recycled lamellar components; after 12h, the percentage is ~97.5%. 2) Membrane recycling establishes a basally directed, non-linear advective PL flow. Velocity is minimal near the COS tip, and maximal (Vmax) near the BE. When n=170, Vmax = 28 nm/s; when n=300, Vmax=38 nm/s. These rates greatly exceed the axial displacement rate of lamellae: ~0.1 nm/s.
Assessing membrane flow rates along the PL may provide a method for characterizing membrane dynamics within the COS.
This PDF is available to Subscribers Only