Diurnal variations in the photoreceptor synaptic terminals of the newt retina

Newt photoreceptor synaptic terminals undergo a variety of morphological changes over a 24-hr (LD 12:12) cycle. During the day, dense-cored synaptic vesicles were found to increase in number and accumulate near the synaptic lamellae; during the dark phase, the dense-cored vesicles decreased in number, while large clear vesicles and profiles of smooth endoplasmic reticulum increased in frequency. The most marked change in photoreceptor synaptic terminal morphology occurred after 10 hr of darkness, at 0730 hr. At this time, photoreceptor synaptic terminal cross-sectional area was found to increase dramatically. Morphometric analysis showed that the number of synaptic vesicles in these terminals remained constant throughout the day, as did the perimeter of photoreceptor terminal profiles. The observed increase in area of synaptic terminals at 0730 hr was found to be due to a decrease in the folding of the terminal plasma membrane. Qualitative observations showed endocytosis to be occurring at a rapid rate at this time as well; and since the number of synaptic vesicles and terminal perimeter did not change, exocytosis of synaptic vesicles was assumed to be occurring at an equally rapid rate. These findings support an extension to the hypothesis of Monaghan and Osborne (1975), suggesting that photoreceptor synaptic vesicles become "supercharged" with transmitter substance in the light.     

Rhythmic changes in metabolism of dopamine in the chick retina: the importance of light versus biological clock

Rhythmic changes in dopamine (DA) content and metabolism were studied in retinas of chicks that were adapted to three different lighting conditions: 12-h light : 12-h dark (LD), constant darkness (DD) and continuous light (LL). Retinas of chicks kept under LD conditions exhibited light-dark-dependent variations in the steady-state level of DA and the two metabolites of DA, i.e. 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Concentrations of DA, DOPAC and HVA were high in light hours and low in dark hours of the LD illumination cycle. In retinas of chicks kept under DD, the content of DA, DOPAC and HVA oscillated in a rhythmic manner for 2 days, with higher values during the subjective light phase than during the subjective dark phase. The amplitudes of the observed oscillations markedly and progressively declined compared with the amplitudes recorded under the LD cycle. In retinas of chicks kept under LL conditions, levels of DA, DOPAC and HVA were similar to those found during the light phase of the LD cycle. Changes in the retinal contents of DA and HVA did not exhibit pronounced daily oscillations, while on the first day of LL the retinal concentrations of DOPAC were significantly higher during the subjective light phase than during the subjective dark phase. Acute exposure of chicks to light during the dark phase of the LD cycle markedly increased DA and DOPAC content in the retina. In contrast, light deprivation during the day decreased the retinal concentrations of DA and DOPAC. It is suggested that of the two regulatory factors controlling the level and metabolism of DA in the retina of chick, i.e. light and biological clock, environmental lighting conditions seem to be of major importance, with light conveying a stimulatory signal for the retinal dopaminergic cells.     

Circadian organization of a subarctic rodent, the northern red-backed vole (Clethrionomys rutilus)

Arctic and subarctic environments are exposed to extreme light: dark (LD) regimes, including periods of constant light (LL) and constant dark (DD) and large daily changes in day length, but very little is known about circadian rhythms of mammals at high latitudes. The authors investigated the circadian rhythms of a subarctic population of northern red-backed voles (Clethrionomys rutilus). Both wild-caught and third-generation laboratory-bred animals showed predominantly nocturnal patterns of wheel running when exposed to a 16:8 LD cycle. In LL and DD conditions, animals displayed large phenotypic variation in circadian rhythms. Compared to wheel-running rhythms under a 16:8 LD cycle, the robustness of circadian activity rhythms decreased among all animals tested in LL and DD (i.e., decreased chi-squared periodogram waveform amplitude). A large segment of the population became noncircadian (60% in DD, 72% in LL) within 8 weeks of exposure to constant lighting conditions, of which the majority became ultradian, with a few individuals becoming arrhythmic, indicating highly labile circadian organization. Wild-caught and laboratory-bred animals that remained circadian in wheel running displayed free-running periods between 23.3 and 24.8 h. A phase-response curve to light pulses in DD showed significant phase delays at circadian times 12 and 15, indicating the capacity to entrain to rapidly changing day lengths at high latitudes. Whether this phenotypic variation in circadian organization, with circadian, ultradian, and arrhythmic wheel-running activity patterns in constant lighting conditions, is a novel adaptation to life in the arctic remains to be elucidated.     

Synaptic activity of frog retinal photoreceptors. A peroxidase uptake study

The uptake of horseradish peroxidase (HRP) into membranous structures, detectable by light and electron microscopy, is used here to monitor the synaptic activity of photoreceptors of isolated frog retinas maintained in the dark or under various illumination conditions. The major findings are: (a) Neurotransmission from photoreceptor terminals seems to involve the same types of endocytic membrane-retrieval processes that occur at other nerve terminals. Presumably, the endocytic processes compensate for exocytic events associated with neurotransmission. The retrieved membrane is "recycled" to form vesicles. Some of these accumulate near the synaptic ribbons, perhaps indicating reutilization for exocytosis. On the other hand, some retrieved membrane evidently is degraded via multivesicular bodies that appear to undergo "retrograde" transport from the receptor synapses to the myoid regions. (b) Photoreceptor terminals take up much HRP in the dark. Steady illumination markedly decreases uptake by rods. Uptake by cones is notably reduced only at illumination intensities higher than those that have maximal effects on rods. (c) The decrease in rod HRP uptake with light is reversible when retinas are allowed to adapt to the dark, if the light exposures used were at intensities that bleach very little visual pigment. Such "recovery" is not observed after light exposures that bleach a considerable amount of visual pigment. The cones recover their dark levels of HRP uptake even after light exposures that bleach considerable amounts of visual pigment. The changes in HRP uptake that we observe parallel expectations for photoreceptor synaptic neurotransmission derived from indirect physiological evidence.     

Movement of spermatozoa in the reproductive tract of adult male Spodoptera litura: daily rhythm of sperm descent and the effect of light regime on male reproduction

Sperm production and movement from the fused testes into the male reproductive tract of the common cutworm Spodoptera litura were studied in insects maintained in a 12 h:12 h light dark (LD) regime. Two types of sperm bundles, eupyrene (nucleated) and apyrene (anucleate) were present in the adult testes. Eupyrene bundles constituted about 25% of the total. Descent of spermatozoa from the testes into the upper vas deferens (UVD) first occurred about 24-30 h before adult eclosion. On entering the reproductive tract, eupyrene spermatozoa remained in bundles while apyrene bundles became dissociated before they reached the UVD. Downward movement of both eupyrene and apyrene spermatozoa within the male tract occurred in a daily rhythm. Sperm descent from the testes into the UVD occurred during the early scotophase, followed by their further descent into the seminal vesicle (SV) during the photophase. Spermatozoa remained in the SV for only a short duration, whence sperm quickly passed through the lower vas deferens into the duplex, which acted as the main sperm storage organ until mating was initiated. During mating 80% of sperm left the duplex, but mating did not influence the number of sperm bundles that subsequently descended into the duplex or the rate of their descent. There was no evidence of sperm reflux. Rearing in constant light (LL) and in constant dark (DD) reduced the number of eupyrene sperm present in the testes of adults that emerged in LL and DD compared to controls (LD), although there was no significant effect on the number of apyrene sperm in the testes. The rhythmic pattern of sperm descent was suppressed in both LL and DD regimes, and the number of sperm in the duplex was adversely affected, with a marked impact in LL reared insects. Male longevity, mating behaviour, oviposition and fertility were found to be more severely affected in LL than in DD.     

Synapsins in the vertebrate retina: absence from ribbon synapses and heterogeneous distribution among conventional synapses

The vertebrate retina contains two ultrastructurally distinct types of vesicle-containing synapses: conventional synapses, made predominantly by amacrine cells, and ribbon synapses, formed by photoreceptor and bipolar cells. To identify molecular differences between these synapse types, we have compared the distribution of the synapsins, a family of nerve terminal phosphoproteins, with that of synaptophysin (p38) and SV2, two intrinsic membrane proteins of synaptic vesicles. We report an absence of synapsin I and II immunoreactivity from all ribbon-containing nerve terminals. These include terminals of rod cells in developing and adult rat retina, rod and cone cells in monkey and salamander retinas, and rat bipolar cells. Furthermore, we show that synapsins I and II are differentially distributed among conventional synapses of amacrine cells. The absence of the synapsins from ribbon synapses suggests that vesicle clustering and mobilization in these terminals differ from that in conventional synapses.     

The effect of dorsal raphe nucleus (DRN) lesions on the locomotor activity rhythm in mice.

The study employed electrical lesions of dorsal raphe nucleus (DRN) to determine the functional significance of those nuclei in the regulation of wheel-running activity rhythm in mice in light/dark (LD 12:12), constant light (LL), and constant dark (DD) conditions. The wheel-running records showed that raphe nucleus lesions resulted in few days' decrease in common activity and amplitude in LD. The activity phase was not compact but in fragmentary form, especially in DD condition. In some animals an earlier onset of activity after DRN lesion in LD was observed. In LL extension of the rhythm period occurred. Destruction of DRN only slightly modulates the wheel-running circadian rhythm in mice.     

Morphologic changes in the pineal gland of rats exposed to continuous darkness

We examined the pineal structure of rats exposed to constant darkness (DD) at light microscopic level. Two groups of rats were exposed to 12:12 light/dark cycle (LD) or DD from their prenatal ontogenesis and then for 3 months after birth. The gland structure of DD rats was observed to have an active appearance. Some of the observed pinealocytes with light nuclei from DD rats were determined to contain double nucleoli. Nuclear area and perimeter of both dark and light types were greater in rats kept in DD than in LD. Rats exposed to DD had more cells with light nuclei and lesser cells with dark ones than rats kept in LD. No significant differences in nuclear characteristics of intermediate type were found between rats kept in LD and those kept in DD. The activity of mammalian pineal can be altered by light conditions to which the animal is exposed.     

Circadian rhythm of locomotor activity in the Japanese honeybee, Apis cerana japonica

Abstract.  To reveal circadian characteristics and entrainment mechanisms in the Japanese honeybee Apis cerana japonica , the locomotor-activity rhythm of foragers is investigated under programmed light and temperature conditions. After entrainment to an LD 12 : 12 h photoperiodic regime, free-running rhythms are released in constant dark (DD) or light (LL) conditions with different free-running periods. Under the LD 12 : 12 h regime, activity offset occurs approximately 0.4 h after lights-off transition, assigned to circadian time (Ct) 12.4 h. The phase of activity onset, peak and offset, and activity duration depends on the photoperiodic regimes. The circadian rhythm can be entrained to a 24-h period by exposure to submultiple cycles of LD 6 : 6 h, as if the locomotive rhythm is entrained to LD 18 : 6 h. Phase shifts of delay and advance are observed when perturbing single light pulses are presented during free-running under DD conditions. Temperature compensation of the free-running period is demonstrated under DD and LL conditions. Steady-state entrainment of the locomotor rhythm is achieved with square-wave temperature cycles of 10 °C amplitude, but a 5 °C amplitude fails to entrain.     

Multi-oscillatory control of eclosion and oviposition rhythms in Drosophila melanogaster: evidence from limits of entrainment studies

The eclosion and oviposition rhythms of flies from a population of Drosophila melanogaster maintained under constant conditions of the laboratory were assayed under constant light (LL), constant darkness (DD), and light/dark (LD) cycles of 10:10h (T20), 12:12h (T24), and 14:14h (T28). The mean (+/- 95% confidence interval; CI) free-running period (tau) of the oviposition rhythm was 26.34 +/- 1.04h and 24.50 +/- 1.77h in DD and LL, respectively. The eclosion rhythm showed a tau of 23.33 +/- 0.63 h (mean +/- 95% CI) in DD, and eclosion was not rhythmic in LL. The tau of the oviposition rhythm in DD was significantly greater than that of the eclosion rhythm. The eclosion rhythm of all 10 replicate vials entrained to the three periodic light regimes, T20, T24, and T28, whereas the oviposition rhythm of only about 24 and 41% of the individuals entrained to T20 and T24 regimes, respectively, while about 74% of the individuals assayed in T28 regimes showed entrainment. Our results thus clearly indicate that the tau and the limits of entrainment of eclosion rhythm are different from those of the oviposition rhythm, and hence this reinforces the view that separate oscillators may regulate these two rhythms in D. melanogaster.     

Changes in melatonin binding sites under artificial light-dark, constant light and constant dark conditions in the masu salmon brain

To test whether the affinity (Kd) and total binding capacity (Bmax) of melatonin receptors exhibit daily and circadian changes in teleost fish whose melatonin secretion is not regulated by intra-pineal clocks, we examined the changes in melatonin binding sites in the brains of underyearling masu salmon Oncorhynchus masou under artificial light-dark (LD), constant light (LL) and constant dark (DD) conditions. In Experiment 1, fish were reared under a long (LD 16:8) or short (LD 8:16) photoperiod for 69 days. Blood and brains were sampled eight times at 3 h intervals. Plasma melatonin levels were high during the dark phase and low during the light phase in both photoperiodic groups. The Bmax exhibited no daily variations. Although the Kd slightly, but significantly, changed under LD 8:16, this may be of little physiological significance. In Experiment 2, fish reared under LD 12:12 for 27 days were exposed to LL or DD from the onset of the dark phase under LD 12:12. Blood and brains were sampled 13 times at 4 h intervals for two complete 24 h cycles. Plasma melatonin levels were constantly high in the DD group and low in the LL group. No significant differences were observed in the Kd and the Bmax between the two groups, and the Kd and the Bmax exhibited no circadian variation either in the LL or DD groups. These results indicate that light conditions have little effect on melatonin binding sites in the masu salmon brain.     

Glycine input induces the synaptic facilitation in salamander rod photoreceptors

Glycinergic synapses in photoreceptors are made by centrifugal feedback neurons in the network, but the function of the synapses is largely unknown. Here we report that glycinergic input enhances photoreceptor synapses in amphibian retinas. Using specific antibodies against a glycine transporter (GlyT2) and glycine receptor β subunit, we identified the morphology of glycinergic input in photoreceptor terminals. Electrophysiological recordings indicated that 10 μM glycine depolarized rods and activated voltage-gated Ca²⁺ channels in the neurons. The effects facilitated glutamate vesicle release in photoreceptors, meanwhile increased the spontaneous excitatory postsynaptic currents in Off-bipolar cells. Endogenous glycine feedback also enhanced glutamate transmission in photoreceptors. Additionally, inhibition of a Cl⁻ uptake transporter NKCC1 with bumetanid effectively eliminated glycine-evoked a weak depolarization in rods, suggesting that NKCC1 maintains a high Cl⁻ level in rods, which causes to depolarize in responding to glycine input. This study reveals a new function of glycine in retinal synaptic transmission.     

State-dependent effects of light-dark cycle on somatosensory and visual cortex EEG in rats

Somatosensory (SSctx) and visual cortex (Vctx) EEG were evaluated in rats under a 12:12-h light-dark (LD) cycle and under constant light (LL) or constant dark (DD) in each sleep or wake state. Under LD conditions during light period, relative Vctx EEG slow-wave activity (SWA) was higher than that of the SSctx, whereas during dark period, relative Vctx EEG SWA was lower than in the SSctx. These effects were state specific, occurring only during non-rapid eye movement sleep (NREMS). Under LL conditions, the duration of REMS and NREMS during the period that would have been dark if the LD cycle had continued (subjective dark period) was greater than under LD conditions. DD conditions had little effect on the duration of NREMS and REMS. SSctx and Vctx EEG SWA were suppressed by LL during the subjective dark period; however, the degree of Vctx SWA suppression was smaller than that of the SSctx. DD conditions during the subjective light period enhanced SSctx SWA, whereas Vctx SWA was suppressed. Under LL conditions during the subjective dark period, Vctx EEG power was higher than that of the SSctx across a broad frequency range during NREMS, REMS, and wakefulness. During DD, SSctx EEG power during NREMS was higher than that of the Vctx in the delta wave band, whereas SSctx power during REMS and wakefulness was higher than that of the Vctx in frequencies higher than 8 Hz. We concluded that the SSctx and Vctx EEGs are differentially affected by light during subsequent sleep. Results provide support for the notion that regional sleep intensity is dependent on prior regional afferent input.     

Developmental plasticity of the locomotor activity rhythm of Drosophila melanogaster

We used four replicate outbred populations of Drosophila melanogaster to investigate whether the light regimes experienced during the pre-adult (larval and pupal) and early adult stages influence the free-running period (τ_DD) of the circadian locomotor activity rhythm of adult flies. In a series of two experiments four different populations of flies were raised from egg to eclosion in constant light (LL), in light/dark (LD) 12:12 h cycle, and in constant darkness (DD). In the first experiment the adult male and female flies were directly transferred into DD and their locomotor activity was monitored, while in the second experiment the locomotor activity of the emerging adult flies was first assayed in LD 12:12 h for 15 days and then in DD for another 15 days. The τ_DD of the locomotor activity rhythm of flies that were raised in all the three light regimes, LL, LD 12:12 h and in DD was significantly different from each other. The τ_DD of the locomotor activity rhythm of the flies, which were raised in DD during their pre-adult stages, was significantly shorter than that of flies that were raised as pre-adults in LL regime, which in turn was significantly shorter than that of flies raised in LD 12:12 h regime. This pattern was consistent across both the experiments. The results of our experiments serve to emphasise the fact that in order to draw meaningful inferences about circadian rhythm parameters in insects, adequate attention should be paid to control and specify the environment in which pre-adult rearing takes place. The pattern of pre-adult and early adult light regime effects that we see differs from that previously observed in studies of mutant strains of D. melanogaster, and therefore, also points to the potential importance of inter-strain differences in the response of circadian organisation to external influences.     

Effect of Different Light Regimes on Pre-Adult Fitness in Drosophila melanogaster Populations Reared in Constant Light for over Six Hundred Generations

Egg to eclosion development time and survivorship were assayed on four laboratory populations of Drosophila melanogaster that had been reared for over 600 generations in continuous light (LL) and constant temperature. The assays were performed in three environments: continuous light (LL), periodically varying light/dark cycles (LD 12:12 hr), and continuous darkness (DD). Development time in LL was significantly less than that in LD, which, in turn, was significantly less than that in DD, whereas survivorship did not differ significantly among the three treatments. The results indicate that individuals from Drosophila populations routinely maintained in LL do not suffer any deleterious effects of LL treatment on pre-adult fitness. Other studies on these populations have shown that free-running period (t) of the eclosion rhythm in DD is greater than that in LD. Our results are, thus, also consistent with the notion that development time may be a function of the free-running period.     

Effect of Different Light Regimes on Pre-Adult Fitness in Drosophila melanogaster Populations Reared in Constant Light for over Six Hundred Generations

Egg to eclosion development time and survivorship were assayed on four laboratory populations of Drosophila melanogaster that had been reared for over 600 generations in continuous light (LL) and constant temperature. The assays were performed in three environments: continuous light (LL), periodically varying light/dark cycles (LD 12:12 hr), and continuous darkness (DD). Development time in LL was significantly less than that in LD, which, in turn, was significantly less than that in DD, whereas survivorship did not differ significantly among the three treatments. The results indicate that individuals from Drosophila populations routinely maintained in LL do not suffer any deleterious effects of LL treatment on pre-adult fitness. Other studies on these populations have shown that free-running period (t) of the eclosion rhythm in DD is greater than that in LD. Our results are, thus, also consistent with the notion that development time may be a function of the free-running period.     

Daily Rhythm of the Response to Noradrenaline in Djungarian Hamsters Acclimated to Cold and Short Photoperiod

We investigated the daily rhythm of the response to noradrenaline injections in Djungarian hamsters (Phodopus sungorus sungorus) at neutral ambient temperature, under long photoperiod (L:D 12:12) and after four weeks of acclimation to cold (10ºC) and short photoperiod (L:D 8:16). Animals were injected with noradrenaline (0.6 mg/kg) every four hours. Body temperature and gross motor activity were measured with MiniMitter transmitters implanted into abdominal cavity. Additionally, we measured body weight and food intake prior to, and after acclimation. After four weeks of acclimation, the experiment was performed under LD cycle and then repeated during one-day of constant light (LL) and constant darkness (DD). In animals acclimated to L:D 12:12 and ambient temperature of 25ºC, noradrenaline injections caused short-lasting increase in body temperature followed by marked decrease. There was no significant difference in the magnitude of the reaction between light and dark phase of the day. After acclimation to cold and L:D 8:16, under LD conditions, we recorded significant differences between the responses to the noradrenaline injections during light and dark phase of the day. Post-injection increase was higher during the day than during the night while following noradrenaline-induced hypothermia was much more pronounced in darkness. In experiments performed after acclimation to cold and short photoperiod but during one day of LL and DD regimes, these differences were attenuated. Data presented here indicate that in cold acclimated hamsters, the response to exogenous noradrenaline depends on the time of injection and it exhibits clear daily rhythm. The rhythmicity is altered under LL and DD regimes. It seems that post-injection increase in body temperature elicits following hypothermia. This hypothermia might be of a great ecological importance. Reasonable lowering of body temperature would be a protective mechanism, allowing for energy charge restoration.     

Circadian rhythms of corneal mitotic rate,retinal melatonin and immunoreactive visual pigments,and the effects of melatonin on the rhythms in the Japanese quail

We investigated circadian ocular rhythms in the Japanese quail, Coturnix coturnix japonica. The birds were placed under light-dark cycles (LD 1212), constant light (LL) and constant darkness (DD), and the retinas were dissected out at four-hour intervals throughout 24 h. Following measurements were performed. (1) Melatonin content in the retina was measured by radioimmunoassay. It was low in light and several folds higher in darkness under LD 1212. The rhythm continued in DD, but disappeared in LL. (2) Mitotic figures in the corneal epithelium were counted. Similar rhythms to the melatonin content were observed in the corneal mitotic rate with a slight phase delay. (3) The retinas were fixed at 4-h intervals and immunostained with anti-bovine rhodopsin serum and anti-chicken iodopsin monoclonal antibodies. The outer segments of photoreceptor cells were stained intensively throughout 24 h in LD 1212, LL and DD. In contrast, the stainability of the locus close to the outer limiting membrane where the Golgi apparatus exists changed diurnally. Scores showing the ratio of cells with positive staining indicated high values from 4 h after the onset of light to the beginning of dark phase under LD 1212. The values were high throughout 24 h in LL and intermediate or low in DD. (4) To investigate the effect of melatonin on the corneal mitotic rate and visual pigments at the Golgi region, melatonin was injected into one eye and saline into the contralateral eye. Melatonin induced a phase advance in the corneal mitotic rate under LD 1212, but did not induce a rhythm under LL. The ratio of photoreceptor cells with positive staining to anti-visual pigment antibodies at the Golgi region was not affected by melatonin injection.Abbreviations ANOVA analysis of variance - BSA bovine serum albumin - DD constant darkness - Io-mAb monoclonal antibodies against chicken iodospin - LD light-dark - LL constant light - mRNA messenger ribonucleic acid - PBS phosphate buffer solution - Rh-As antiserum against bovine rhodopsin - SCN suprachiasmatic nucleus - T transducin - T transducin      

Differential regulation of arylalkylamine N-acetyltransferase activity in chicken retinal ganglion cells by light and circadian clock

Retinal ganglion cells (RGCs) contain circadian clocks driving melatonin synthesis during the day, a subset of these cells acting as nonvisual photoreceptors sending photic information to the brain. In this work, the authors investigated the temporal and light regulation of arylalkylamine N-acetyltransferase (AA-NAT) activity, a key enzyme in melatonin synthesis. The authors first examined this activity in RGCs of wild-type chickens and compared it to that in photoreceptor cells (PRs) from animals maintained for 48?h in constant dark (DD), light (LL), or regular 12-h:12-h light-dark (LD) cycle. AA-NAT activity in RGCs displayed circadian rhythmicity, with highest levels during the subjective day in both DD and LL as well as in the light phase of the LD cycle. In contrast, AA-NAT activity in PRs exhibited the typical nocturnal peak in DD and LD, but no detectable oscillation was observed under LL, under which conditions the levels were basal at all times examined. A light pulse of 30-60?min significantly decreased AA-NAT activity in PRs during the subjective night, but had no effect on RGCs during the day or night. Intraocular injection of dopamine (50 nmol/eye) during the night to mimic the effect of light presented significant inhibition of AA-NAT activity in PRs compared to controls but had no effect on RGCs. The results clearly demonstrate that the regulation of the diurnal increase in AA-NAT activity in RGCs of chickens undergoes a different control mechanism from that observed in PRs, in which the endogenous clock, light, and dopamine exhibited differential effects. (Author correspondence: mguido@fcq.unc.edu.ar ).     

Myeloid bodies in the pigment epithelium of a teleost embryo, the viviparous Poecilia reticulata

Appearance of myeloid bodies (MB) in the retinal pigment epithelium (RPE) precedes photoreceptor outer segment development in Poecilia reticulata embryos reared under a 12 hrs LD cycle, in constant darkness (DD) and constant light (LL). When first formed, MB are predominantly continuous with rough endoplasmic reticulum (RER). The same is observed in the peripheral growth zone of the developed eye, whereas in differentiated parts, MB are continuous with the smooth endoplasmic reticulum (SER). At onset of photomechanical movements, wavy MB predominate in light-adapted LD embryos, are exclusively present in LL and are located in the RPE processes. SER abounds. Straight MB predominate in dark-adapted LD embryos, are exclusively present in DD and contain electrondense material between lamellae. Diurnal appearance of electrondense material may be coupled with transfer of retinol, mediated by various transport proteins.