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Morphogenesis of the retina of the Sprague-Dawley albino rat was studied by light microscopy from day 11 of gestation until 225 days after birth. A quantitative analysis during development of retinal volume, thickness of the entire neural retina and thickness of each of the retinal layers, both posteriorly and peripherally, was made. The results indicate that initially a single neuroblastic layer forms and continually thickens by mitosis at its outer border. The retinal layers then form in sequence, moving from the inner retinal border outward and always beginning posteriorly and then spreading peripherally. The transient layer of Chievitz does not appear. All adult layers are present by eight days after birth and each layer thins after reaching its maximal thickness. Total thickness of the retina excluding pigmented epithelium, is greatest on postnatal day 5, but retinal volume only reaches a peak on postnatal days 7 to 12. The nerve fiber, inner plexiform, outer plexiform and bacillary layers all continue to increase in thickness after the ganglion cell and inner and outer nuclear layers reach their maximal width and are beginning to become thinner.The principal features of development of the mammalian retina have been described for man (Barber, '55; Duke-Elder, '58; Mann, '64) and a number of laboratory animals (Mann, '28; Tansley, '51; Parry, '53; Noell, '58; Donovan, '66) including the rat (Detwiler, '32; Tansley, '33; Bourne et al., '38; Paik and Chung, '66). Particular attention has been paid to the relationship between the structural changes underlying retinal development and functional maturity monitored by electroretinography (Parry, '53; Hellstrom and Zeterstrom, '56; Noell, '58; Horsten and Winkelman, '60).Despite the fact that many investigations of retinal development have been published, a comprehensive, sequential analysis of development of the retinal layers in mammals is not available, nor has a detailed description of the morphogenesis of the rat retina been published. The present investigation explores, quantitatively, the successive development of the intrinsic retinal layers in the rat, from day 11 of gestation until 225 days after birth. The morphology of the developing rat retina has been studied and a timetable indicating the time of first appearance and relative changes in thickness of each of the retinal layers has been prepared. The timing of development in this AM. J. ANAT., 127: 281-302. strain of rat is correlated with the standard developmental stages of Christie ('64). The results provide necessary background information for future studies of retinal ultrastructure, experimental embryology and teratogenesis. MATERIALS AND METHODSThe animals used were of the Wisconsin strain of the Sprague-Dawley albino rat (Rattus norvegicus). Rats were fed Purina lab chow and water ad libitum and, in addition, nursing females were fed rolled oats. To determine the age of the prenatal rats, daily vaginal smears of the breeding females were used. The first day of gestation was taken to be...

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The morphogenesis of the outer human retina from 6.5 to 18 weeks of gestational age (16-156 mm) was studied by light and electron microscopy. During this period the retinal pigment epithelium changes from a pseudostratified columnar epithelium, with mitoses at its inner edge, to a single layer of cuboidal cells. Initially, mature pigment granules, premelanosomes, polysomes, rough endoplasmic reticulum and a Golgi complex occupy the epithelial cytoplasm. Lateral epithelial surf aces show infoldings and neighboring epithelial cells are bound at their inner borders by a junctional complex consisting of a zonula occludens and zonula adherens. Later, pigment epithelial development is marked by an increase in cytoplasmic organelle content and the appearance of slender processes at the inner epithelial border. Initially, the outer neural retina is formed by a uniform population of elongated cells containing oval nuclei with multiple nucleoli and, in the cytoplasm, mitochondria, polysomes and the Golgi complex. Cell division is common in the proliferative layer at the outer edge of the posterior neural retina until 120 mm crown-rump length, and cell membrane junctions between outer neuroblasts remain intact during mitosis.Cilia project outwards from outer neuroblasts and invaginate the immediately adjacent pigment epithelium. At 120 mm crown-rump length, differentiating rods, cones and Miiller cell processes are distinguishable and developing ribbon synapses and surface contacts are present at the base of the cone prior to outer segment formation.

The photoreceptors of the adult Anableps anableps have been investigated by light microscopy, scanning and transmission electron microscopy. The fish is a surface swimmer and the eye is divided by the water meniscus. In general, the photoreceptors (rods, single cones, and double equal cones) resemble those of other vertebrates, but there are several unusual features: (1) The outer segment discs of the double cones differ in the two members. (2) All cones have a prominent accessory outer segment derived from the single connecting cilium, and there is no second centriole. (3) The exterior of the inner segments is ridged and grooved longitudinally, most markedly so in the cones. (4) A membranebounded oil droplet is present in the distal cone inner segment, formed from mitochondria which enlarge, fuse and transform in a vitreal‐scleral gradient. (5) There are knob‐like invaginations of rod cytoplasm into the cones immediately scleral to the external limiting membrane. (6) Subsurface cisterns underlie apposed plasma membranes of double cone inner segments and direct rod‐cone inner segment contacts. (7) Fine “fins” on the cones interdigitate, with Müller cell cytoplasm between, just scleral to the external limiting membrane. (8) In the rod spherule there is a greater density of vesicles and the cytoplasm is darker than in the cone pedicle. The well‐defined cone mosaic has a linear pattern peripherally and a square pattern centrally. The photoreceptors undergo photomechanical movements. Photoreceptor ultrastructure is alike in both dorsal and ventral parts of the retina, but the ventral retina contains more cells and is thicker than the dorsal retina. The adjustments necessary for simultaneous air and water vision are found mainly in lens shape, corneal thickness and curvature, and the greater number of cells in the ventral retina.