Morphogenesis and renewal of cone outer segments

Abstract

Abstract This chapter describes details of the morphogenesis and the renewal of outer segments (OS) in vertebrate cones that distinguish them from rods. The findings suggest that in cones new membrane is distributed, via the ciliary stalk, to partial growing lamellae that occur both as evaginations at the base and as invaginations throughout distal levels of the OS. During photoreceptor morphogenesis in the developing retina of the amphibian Xenopus , cone outer segments (COS) continually decrease in taper (basal width divided by length) and their distal lamellae contain partial membrane infoldings, termed distal invaginations (DI). Assuming that the growth of a DI splits one pre-existing lamellae into two or more daughter lamellae, the formation of DI at various levels throughout developing COS can decrease their taper. Accordingly, a model has been proposed whereby new membrane entering developing COS is distributed into basal evaginations (BE) at the base and also flows distally along the ciliary membrane to enter lamellae containing growing DI; in developing rod outer segments (ROS) all new membrane enters BE. During OS renewal in the adult Xenopus retina, the shape of mature COS is remodeled in a somewhat different way. During the night COS become short truncated frustums by shedding their narrow tips, during the day COS expand in size and become conical by re-forming narrow tips. Mature COS contain DI, whose formation could participate in remodeling COS shape. Light microscopic observation showed step-like irregularities in the external outline of many COS during day times. Electron microscopic observation showed that these irregularities were on the non-ciliary side of COS, and that the number of DI per COS varied with time but was always greater than the number of BE. These findings suggest that the growth of DI at various levels can remodel the shape of a COS, re-establishing a smooth external outline and generating a new narrow tip. Thus, the model proposed for how membrane is distributed within developing COS can, with slight modification, also explain how the shape of mature COS is remodeled during renewal. New lamellae are likewise added to the COS by the formation of BE and DI, but in mature COS the formation of DI can decrease lamellar width. Because BE formation does not change the local OS width, the taper of vertebrate photoreceptor OS presumably depends upon the relative amount of membrane that is distributed into BE versus DI. The proposed model for membrane distribution within COS is supported by an autoradiographic study showing a shift in the distribution of labeling over COS in the Xenopus retina after short survival times. The model is also indirectly supported by immunocytochemical and ultrastructural observations of the ciliary axoneme microtubules in photoreceptor OS from this retina. In rods the axoneme is not as long as the OS, but in cones the axoneme extends from the base to the distal end of COS at all diurnal times and axonemal microtubules were discarded when the COS tip was shed. During OS renewal the ciliary axoneme is thus replaced in cones, but apparently not replaced in rods. These observations are consistent with the proposed model because elongation of the axoneme can participate in moving membrane along the ciliary stalk into distal regions of the OS where DI are growing; both of these occur in cones but not in rods. These findings provide convincing evidence that in developing and mature cones some new membrane entering the OS is distributed into BE and moves distally along the ciliary membrane to enter pre-existing lamellae in which DI are expanding, but that in ROS all new membrane enters BE. These fundamentally different features of the OS in rods and cones may help to clarify their different photosensitivities, their evolutionary origins, and certain photoreceptor degredations.