Submicroscopic Organization of Retinal Cones of the Rabbit

Abstract

The fine structure of the cone cell of the rabbit is described and compared wtih that of the rod. The cone outer segment consists of a pile of flattened sacs with two membranes 30 A thick and a regular clear space in between of about 30 A. The membrane of the rod sacs is slightly thicker (~-~40 A) and the clear space is less regular and frequently absent in the deeper regions. The distance between sacs is from 85 to 95 A in the cone and from 110 to 120 A in the rod, and the total repeating period is about 190 A and 210 A, respectively. These results are discussed in relation to the concentration of solids in both photoreceptors.A connecting cilium was observed in the cone cell and compared with that previously described in rods (4). This finding suggests that morphogenetically the cone may also result of the differentiation of a primitive cilium (5).The inner segment of the cone shows a distal portion with large concentration of elongated mitochondria and a proximal one with a large Golgi complex in the axis surrounded by components of the endoplasmic retieulum. It is concluded that both photoreceptors have a similar general plan of submicroscopic organization, with some minor difference in fine structure probably related to their specific chemical composition and function.Recent studies on the submicroscopic morphology of photoreceptors (2, 4-6, 8, 11-14, 19, 20) refer mainly to the rod cells, while very little has been published on the structure of the cones (12,14). This may be due to the fact that in mammalian retinas there are fewer cones in relation to rods and the cone outer segment is extremely fragile and difficult to observe in good conditions of preservation. Sj6strand (12) described the outer segment of the cone in the perch as made up of many superimposed discs, which he interpreted as formed by single membranes. This was at variance with his finding of "double membrane discs" in the rod outer segment. However, in a more recent publication (14) he describes the cone outer segment of the toad as having a structure similar to that of the rod, although of different dimensions. The synaptic region of the cone with the complex relationship of the enlarged ending and the bipolar *

The ciliary pocket: a once-forgotten membrane domain at the base of cilia

“…Thereafter, the CC emerges at the apical surface in a configuration very similar to the one found for PC in kidney cells (Figure 3, B1), with the exception of its distal region that enlarges and differentiates to form the outer segment (De Robertis and Lasansky, 1958;Galbavy and Olson, 1979). At later stages, when the outer segment is mature (Figure 3, B4), a ciliary pocket-like domain is present at the base of the CC (De Robertis and Lasansky, 1958;Tokuyasu and Yamada, 1959;Watanabe et al, 1999). This domain was first called the periciliary ridge complex in the frog (Peters et al, 1983) and, due to structural differences (Watanabe et al, 1999), the periciliary complex (Maerker et al, 2008) or periciliary membrane complex (Yang et al, 2010) in mammals.…”

“…Ciliogenesis in photoreceptor cells remains poorly characterized but evidence indicates that the CC is assembled in the cytoplasm similarly to PC in fibroblasts with images indicating the presence of a pocket at the base of the CC once it has reached the ap-ical membrane that will become the inner segment . Thereafter, the CC emerges at the apical surface in a configuration very similar to the one found for PC in kidney cells (Figure 3, B1), with the exception of its distal region that enlarges and differentiates to form the outer segment (De Robertis and Lasansky, 1958;Galbavy and Olson, 1979). At later stages, when the outer segment is mature (Figure 3, B4), a ciliary pocket-like domain is present at the base of the CC (De Robertis and Lasansky, 1958;Tokuyasu and Yamada, 1959;Watanabe et al, 1999).…”

“…A possibly related membrane domain has been reported at the CC (connecting cilium) of photoreceptor cells. At the apical face of the inner segment, the CC corresponds to the proximal part of a modified PC (De Robertis and Lasansky, 1958), which adopts all the characteristics of an elongated ciliary necklace/transition zone (Rohlich, 1975). The distal part of the modified PC is organized as membrane stacks enriched in rhodopsin and is involved in light sensing (review by Insinna and Besharse, 2008).…”

“…The distal part of the modified PC is organized as membrane stacks enriched in rhodopsin and is involved in light sensing (review by Insinna and Besharse, 2008). In contrast with the pocket found at PC, this microdomain looks asymmetric, since the membrane of the inner segment preferentially interacts with one ‘side’ of the CC (De Robertis and Lasansky, 1958; Tokuyasu and Yamada, 1959; Watanabe et al, 1999). Moreover, a ciliary pocket is also present at the base of motile cilia of ependymal cells (Brightman and Palay, 1963; Molla‐Herman et al, 2010) and at the base of the flagellum in elongating spermatids (Fawcett and Ito, 1958; Molla‐Herman et al, 2010), highlighting its presence at the base of all main cilia subtypes.…”

Photoreceptor Organelles in Animals

“…These observations would seem to explain the fibre observed by light microscopists (p. 47). The cilium has also been found in cones (Sjostrand & Elfvin, 1956;De Robertis & Lasansky, 1958).…”