Scanning electron microscopy of developing photoreceptors in the chick retina

Photoreceptor calyceal processes accompany the developing outer segment, adopting a stable length despite a dynamic core

Vertebrate photoreceptors detect light through a large cilium-based outer segment, which is filled with photopigment-laden membranous discs. Surrounding the base of the outer segment are microvilli-like calyceal processes (CPs). Although CP disruption has been associated with altered outer segment morphology and photoreceptor degeneration, the role of the CPs remains elusive. Here, we used zebrafish as a model to characterize CPs. We quantified CP parameters and report a strong disparity in outer segment coverage between photoreceptor subtypes. CP length is stable across light and dark conditions, yet heat-shock inducible expression of tagged actin revealed rapid turnover of the CP actin core. Detailed imaging of the embryonic retina uncovered substantial remodeling of the developing photoreceptor apical surface, including a transition from dynamic tangential processes to vertically oriented CPs immediately prior to outer segment formation. Remarkably, we also found a direct connection between apical extensions of the Müller glia and retinal pigment epithelium, arranged as bundles around the ultraviolet sensitive cones. In summary, our data characterize the structure, development and surrounding environment of photoreceptor microvilli in the zebrafish retina.

The morphological and functional diversity of apical microvilli

Sensory neurons use specialized apical processes to perceive external stimuli and monitor internal body conditions. The apical apparatus can include cilia, microvilli, or both, and is adapted for the functions of the particular cell type. Photoreceptors detect light through a large, modified cilium (outer segment), that is supported by a surrounding ring of microvilli-like calyceal processes (CPs). Although first reported 150 years ago, CPs remain poorly understood. As a basis for future study, we therefore conducted a review of existing literature about sensory cell microvilli, which can act either as the primary sensory detector or as support for a cilia-based detector. While all microvilli are finger-like cellular protrusions with an actin core, the processes vary across cell types in size, number, arrangement, dynamics, and function. We summarize the current state of knowledge about CPs and the characteristics of the microvilli found on inner ear hair cells (stereocilia) and cerebral spinal fluid-contacting neurons, with comparisons to the brush border of the intestinal and renal epithelia. The structure, stability, and dynamics of the actin core are regulated by a complement of actin-binding proteins, which includes both common components and unique features when compared across cell types. Further, microvilli are often supported by lateral links, a glycocalyx, and a defined extracellular matrix, each adapted to the function and environment of the cell. Our comparison of microvillar features will inform further research into how CPs support photoreceptor function, and also provide a general basis for investigations into the structure and functions of apical microvilli found on sensory neurons.

Neurochemical phenotype and birthdating of specific cell populations in the chick retina

The chick embryo is one of the most traditional models in developing neuroscience and its visual system has been one of the most exhaustively studied. The retina has been used as a model for studying the development of the nervous system. Here, we describe the morphological features that characterize each stage of the retina development and studies of the neurogenesis period of some specific neurochemical subpopulations of retinal cells by using a combination of immunohistochemistry and autoradiography of tritiated-thymidine. It could be concluded that the proliferation period of dopaminergic, GABAergic, cholinoceptive and GABAceptive cells does not follow a common rule of the neurogenesis. In addition, some specific neurochemical cell groups can have a restrict proliferation period when compared to the total cell population.

Morphogenesis of the different types of photoreceptors of the chicken (Gallus domesticus) retina and the effect of amblyopia in neonatal chicken

Despite the great variety in chicken photoreceptors, existing morphogenetic studies only deal with two types: rods and cones. We have therefore examined by scanning electron microscopy the first appearance and maturation of different retinal photoreceptors in 36 chicken embryos (Gallus domesticus), aged 5-19 days prehatching. On day 5 of incubation, chicken retinae were only composed of proliferating ventricular cells devoid of photoreceptors. On day 8, outer mitotic cells were separated from inner differentiating photoreceptors, by the transient layer of Chievitz. Ball-like protrusions appeared at the ventricular surface, representing the first signs of photoreceptor inner segment formation. From day 10 onward, double cones, single cones, and rods could be clearly distinguished, and occasional cilia were detected at their tip. On day 12, inner segments had increased in length and diameter, and frequently carried a cilium representing the beginning of outer segment formation. On day 14, most photoreceptors displayed a distinct outer segment. On day 19, photoreceptors had essentially assumed adult morphology. Based on the shape of their outer segments, two subtypes of cones and three subtypes of double cones could be distinguished. Throughout development, we observed microvilli close to maturing photoreceptors, either originating from their lateral sides, from their tip, or from Müller cells. Microvillus density peaked between day 12 and 14, indicating an important role in photoreceptor morphogenesis. Unilateral occlusion of the eyes of posthatching chicken reduced the proportion of double cones to single cones in the retina, indicating dependence of retinal morphogenesis upon functional activity of visual cells.

Spatial and temporal patterns of growth and differentiation of cone oil droplets in the chick retina

Avian cone photoreceptors have an oil droplet in the outer portion of their inner segment that acts as a long-pass cut-off filter between incident light and visual pigment. Chick cone droplets are mainly red, orange, yellow, green, and colorless, and the colors are due to three carotenoid pigments with characteristic absorption spectra. Little is known of the differentiation of this organelle, the natural marker of cones, and the little that is known is largely controversial. We used flat whole-mounts of fresh retinas to study the time and place of the appearance of droplets, their growth rates, the sequence of droplet color differentiation, and the spatial distribution of these colors. We show that droplet differentiation starts on embryonic Day 10 (E10) in a relatively small area above the optic nerve head. The differentiation spreads to the rest of the retina in a manner similar to that of photoreceptor neurogenesis, with three decreasing gradients of droplet size and color between E13-E20: from central to peripheral, dorsal to ventral, and temporal to nasal. The rate of growth of the droplets was not constant, but showed a maximum between E17 and postnatal Day 1 (P1) in most of the retinal zones. Color differentiation started at E16-E17, 5-6 days after their appearance, when the droplets were already of considerable size. Initially, all droplets were colorless, and then turned pale green or yellow to acquire progressively the mature colors. Differentiation ended in the whole retina by P15, with ventral droplets of larger diameter than dorsal ones, the peripheral ones generally larger than the central ones, and with the color distribution varying with the retinal area. Our results show that growth and color differentiation of the droplets is regulated temporally and spatially, and the cones complete differentiation at P15 rather than at prenatal stages, as is thought generally.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Photoreceptor morphogenesis in the human retina: a scanning electron microscopic study

There are a number of scanning electron microscopic (SEM) studies on retinal photoreceptors of vertebrates. However, most of these are concerned with the adult retina, and only a very few deal with developing photoreceptors. In man, SEM studies have not been carried out on photoreceptor morphogenesis during fetal or postnatal stages. Hence, the present study was undertaken to examine the sequential morphological changes in developing photoreceptors during different gestational ages in the human retina. Retinas of human fetuses of gestational ages of 10-25 weeks and from autopsy of a 5-month-old infant were processed for SEM. The observations show some new information on the morphogenesis of photoreceptors. At 10-11 weeks, the outer and inner neuroblastic zones are well developed and separated from each other by the layer of Chievitz. By 15-16 weeks, the photoreceptor precursors appear as spherical inner segments on the scleral surface of the outer neuroblastic zone. Cilia develop as small protrusions from the apical ends of the inner segments. Photoreceptor inner segments become arranged in mosaic pattern by 18-19 weeks. In the mosaic, large cone inner segments (putative blue cones) stand out prominently from the remaining small cone inner segments (prospective red/green cones). The rod inner segments are identifiable and show cilia. Between 19-20 and 24-25 weeks, the cone inner segments elongate and change in shape from spherical to oval. At 24-25 weeks, the outer segments develop from the distal ends of rod cilia. At this period, the inner segments of rods and cones are interconnected by protoplasmic projections. Although the precursors of both rods and cones appear to be in a similar state of development at 14-15 weeks gestation, the rods undergo morphological maturation earlier than do the cones. Photoreceptor development in the anterior retina lags behind that of the posterior retina by about 10 weeks. At 5 months after birth, the posterior retina possesses fully developed photoreceptors that are comparable to those of the adult. However, the photoreceptors in the ora serrata resemble those in the posterior retina of 24-25 weeks gestation.

Remodeling of junctional complexes during the development of the outer blood-retinal barrier

BACKGROUND

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by separating the neural retina from fenestrated capillaries in the choroid. The barrier depends upon tight junctions within the apical junctional complexes that bind neighboring cells. During development, permeability decreases as the apical junctional complex gradually matures. To investigate this process, the composition of the apical junctional complex was monitored during RPE development in chicken embryos.

METHODS

Permeability was monitored by incubating freshly isolated RPE/choroid in medium containing horseradish peroxidase followed by histochemical staining and electron microscopy. The expression of the tight junction proteins, ZO-1 and occludin, was determined by immunofluorescence and immunoblotting. Development of the RPE apical junctional complex was to compared to the homologous complex that forms the outer limiting membrane of the neural retina.

RESULTS

The apical junctional complex of the RPE was permeable to horseradish peroxidase until embryonic day 10-12. Two putative forms of ZO-1 had approximately the same molecular mass as mammalian ZO-1 and were present in the apical junctional complexes at different stages of development. We identified one form as ZO-1, because it was present in mature RPE and shared an epitope with the rodent isoforms, ZO-1 alpha+ and ZO-1 alpha-. The second form lacked this epitope but was identified by a polyclonal antibody to ZO-1. It was designated the ZO-1-like protein (ZO-1LP). On embryonic day 3, occludin and ZO-1LP were observed along the apical surface of the neuroepithelium that gave rise to the RPE and the neural retina. In the neural retina, occludin expression decreased just before inner segments were formed, but ZO-1LP expression continued in the outer limiting membrane throughout development. During RPE development, occludin expression was constant or increased slightly. By contrast, ZO-1LP was gradually replaced by ZO-1 and total ZO-1 immunoreactive proteins decreased more than 10x.

CONCLUSIONS

A gradual change in the composition of the apical junctional complexes accompanied the period of barrier formation. In RPE, ZO-1 gradually replaced ZO-1LP, but the decrease in ZO-1 expression suggests its functions during junction formation are not directly related to junction permeability. By contrast, occludin was lost and ZO-1LP retained where an adherens junction forms the permeable, outer limiting membrane.

Fatty acid composition of phospholipids from chick neural retina during development

The fatty acid composition of retina phospholipids from developing chicks was investigated to determine what changes, if any, occur in the relative levels of polyunsaturated fatty acids. Embryonic chicks were killed at 3-day intervals from day 6 through hatching (day 21), and at 1 week post-hatch. Fatty acids were prepared from retina phospholipids and were analysed by capillary gas-liquid chromatography. A comparison of the composition of yolk taken on day 6 with retinas isolated on that day revealed a much greater proportion of polyunsaturated fatty acids in the latter, suggesting an ability of the embryo to metabolize selectively unsaturated fatty acids at this early stage of development. Throughout the time course studied, saturated fatty acids constituted 50% of all fatty acids, most of which was due to palmitic acid (16:0; 33-41%). Among other saturated fatty acids, myristic acid (14:0) increased to maximal levels by day 18, then declined, while stearic acid (18:0) was minimal on day 12 and then increased. Polyunsaturated fatty acids varied between 14 and 23% of total fatty acids, depending on the developmental stage. One of the most remarkable changes in polyunsaturated fatty acids occurred in the levels of 22:4 (n-6). The proportion of this single fatty acid decreased from 9.4 to 2.4% between days 15 and 18. Relative levels of 22:5 (n-6) increased significantly between day 21 and 1 week post-hatch, from 1.1 to 3.2%. In this same time period, the proportion of 22:6 (n-3), the fatty acid known to be prominent in the outer segments of rod-dominant retinas, did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Establishment of photoreceptor cell polarity in culture revealed by mushroom lectin binding

Differentiation of photoreceptor cells dissociated from chick embryo was investigated in a monolayer culture at low cell density and low serum concentration. The precursor cells were initially round and finally adopted the elongated and monopolar shape characteristic of such photoreceptor cells in vivo. They exhibited the primitive inner segment, and on its distal side showed a crest-like membrane expansion instead of the developed outer segment. The cell polarity of these cultured photoreceptor cells was examined using mushroom lectin, which has specific affinity for the D-galactose-beta(1,3)-N-acetylgalactosamine moiety. Rhodamine-labeled mushroom lectin uniformly initially stained the surface of early round photoreceptor cells, but with time the staining became restricted to the inner segment and its distal membrane expansion in all elongated photoreceptor cells at 12 days from the beginning of development. In semithin sections of 12-day-old chick embryo retina, mushroom lectin also stained the immature inner segment that had emerged at this stage. After hatching, light and electron microscopic observations revealed that the entire scleral surface of retinal plasma membranes as far as the outer limiting membrane, i.e. the inner and outer segments of all types of photoreceptor cells and also Müller cell processes, were stained with the mushroom lectin. These results clearly show that all photoreceptor cells can establish their inherent cell polarity even in culture, and that mushroom lectin is a valuable marker, for the investigation of photoreceptor cell polarity.

The structure of the developing chick retinal pigment epithelium revealed by high resolution scanning electron microscopy

The retinal pigment epithelium (RPE) in the developing eye of chick embryos has been studied during the early stages of development by high resolution scanning electron microscopy (HRSEM). Specimen preparation techniques which involve removal of the cytoplasmic matrix permitted visualization of organelles and other subcellular structures within RPE cells in detail and in three dimensional (3-D) stereo HRSEM. Using this technique, we were able to examine changes in melanosome structures during development and demonstrate that pigmentation in the RPE was present by day 4 of development. RPE plasma cell membranes showed extensive folding of the apical portion of the membrane closest to the developing neural retina by day 9. Examination of RPE photoreceptor junction revealed photoreceptor inner segments by day 6 and an outer segment by day 9. Mitochondria in the RPE were found to contain tubular cristae only. The ultra-structure in 3-D of the Golgi apparatus, smooth and rough endoplasmic reticulum, lysosomes and nuclear chromatin of the RPE, and Bruch's layer was revealed by the HRSEM method.

Development of synapses between chick retinal neurons in dispersed culture

Morphological criteria allow several kinds of synapse to be recognized in the vertebrate retina. It is, however, not presently known if, or how, these morphological differences reflect physiological distinctions. Since a proper investigation of synaptic physiology in the intact retina is compromised by technical difficulties, we have examined dispersed cultures to discover if they are likely to provide a more tractable physiological preparation. The chief question addressed here concerns the extent to which normal synaptic development takes place in the impoverished conditions of dispersed cell culture. Cultures were established from embryonic day 8 chick retina and fixed for microscopy on embryonic equivalent (E.E.) days 12, 14, 16, and 18. Neuronal processes appeared shortly after plating and continued to increase in number and extent through E.E. 16. Cone cells were recognizable by virtue of their distinctive oil droplets. Two classes of cone could be distinguished on the basis of the density of their cytoplasmic staining. Presynaptic ribbons could be observed in cone cells on E.E. 12, but characteristic dyad and triad postsynaptic organization was seldom present at this stage nor was it often observed at subsequent times. An increase in the number of ribbon synapses in culture was seen on E.E. 18. These synapses may represent those of bipolar cells. Conventional synapses were found at all times examined but the number of these increased greatly between E.E. 14 and 16. Of these conventional synapses, we found some whose anatomy was characteristic of synapses made by amacrine cells as well as some whose anatomy was characteristic of synapses made by bipolar cells.

Detection of alpha-transducin in retinal rods but not cones

The distribution in chicken retina of the alpha subunit of transducin, the guanine nucleotide--binding protein that couples light-dependent activation of rhodopsin with activation of guanosine 3',5'-monophosphate phosphodiesterase, was determined with the aid of a specific antiserum. alpha-Transducin was found in rod photoreceptor cells but was not detected in cones. These results show that rods and cones differ with respect to alpha-transducin content and suggest that the processes of phototransduction may differ correspondingly in rods and cones.

Localization of chick retinal visinin-like immunoreactivity in the rat forebrain and diencephalon

The present study is an examination, using an indirect immunofluorescence method, of the distribution of visinin, a 24,000 dalton peptide, in the rat forebrain and diencephalon. Immunoreactive structures were localized in the neuronal elements showing an uneven distribution. Immunoreactive neurons were found in the olfactory bulb, anterior olfactory nucleus, cerebral cortex, amygdaloid complex, ventral portion of the nucleus caudatus putamen, septal area, nucleus accumbens, nucleus paratenialis, nucleus rhomboideus, nucleus reuniens, nucleus paraventricularis hypothalami, nucleus supraopticus, nucleus anterior hypothalami, preoptic area, hypothalamic periventricular nucleus, nucleus mammillaris medialis, medial habenular nucleus, zona incerta, nucleus lateralis thalami, nucleus tractus optici and gyrus dentatus. Immunoreactive fibers were observed in the above areas, particularly near the labelled cells, forming fiber plexuses of varying density. In addition, dense plexuses were also seen in the globus pallidus, anteroventral nucleus of the thalamus, substantia nigra and hippocampus. In the former three structures, no labelled cells were present and in the latter, a few scattered neurons were found, indicating that these fibers originate from extrinsic sources.

Immunohistochemical localization of chick retinal 24 kdalton protein (visinin) in various vertebrate retinae

Antiserum against a protein (24,000 daltons, visinin) of chick retina has been provided for immunohistochemical study on the localization of visinin in chick retinae during development, as well as in various vertebrate retinae. The photoreceptor cells were stained with anti-visinin serum from 7th day embryonic retinae and its intensity was gradually increased with embryonic age. In addition, visinin-like immunoreactivity was found in some kinds of amacrine and displaced amacrine cells from 11th-day embryonic retinae. When human, cat, frog and carp retinae which contain both rods and cones were examined, staining of cone cells was clearly observed in the photoreceptor cell layer, but not in the rods. Furthermore visinin-like immunoreactivity was barely detectable in the photoreceptor cells of bovine, rat and mouse retinae containing mostly rod cells. These results suggest that visinin is mainly located in the cone cells in various vertebrate retinae and is a good marker for the cone cells.

Localization of chick retinal 24,000 dalton protein (visinin)-like immunoreactivity in the rat lower brain stem

The distribution of visinin, a 24,000 dalton peptide, in the lower brain stem of the rat was examined by means of an indirect immunofluorescent method. Visinin-immunoreactive structures were found to be unevenly distributed only in the neuronal elements. The following neuronal systems were strongly labeled by the antiserum; the Purkinje cell system, mammillotegmental system, habenulointerpeduncular system, the second layer of the superior colliculus, ventral tegmental area, substantia nigra pars lateralis, area medial to the medial geniculate body, parabrachial area, dorsal and ventral nuclei of the lateral lemniscus, pontine reticular formation just medial to the trigeminal principal nucleus, superior olivary nucleus, solitarii nucleus, external layer of the inferior colliculus and spinal trigeminal nucleus. The densities of the labeled fibers in these areas paralleled those of the labeled cells. In addition, highly dense visinin-immunoreactive fiber plexuses were seen in the zona compacta of the substantia nigra, lateral portion of the interpeduncular nucleus, ventral tegmental nucleus of Gudden and vestibular nucleus.

Cilia in the fetal and neonatal canine retina

Cilia in the canine retina were examined at 40, 46 and 50 days of gestation and at birth by scanning electron microscopy, transmission electron microscopy, and by the freeze-fracture technique. Cilia were similar in all age groups examined. Scanning electron micrographs showed them to be smooth-surfaced conical to tubular extensions arising from putative photoreceptor inner segments. Cilia when freeze-fractured contained variable numbers of circumferential rows of 10 nm P-face particles: these constitute the ciliary necklace. Transmission electron micrographs showed the ciliary membrane to contain electron-dense beads which corresponded to the ciliary necklace seen in freeze-fracture replicas. The ciliary necklace identified in the developing canine retina was similar to those found in other types of motile and sensory cilia.

Appearance of a Purkinje shift in the developing retina of the chick

The development of rod and cone function was studied in late embryonic and posthatch chicks by measuring the spectral sensitivity of the electroretinogram (ERG) of opened eyecups to 100-msec flashes and 25-Hz stimuli. At the time of hatching the ERG is dominated by cone activity: the response-energy curves for different wavelengths are parallel, and the spectral sensitivity of the dark-adapted eye is maximal near 580 nm. During the first week after hatching, rods make a progressively greater contribution to the ERG: the response-energy functions cease to be parallel, and the spectral sensitivity maximum to dim flashes moves to about 510 nm while the maximum for brighter lights remains at 580 nm. Only the cones are able to follow the 25-Hz stimulus. Spectral sensitivity of the eye to 25-Hz stimuli, measured on red- or orange-adapting backgrounds, reveals the presence at the time of hatching of two photopic processes with lambda max at 415 nm and 580 nm, with evidence for a third in the region 470-510 nm.

Analysis of a distinctive protein in chick retina during development

Soluble proteins from the chick retina were analyzed at various developmental stages by SDS-polyacrylamide gel electrophoresis. A peptide of about 24,000 daltons (24 Kd protein) appeared in the 14-day embryo and gradually increased with embryonic age, maintaining a fairly steady level after hatching. Polypeptides which correspond to actin and tubulin, however, remained almost unchanged during development. The 24 Kd protein was not detected in the cerebrum, tectum, pigment epithelium or vitreous body at any age. To characterize this protein, it was partially purified by gel filtration and ion exchange column chromatography, and its isoelectric point was measured. It was focused in a diffuse spot at about pH 5.5. In the bovine retina, a protein was observed at 24,000 daltons on SDS-polyacrylamide gel, but its isoelectric point was more basic than that of chick retina. It is suggested that the 24 Kd protein is one of the distinctive proteins that increase in concentration during the chick retinal development, and would be closely associated with retinal functions.

Scanning electron microscopy of monkey foveal photoreceptors

Rhesus monkey retina and especially the foveal photoreceptors (PR) were investigated by scanning electron microscopy (SEM). There are a few scattered SEM photomicrographs of the primate retina in the literature but this is the first detailed and comprehensive view by SEM of a primate retina. Some new aspects of surface morphology are displayed and the study also highlights and emphasizes some aspects of photoreceptor structure that have either been overlooked or not clearly displayed in studies using transmission electron microscopy only. For examination by SEM retinas were fixed in glutaraldehyde-formaldehyde, osmicated, immersed in thiocarbohydrazide, dehydrated in alcohols, and gold-coated. The fovea appears as a sharply defined pit with steep slopes, and its vitreal surface looks different from that of the rest of the retina. It appears to have a matted surface. The rest of the vitreal surface is relatively smooth and displays distinct lines which diverge in a radiating pattern from the foveal slopes. The choroid has a spongelike appearance; the sclera appears fibrous with the fibers running parallel to the vitreal surface. Photoreceptor nuclei are sometimes lost during tissue processing. They leave a discrete "nuclear nest" formed from Müller cell processes. Henle fibers turn at a sharp angle from the cones to run parallel to the vitreal surface. The external limiting membrane is seen as a clear line. Immediately vitreal to it, the Müller cell microvilli surround the proximal inner segments. The cone inner segment (CIS) narrows toward the cilium where the cell is markedly constricted. The ciliary connectives are aligned and appear as a shadowy, slightly wavy zone when the retina is viewed in vertical section. The freestanding, tapering calycal processes (CP) arise from and are continuous with longitudinal CIS ridges. CP surround the proximal parts of the outer segments (OS), but there are no CP around the ciliary backbone. Some CP bear small protrusions. OS break off and remain embedded among the pigment epithelium microvilli (PEM) more often than PEM remain attached to OS distal ends. The foveal OS tapers slightly from its proximal to its distal end. The OS may bear knoblike swellings and convolutions in their more distal regions but not at their tips.

Morphogenesis of rod cells in the retina of the albino rat: a scanning electron microscopic study

This study presents scanning electron microscopic (SEM) observations of topographical changes that occur during morphogenesis of rod cells in the albino rat. Correlative transmission electron microscopy (TEM) was also utilized. Albino rats ranging in age from birth to three weeks were used for the study. Tissues were prepared by conventional methods for SEM and TEM. At birth, numerous irregularly arranged inner segments extend from the external surface of the sensory retina. They are spherical, smooth surfaced and possess a randomly oriented cilium. The internal morphology of these immature inner segments is comparable to that observed in other vertebrate species. Statistical analysis reveals a rapid increse in the number of rod cells during the first week. This period is characterized by the elongation of inner segments and their associated cilia. Microvilli project from the apices of Müller cells, but not from adjacent inner segments. By day 5, cilia occasionally display small bulbous outer segments. They are more numerous by day 8 and are usually eccentrically positioned at the tips of cilia. By day 11, outer segments are abundant and frequently obscure from view the underlying inner segments and associated cilia. Elongated cylindrical outer segments are present within the posterior retina at the end of the second week. However, rod cell morphogenesis lags in the peripheral retina. Topographical variations between developing photoreceptor cells in mammalian and non-mammalian retinas are discussed.