Figure 3.

Morphology of cone and rod photoreceptors. The development [a] and structure [b] of a “typical” vertebrate cone (C) and rod (R) photoreceptor. [a] Development of the outer segment. The visual pigment is in membranous discs of the outer segment, which derive from infoldings of the plasma membrane of a microtubule-based, ciliated appendage. In rods, the discs pinch off completely from the plasma membrane, but in cones the discs are open to the extracellular environment. [b] Differentiated rod and cone photoreceptors. Note that the synaptic terminals of cones and rods are usually more distinct than in this illustration, in that cone pedicles are larger with multiple ribbon synapses and rod spherules are smaller with a single ribbon synapse. Abbreviations: F, flagellum (more accurately, cilium); FM, folds of the flagellar (i.e., ciliary) membrane; BB, basal body; PD, presumptive disc; OS, outer segment; D, disc; OD, oil droplet; Dn, dendrite (more accurately, calycal process); EM, ellipsoid mitochondria; IS, inner segment; P, paraboloid; N, nucleus; S, synapses. (c) The lamprey retina offers examples of ‘hybrid’ photoreceptor types. All photoreceptors have cone-like morphology, with open outer segment discs and synaptic terminals with the morphology typical of cone pedicles. However, the shorter type of photoreceptor has rod-like properties (rhodopsin visual pigment and scotopic sensitivity and kinetics). Abbreviations: PC, pigment cells; Dm, desmosome; Ds, disc stratification; PCP, pigment cell process; OBM, outer boundary membrane (outer limiting membrane); MF, Müller fiber; OSL, outer synaptic layer. Reprinted with permission from Vinnikov, Y.A. 1982. Evolution of Receptor Cells. Cytological, Membranous and Molecular Levels. (Translated from Russian by Nicholas Bobrov). Molecular Biology, Biochemistry and Biophysics, vol. 34, Springer-Verlag, Berlin Heidelberg New York, 141 pp.