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The fine structural features of a retinal: melanophage, observed in degenerating retinae of Notophthalmus viridescens is described. The melanophage, believed to originate from the vascular supply, serves for the stepwise degradation of extruded single melanin granules from cellular elements of the pigment epithelium which are forming the regenerated retina.

The process of retinal regeneration in adult Triturus cristatus carnifex was analyzed autoradiographically following tritiated thymidine administration at closely spaced postoperative intervals. In unoperated control animals given five daily doses of labeled thymidine, a continuous input of two types of cells was observed from the ora serrata into the ganglion cell and inner nuclear layers. One cell type appeared to represent retinal neurons with dispersed nuclear chromatin while the second cell type, containing a nucleus with dense heterochromatic rim, was displaced into all retinal layers with time, ultimately appearing within the layer of photoreceptor outer segments approximating the pigment epithelium. In operated animals, both control and operated (enucleated) eyes showed a loss of proliferative capacity from the ora serrata by two days postoperative. Retinal degeneration and regeneration in this species differed from that observed earlier for Notophthalmus viridescens in that: (1) invading pigment epithelia dissected the degenerating retina sequestering the outer nuclear layer between two epithelial sheets for phagocytosis by day 8; (2) degenerating inner retinal cells demonstrated labeling with thymidine and mitotic division before degeneration; (3) the process of removal of retinal and lens debris was assisted by vascularly derived phagocytes. The restoration of the retina involves both cellular addition from the anterior pars ciliaris retina and ora serrata, forming the anterior half of the new retina, and the posterior pigment epithelium, forming the posterior half of the retina from the innermost lamina. Sites of regional cellular proliferation at the vortex veins do not develop in this species. The basal pigment epithelium, upon Bruch's membrane, does not contribute to the retinal regenerate. The species differences presented in the process of retinal regeneration and the implications for the understanding of the generation of retinotectal specificity are discussed. It is concluded that the specificity engendered during both development and regeneration in this system must: (1) be derived from the ganglion cells; (2) be responsible for not only tectal connections but intra-retinal position and connectivity; and, (3) not be temporally related.

The process of retinal degeneration and regeneration in retinectomized or enucleated and reimplanted eyes of adult Notophthalmus viridescens has been studied following staged administrations of tritiated thymidine or colchicine. In control left eyes, no labeled cells were observed in either the retina, pigment epithelium, or ora serrata. In retinectomized or enucleated eyes, retinal degeneration proceeded from the posterior central pole circumferentially to the ora serrata, with complete retinal degeneration being consistently observed. Retinal replacement originated from two cellular sources: the posterior pigment epithelium and the anterior complex, consisting of cells in the ora serrata and pars ciliaris retina. The posterior pigment epithelium gave rise to an initial central retinal portion which differentiated in an annular pattern which extended from the optic nerve to the vortex veins. Cellular addition from the pigment epithelium became restricted to these two regions by day 25. The anterior retinal mass was regenerated from the anterior elements (ora serrata/pars ciliaris retina) with a minor contribution from the underlying anterior pigment epithelium. Data concerning the relative numbers of cellular types involved in the regenerative sequence and the times of their function are presented. Peak labeling occurred in the posterior pigment epithelium at day 20, the pars ciliaris retina at days 18 and 40, and the new retina at days 28 and 41. By 30 days regional sites of cellular proliferation became established that coincided with the pattern of ocular blood vessels. Retinal differentiation followed the pattern commonly observed during retinal development. The significance of these observations with respect to earlier work demonstrating a different sequence for retinal restoration is discussed.

Since the foveal photoreceptors in the duplex retina of adult human and other diurnal primates are closely associated with high visual acuity and spectral sensitivity, observations were made on the pre- and postnatal morphologic development of the macula and fovea in the retina of the diurnal rhesus monkey. Histologic findings on the level of development of the retina at birth revealed a macula with the characteristic foveal depression. Observations on the ultrastructural development of the foveal photoreceptors revealed that their outer and inner segments were relatively well differentiated even at birth. An examination of the prenatal morphogenesis of the photoreceptors in the eye of the fetus indicated that the initial differentiation of the receptors may take place as early as 100 to 125 days after conception. However, the receptors in the specialized fovea continued to develop for 1 to 2 months after birth, whereas the photoreceptors outside the central region of the retina had completed their fine structural development at birth. By the end of the second month after birth, the macular and foveal areas essential for high visual acuity and spectral sensitivity appeared structurally well completed. In general, major maturational changes observable after birth were apparent predominantly in the pigment epithelium of the retina.

The normal adult retina of the North American Urodele, Triturus viridescens, has been studied with the electron microscope. Three basic types of photoreceptors are observed: rods, possibly distinguishable as either red or green rods, single cones, and double cones. The photoreceptors differ from those reported in other amphibia in that: (1) none of photoreceptors contain an oil droplet; (2) all photoreceptors possess an enlarged paraboloid, and, (3) the double cones consist of two complete photoreceptors, a principal cone member and an accessory cone member. The two members are closely related to one another along the length of the inner segment through a clasping of the principal member within cytoplasmic expansions of the accessory member. The principal member inner segment bears strong resemblance to the single cone while the accessory member inner segment is rod-like in nature. The sustentacular elements of this avascular retina display a diversity of scleral processes. The presence of specific collar-like glial ensheathments of the photoreceptors at the external limiting membrane contrasts with the extensive interreceptor contacts between all photoreceptor elements throughout the outer nuclear layer. The synaptic bases are characterized by the presence of three types of specific contact sites: (1) interreceptor contacts among all receptor bases; (2) clusters of fine horizontal cell dendrites penetrate the receptor base and terminate in a superficial spray of typical axodendritic synapses; (3) prominent bipolar dendrites enter within the horizontal cell processes terminating in the typical retinal ribbon synapse. The significance of these observations are discussed with respect to the origin of the amphibian double cone, the metabolic support of the retinal, and the potential integration among the receptor cells.