Cone Outer Segment Morphogenesis: Taper Change and Distal Invaginations

Because cone outer segments (COS) are now known to be continually renewed, I reexamined COS morphogenesis in retinas of Xenopus tadpoles (prepared by standard histologic techniques and viewed by light and electron microscopy) to clarify how COS incorporate new membrane. I observed that developing COS underwent an unexpected shape change: they were always conical, but their taper (width divided by length) continually decreased. Ultrastructural examination revealed that many of the membrane foldings within distal COS were partial or incomplete, not extending across the full COS width but ending at variable distances from the ciliary side. Because these partial folds represented infoldings of the plasma membrane of an existing lamella, and they occurred at all COS levels except the base, I have termed them distal invaginations (DI). The completion of each DI increased COS length by one lamella but caused no noticeable change in local COS width; thus the formation of many DI throughout the distal COS presumably resulted in the observed decrease in overall COS taper. Based on these findings, I suggest that DI indicate growing membrane fronts and may represent sites where newly synthesized membrane is incorporated into COS. Because DI occur in developing and adult COS of various vertebrate species, I propose that DI formation plays an important role in the generation of COS taper during development and the remodeling of COS taper in mature cones after tip shedding. THOtJGI-I it is now accepted that the light-sensitive outer segments (OS) ~ of both vertebrate rods and cones are regularly renewed (Anderson et al., 1978; Young, 1978; Bok, 1985; Roof, 1986), numerous aspects of cone outer segment (COS) renewal remain problematical. I have reexamined the morphogenesis of COS to clarify how new membrane is incorporated into COS. Rod outer segment (ROS) membranes are renewed in a very orderly manner, as first revealed by autoradiography (Young, 1967): radioactive protein molecules become trapped in new membranous disks generated at the OS base, producing an autoradiographic band of label; the unchanged band is displaced sclerally as additional disks form below, and finally discarded from the OS tip and phagocytized by the pigment epithelium (Young and Bok, 1969). This pattern of membrane renewal is consistent with the ultrastructural morphology and cylindrical shape of mature ROS (Young, 1976; Steinberg et al., 1980; Bok, 1985). It is thus clear how new membrane is incorporated into ROS: each membrane entering via the connecting cilium becomes distributed into successive new membrane folds that evaginate from the cilium at the OS base, these evaginations expand to the full OS width and are displaced away from the base, then lose their 1. Abbreviations used in thispaper: BE, basal evagination; COS, cone outer segment; DI, distal invagination; LM, light microscopy; OS, outer segment; ROS, rod outer segment. connections and become isolated into separate disks surrounded by the plasma membrane. COS differ from ROS in structural organization, autoradiographic labeling pattern, and three-dimensional shape. COS likewise consist of numerous parallel membrane foldings oriented at right angles to the connecting cilium, but in COS all the membrane foldings apparently retain continuity with each other and with the plasma membrane, forming a single topologically continuous membrane system. To emphasize the topologic distinction between ROS and COS membranes, I will refer to the cytoplasm-enclosing membrane foldings in COS as lamellae, rather than disks. Thus, although different COS vary in external form, their membranes are believed to have the same structural organization (Young, 1970; Fein and Szuts, 1982; Papermaster and Schneider, 1982; Bok, 1985), illustrated in Fig. 1. Autoradiographic experiments produce a diffuse labeling over COS, which was originally taken as evidence that they undergo no analogous membrane renewal (Young, 1969): when COS attained their final size, the addition of new membrane was believed to stop. The tapered conical shape of COS was taken as circumstantial evidence that their membranes are not renewed (Young, 1969), and attributed (Young, 1971) to the way they form during development (Nilsson, 1964): the earliest membrane foldings are small and successive folds formed below become progressively larger. 9 The Rockefeller University Press, 0021-9525/87/11/2267/11 $2.00 The Journal of Cell Biology, Volume 105, October 1987 2267-2277 2267 on Jne 6, 2017 D ow nladed fom Published November 1, 1987