The ultrastructure of monkey foveal photoreceptors, with special reference to the structure, shape, size, and spacing of the foveal cones

Abstract

A systematic electron microscopic study was made of the structure of foveal cones of Macaca spp. Transverse sections of inner (IS) and outer segments (OS) were made in sequence, from the pigment epithelial zone (PEZ) to the outer limiting membrane (OLM). The smallest diameters of hundreds of cone sections were measured from electron micrographs with a Zeiss particle-size analyzer, and analyzed statistically. Some details are also included about Cebus photoreceptors. It is claimed in the literature that foveal cones are rod-like (cylindrical) and untapered. Our study shows the foveolar cone to be a tapered structure. There has been some confusion between the foveola, which is rod-free, and the fovea, which has a high concentration of cones, but is not rod-free. Within the fovea, as the ratio of cones to rods falls from infinity to 1, with distance from the central bouquet of cones, the cone center-to-center distances increase, the inner segment diameters increase, and the number of cones/sq mm decreases. The tapered calycal processes are more massive in M. irus than M. mulatta, and the lateral fins are better developed. Lateral fins are not present in the foveola. The cones are arranged in straight lines.

A note about retinal structure and visual acuity. A light microscopic study of the cones in fovea centralis

“…The size of one cone cell equals 3.8 mm or a subtending angle of 0.8 arc minute Taking the focal length of the human eye to 17 mm, the subtending angle yø to a cone-to-cone-center distance of 3.8 mm equals (Fig. 4a 1989;Borwein et al 1980). In the present study a slightly larger dimension of 3.8 mm was found.…”

“…A linear shrinkage of about 30% during conventional procedures for electron microscopy is not uncommon (Luft 1973). In previous studies (Krebs & Krebs 1989;Borwein et al 1980), immersion-fixation, hypertonic fixative and stepwise dehydration/infiltration pro-cedures were used, versus our in vivo perfusion-fixation technique with an isotonic fixative followed by a dehydration/filtration preparative procedure in small increments, known to minimize cell shrinkage in other tissues (Rostgaard & Tranum-Jensen 1980;King 1991).…”

Human Cone Visual Pigment Deletions Spare Sufficient Photoreceptors to Warrant Gene Therapy

“…2C). At the foveola, which normally is dominated by L/M cones and lacks rods (Borwein et al, 1980;Youdelis and Hendrickson, 1986), the ONL was abnormally thin in all patients but one (Fig. 2D, upper panel).…”

“…The foveola refers to a region at the center of the fovea packed with L/M cones and lacking rod photoreceptors (Borwein et al, 1980;Youdelis and Hendrickson, 1986). Foveola is thus a key retinal location to answer questions about L/M cones in BCM.…”

The Shape of Color: Retinal Cones and Spectral Dispersion

“…However, the central cones are, in fact, conical. Direct measurements (Borwein, et al, 1980) show that the foveal COSs typically have a proximal diameter (near their junction with the inner segment) of From left-to-right the six cone types depicted are from the periphery in to the fovea. The locations are: I -edge of the ora serrata, II -3 mm in from the ora serrata, III -midway between the ora serrata and the visual center, IV -periphery of the macula, V -within the macula, and VI -at the foveal center.…”