Immunocytochemical localization of serotonin and photoreceptor-specific proteins (rod-opsin,S-antigen) in the pineal complex of the river lamprey,Lampetra japonica,with special reference to photoneuroendocrine cells

Summary The pineal complex of the river lamprey, Lampetra japonica, was examined by means of immunocytochemistry with antisera against serotonin, the precursor of melatonin, and two photoreceptor proteins, rod-opsin (the apoprotein of the photopigment rhodopsin) and S-antigen. Serotonin-immunoreactive cells were observed in both the pineal and the parapineal organ. The proximal portion of the pineal organ (atrium) comprised numerous serotonin-immunoreactive cells displaying spherical somata. In the distal end-vesicle of the pineal organ, the serotonin-immunoreactive elements resembled photoreceptors in their size and shape. These cells projecting into the pineal lumen and toward the basal lamina were especially conspicuous in the ventral portion of the end-vesicle. In addition, single serotonin-immunoreactive nerve cells were found in this location. Retinal photoreceptors were never seen to contain immunoreactive serotonin; amacrine cells were the only retinal elements exhibiting serotonin immunoreaction. Strong S-antigen immunoreactivity was found in numerous photoreceptors located in the pineal end-vesicle. In contrast, the S-antigen immunoreactivity was weak in the spherical cells of the atrium. Thus, the pattern of S-antigen immunoreactivity was roughly opposite to that of serotonin. Similar findings were obtained in the parapineal organ. The rod-opsin immunoreaction was restricted to the outer segments of photoreceptors in the pineal end-vesicle and parapineal organ. No rodopsin immunoreactive outer segments occurred in the proximal portion of the atrium. Double immunostaining was employed to investigate whether immunoreactive opsin and serotonin are colocalized in one and the same cell. This approach revealed that (i) most of the rodopsin-immunoreactive outer segments in the end-vesicle belonged to serotonin-immunonegative photoreceptors; (ii) nearly all serotonin-immunoreactive cells in the end-vesicle bore short rod-opsin-immunoreactive outer segments protruding into the pineal lumen; and (iii) the spherical serotonin-immunoreactive cells in the pineal stalk lacked rod-opsin immunoreaction and an outer segment. These results support the concept that multiple cell lines of the photoreceptor type exist in the pineal complex at an early evolutionary stage.     

The pineal organ is the first differentiated light receptor in the embryonic salmon,Salmo salar L.

Summary The initial appearance of S-antigen, -transducin, opsin and 5-HT during embryogenesis of the pineal organ and retina was studied by means of immunocytochemistry in the Atlantic salmon, Salmo salar L. The presence of these substances may be taken as a good indication of photoreceptor differentiation; -transducin and S-antigen are involved in the phototransduction process, opsin is the proteinaceous component of the photopigment rhodopsin, and 5-HT is a neurotransmitter or neurohormone produced by pineal photoreceptors. Two days after the retinal pigment layer became visible in the eggs, the outer segments of a few pineal photosensory cells showed immunoreactivity to opsin and -transducin. At the same time S-antigen and serotonin were present in pineal cells of the photoreceptor type. The number of immunoreactive cells in the pineal organ increased up to hatching. In the differentiating retina of the salmon, no immunoreactivity to antibodies raised against the mentioned substances was detectable until after hatching. These results indicate that in ontogeny the developing pineal organ of the salmon embryo has the ability to perceive light information much earlier than the retina.A preliminary account of this work was presented at the Tenth European Neuroscience Congress, Marseille, France, September 14–18, 1986     

Immunocytochemical and electron-microscopic investigations of the pineal organ in adult agamid lizards,Uromastix hardwicki

Summary Lacertilian species display a remarkable diversity in the organization of the neural apparatus of their pineal organ (epiphysis cerebri). The occurrence of immunoreactive S-antigen and opsin was investigated in the retina and pineal organ of adult lizards, Uromastix hardwicki. In this species, numerous retinal photoreceptors displayed S-antigen-like immunoreactivity, whereas only very few pinealocytes were labeled. Immunoreactive opsin was found neither in retinal photoreceptors nor in pinealocytes. Electron microscopy showed that all pinealocytes of Uromastix hardwicki resemble modified pineal photoreceptors. A peculiar observation is the existence of a previously undescribed membrane system in the inner segments of these cells. It is evidently derived from the rough endoplasmic reticulum but consists of smooth membranes. The modified pineal photoreceptor cells of Uromastix hardwicki were never seen to establish synaptic contacts with somata or dendrites of intrapineal neurons, which are extremely rare. Vesiclecrowned ribbons are prominent in the basal processes of the receptor cells, facing the basal lamina or establishing receptor-receptor and receptor-interstitial type synaptoid contacts. Dense-core granules (60–250 nm in diameter) speak in favor of a secretory activity of the pinealocytes. Attention is drawn to the existence of receptor-receptor and receptor-interstitial cell contacts indicating intramural cellular relationships that deserve further study.Supported by the Deutsche Forschungsgemeinschaft (Ko 758/31) and the Deutscher Akademischer Austauschdienst (Senior DAAD Research Fellowship to M.A.H.)     

Differential Expression of mRNA and Protein Encoding Retinal and Pineal S-Antigen During the Light/Dark Cycle

S-Antigen is a soluble cell protein unique to the retina and pineal gland. In the former, it is a well-characterized molecule that participates in light-induced signal transduction in photoreceptor cells. In the latter, the functional role is presently not known. The expression of S-antigen and its mRNA was examined in the rat retina and pineal gland throughout the diurnal cycle and with light interruption of the dark cycle. A cDNA for rat S-antigen was isolated from a pineal gland library to examine the mRNAs. A 1.7-kb mRNA for S-antigen was observed in both the pineal gland and the retina. Retinal S-antigen mRNA was expressed throughout the diurnal cycle and increased with light interruption of the dark cycle. In contrast, pineal gland S-antigen mRNA levels were detectable only during the dark and were absent preceding and during light. The phenotypic expression of immunoreactive S-antigen, identified with two S-antigen monoclonal antibodies (MAbs), MAb A9C6 and MAb C10C10, was analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel (PAGE) and isoelectric focusing (IEF) electrophoresis. Immunoblot analysis of gels after SDS-PAGE revealed a single 46-kDa protein in retina. In contrast, two bands of approximately 43 and 46 kDa were identified in the pineal gland. Immunoblots of the retinal extracts separated by IEF electrophoresis revealed five S-antigen isomers, which vary quantitatively throughout the diurnal cycle and when light interrupted the dark cycle. Immunoblots of the pineal gland samples separated by IEF electrophoresis indicated that the pineal gland possesses four pineal gland-specific forms of S-antigen in addition to the five forms present in the retina. The differences observed in the mRNA and protein analyses suggest tissue-specific structural components for S-antigen in the retina and pineal gland that are not regulated in the same manner.     

Rod-opsin immunoreaction in the pineal organ of the pigmented mouse does not indicate the presence of a functional photopigment

The aim of the present study was to characterize the rod-opsin immunoreaction in the mammalian pineal organ. Pigmented mice (strain C57BL) were selected as the animal model. Immunocytochemical investigations involving the use of highly specific polyclonal and monoclonal antibodies against bovine rod-opsin (the apoprotein of the photopigment rhodopsin) showed that approximately 25% of all pinealocytes were rod-opsin immunoreactive. Immunoblotting techniques revealed three protein bands of approximately 40, 75, and 110 kDa; these were detected by the monoclonal antibody and the polyclonal antiserum in retinal and pineal extracts. These protein bands presumably represented the monomeric, dimeric and trimeric forms of rod-opsin. The amount of rod-opsin in retina and pineal organ was quantified by means of an enzyme-linked immunosorbent assay. This yielded 570±30 pmoles rod-opsin per eye and 0.3±0.05 pmoles rod-opsin per pineal organ. High pressure liquid chromatography analysis of whole eye extracts demonstrated the chromophoric group of the photopigment rhodopsin, 11-cis retinal, and its isomer, all-trans-retinal. A shift from 11-cis retinal to all-trans-retinal was found upon light adaptation. No retinals were detected in the pineal organ. Autoradiographic investigations showed that ³H-retinol, intraperitoneally injected into the animals, was incorporated into the outer and inner segments of retinal photoreceptors, but not into the pineal organ. It is concluded that the mouse pineal organ contains the authentic apoprotein of rhodopsin but that it lacks retinal derivatives as essential components of all known vertebrate photopigments. Consequently, the photoreceptor-specific proteins of the mammalian pineal organ are not involved in photoreception and phototransduction, but may serve other functions to be explored in future studies.     

Immunocytochemical markers revealing retinal and pineal but not hypothalamic photoreceptor systems in the Japanese quail

Summary The retinal proteins opsin,-transducin, S-antigen and interstitial retinol-binding protein (IRBP) are essential for the processes of vision. By use of immunocyto-chemistry we have employed antibodies directed against these photoreceptor proteins in an attempt to identify the photoreceptor systems (retina, pineal and deep brain) of the Japanese quail. Opsin immunostaining was identified within many outer (basal portion) and inner segments of retinal photoreceptor cells and limited numbers of photoreceptor perikarya. Opsin immunostaining was also demonstrated in limited numbers of pinealocytes with all parts of these cells being immunoreactive. These results differ from previous observations. In contrast to the results obtained with the antibody against opsin, S-antigen and-transducin immunostaining was seen throughout the entire outer segments and many photoreceptor perikarya of the retina. In the pineal organ immunostaining was seen in numerous pinealocytes in all follicles. These results conform to previous findings in birds. In addition, IRBP has been demonstrated for the first time in the avian retina and pineal organ. These findings underline the structural and functional similarities between the retina and pineal organ and provide additional support for a photoreceptive role of the avian pineal. No specific staining was detected in any other region of the brain in the Japanese quail; the hypothalamic photoreceptors of birds remain unidentified.     

Immunocytochemical demonstration of rod-opsin,S-antigen,and neuron-specific proteins in the human pineal gland

Summary The aim of this study was to examine whether rod-opsin and S-antigen immunoreactions were present in the pineal organ of adult man and how these immunoreactions were correlated with neuronal markers, e.g., synaptophysin, and neurofilaments L, H and M. Three perfusion-fixed epithalamic regions including the pineal organ and five pineal glands obtained at routine autopsy were used. The specimens were taken from female or male patients, 25 to 85 years of age. All immunoreactions were performed using highly specific, well-characterized antibodies. Rod-opsin and S-antigen-immunoreactive pinealocytes occurred in all pineal organs investigated; however, the immunoreaction was restricted to small subpopulations of pinealocytes (rod-opsin immunoreaction: approximately 3%–5%; S-antigen immunoreaction: approximately 5%–10% of the total population). In contrast, immunoreactions for synaptophysin and neurofilaments M and H were present in numerous pinealocytes. Immunoreactivity for neurofilament L was not found. These data suggest that the cellular composition of the human pineal organ is heterogeneous. Moreover, the presence of rod-opsin and S-antigen immunoreactions in the human pineal organ indicates that it may be affected by autoimmune retinal diseases that are provoked by antibodies against these proteins, as is the case in rodents and non-human primates.     

Immunocytochemical demonstration of retinal S-antigen in the pineal organ of four mammalian species

By means of immunocytochemistry retinal S-antigen is selectively demonstrated in retinal photoreceptor cells of the rat and in pinealocytes of the hedgehog, rat, gerbil and cat. Brain areas surrounding the pineal organ are immunonegative. The immunoreactive material is evenly distributed in the perikarya of the cells. Occasionally, inner segments of retinal photoreceptors and processes of pinealocytes are also stained. The outer segments of retinal photoreceptors display a strong immunoreaction. In both pinealocytes and retinal photoreceptors the intensity of the immunoreaction varied considerably among individual cells. The immunocytochemical demonstration of retinal S-antigen in mammalian pinealocytes indicates that these cells still bear characteristics of photoreceptors. This finding is in accord with the concept that mammalian pinealocytes are derived from pineal photoreceptor cells of poikilothermic vertebrates.     

Analysis of pore-rich areas on the nuclear envelope of spermatocytes

The S-antigen is a protein of photoreceptors, mainly known for its autoantigenic properties in mammals, which is widely distributed in the retina of vertebrates and in photoreceptor organs of invertebrates. Using three monoclonal antibodies specific for different epitopes of S-antigen, this study complements our previous data on retinal rods and cones and presents new results on the photosensory cells of the pineal complex. Immunoreactivity was found in (i) retinal rods and cones, (ii) cone-like and modified photoreceptor cells, and pinealocytes of the pineal organ of vertebrates, (iii) cone-like photoreceptors of the frontal organ of the frog and of the third eye of the lizard. According to the species and the antibody used, some differences were found at the level of the cellular compartments of the pineal photoreceptor cells.     

The pineal eye inXenopus laevis embryos and larvae: A photoreceptor with a direct excitatory effect on behaviour

Summary Behavioural experiments demonstrate that embryos and young larval stages ofXenopus laevis when exposed to a sudden drop in light intensity may show locomotor activity. Recordings from motoneurone axons demonstrate that the integrity of the pineal eye photoreceptors are essential for this response to occur. Thus the pineal has a direct excitatory effect on behaviour.The pineal eye arises embryologically as a single dorsal vesicular evagination of the diencephalon and anatomically it is very similar to that of other developing amphibians.Recording from the pineal eye using a suction electrode demonstrates that suddenly lowering the light intensity evokes a burst of impulses followed by a raised firing frequency. Conversely, increasing the light intensity leads to a lowered firing frequency. With prolonged exposure to white light at a range of intensities, the frequency of spike discharge is dependent upon the light intensity. The pineal eye can therefore act as a luminance detector.The pineal photoreceptors are most sensitive to light of a wavelength near 520 nm, this probably enables maximum sensitivity to the wavelengths of light that penetrate the freshwater environment.The possible role of the pineal eye in controlling locomotor activity is discussed.Abbreviations HRP horse raddish peroxidase - SEM scanning electron microscopy     

The gene for retinal S-antigen (48-kDa protein) maps to the centromeric portion of mouse chromosome 1 near Idh-1

S-antigen (48-kDa protein) is a soluble protein of the retina and the pineal gland that is believed to play an important role in the visual process. S-antigen is involved in the regulation of the activity of rod photoreceptor-specific cGMP-phosphodiesterase (cGMP-PDE). The activity of this enzyme has been shown to be deficient in the retina of the rd mouse, which is affected by an autosomal recessive disease characterized by degeneration of the photoreceptor cells. The abnormal cGMP-PDE activity could result from, among other things, a lesion in the enzyme itself or in any of the proteins that regulate it, such as the S-antigen. We have used a mouse cDNA clone for the S-antigen to map the corresponding gene, Sag, to mouse chromosome 1 near Idh-1. Since the rd gene is located on mouse chromosome 5, our results suggest that Sag is not the site of the rd mutation.     

Pertussis toxin blocks melatonin-induced pigment aggregation inXenopus dermal melanophores

Summary The molecular mechanism of action for the pineal hormone melatonin was explored by testing melatonin interaction with the components of the hormone-sensitive adenylate cyclase complex in aXenopus dermal melanophore bioassay. Forskolin was employed to stimulate melanosome dispersion. The ability of melatonin to reverse forskolin-stimulated pigment dispersion was assessed, as was the effect of pertussis toxin on the ability of melatonin to aggregate dispersed pigment.Forskolin elicited dispersal of melanosomes in a dose dependent manner (EC₅₀=12 nM) in meninges from stage 52–56 tadpoles ofXenopus laevis. Maximal pigment dispersion was obtained with 100 nM forskolin. Melatonin reversed this effect of forskolin (EC₅₀=1.5 nM), causing pigment aggregation. Pertussis toxin blocked the melatonin-induced aggregation (EC₅₀=358 ng/ml).Prior treatment of the melanophore containing meningeal explants with pertussis toxin results in blockade of melatonin induced pigment aggregation. A 41 kDa pertussis toxin substrate is found in explant homogenates treated with³²P-NAD and pertussis toxin. The availability of this substrate is reduced by prior treatment of intact explants with pertussis toxin and depletion of melatonin responsiveness corresponds to depletion of the 41 kDa substrate. Together, these data suggest that melatonin action upon amphibian dermal melanosomes is mediated by a system requiring a protein similar to the regulatory protein Ni used by mammalian cells to mediate the action of hormones which inhibit adenylate cyclase through a cell surface receptor.Abbreviations MI melanophore index - MSH melanocyte stimulating hormone - FCS fetal calf serum     

Interaction and developmental activation of two neuroendocrine systems that regulate light‐mediated skin pigmentation

Lower vertebrates use rapid light‐regulated changes in skin colour for camouflage (background adaptation) or during circadian variation in irradiance levels. Two neuroendocrine systems, the eye/alpha‐melanocyte‐stimulating hormone (α‐MSH) and the pineal complex/melatonin circuits, regulate the process through their respective dispersion and aggregation of pigment granules (melanosomes) in skin melanophores. During development, Xenopus laevis tadpoles raised on a black background or in the dark perceive less light sensed by the eye and darken in response to increased α‐MSH secretion. As embryogenesis proceeds, the pineal complex/melatonin circuit becomes the dominant regulator in the dark and induces lightening of the skin of larvae. The eye/α‐MSH circuit continues to mediate darkening of embryos on a black background, but we propose the circuit is shut down in complete darkness in part by melatonin acting on receptors expressed by pituitary cells to inhibit the expression of pomc, the precursor of α‐MSH.     

Sensory cells of the “rod-” and “cone-type” in the pineal organ of Rana esculenta,as revealed by immunoreaction against opsin and by the presence of an oil (lipid) droplet

Summary The pineal organ of the frog, Rana esculenta, was studied by use of light- and electron-microscopic methods including immunoreaction against opsin. Most of the morphologically classified cone-type outer segments of the pineal photoreceptors reacted with antisera against opsin of the bovine retina that is dominated by rods. Some of the outer segments of pineal photoreceptor cells remained unstained in accord with the reference tissue, the frog retina, where generally the rods were opsin-positive and most of the cones opsin-negative.The opsin-negative outer segments of pineal photoreceptors were found in continuity with inner segments each containing a large oil (lipid) droplet. These oil droplets stained intensely with osmic acid, Sudan III, Sudan Black B or Scharlach R in cryostat sections, and were soluble in lipid solvents. In ultrathin sections of osmicated material, the oil droplets were homogeneous and of varying electron density. Approximately one tenth of the pineal photoreceptors contained oil droplets and at the same time possessed opsin-immunonegative outer segments.Since in the retina oil droplets and a negative immunoreaction against bovine opsin are characteristic of cones, we suggest that in the pineal organ they also mark conetype photoreceptors scattered among rod-type photo-receptors, the latter displaying a positive immunoreaction with the antisera used.Support from the Deutsche Forschungsgemeinschaft (Ok 1/25-3) is gratefully acknowledged     

Long-term effects of constant light or darkness on chicken pineal hydroxyindole-O-methyltransferase expression: biochemical and cellular aspects

1. Chickens kept in constant light, as opposed to constant darkness, display a twofold increase in the activity of pineal hydroxyindole-O-methyltransferase (HIOMT), the last acting enzyme in the melatonin pathway. 2. Using an immunological approach, we presently show that this regulation of HIOMT activity reflects changes in the concentration of a single molecular form of the enzyme protein (a 38 kDa polypeptide). Immunohistofluorescence indicates that these concentration changes concurrently affect modified photoreceptors and pinealocyte-like cells in the chicken pineal organ. 3. Together, the present data support the hypothesis that environmental lighting might regulate the expression of the HIOMT gene.     

The pineal gland in wild-type and two zebrafish mutants with retinal defects

Light and transmission electron microscopy were used to characterize the ultrastructural features of the pineal glands of wild-type and two mutant zebrafish strains that have retinal defects. Particular attention was given to the pineal photoreceptors. Photoreceptors in the pineal gland appear quite similar to retinal cone photoreceptors, having many of the same structural characteristics including outer segment disk membranes often confluent with the plasma membrane, calycal processes surrounding the outer segments, and classic connecting cilia. The pineal photoreceptor terminals differ from photoreceptor terminals in the retina in that they have short synaptic ribbons and make dyad synapses which may or may not be invaginated. Pineal photoreceptors in two zebrafish mutants with abnormal retinal photoreceptors were also studied. Pineal photoreceptors in the niezerka (nie) mutant degenerate, as they do in the retina, indicating that pineal and retinal photoreceptors share at least some genes. However, the synaptic terminals of no optokinetic response c (nrc) pineal photoreceptors are normal, suggesting that this mutation is specific to the retina.     

Pineal organ-like organization of the retina in megachiroptean bats

Phylogenetically originated from photoreceptive structures, the pineal organ adapts the organism to circadian and circannual light periodicity of the environment, while the retina develops to a light-based locator. Bats have a nocturnal life and an echolocator orientation presumably modifying the task of photoreception. Looking for morphological basis of the special functions, in the present work we compared the fine structure and immunocytochemistry of the retina and pineal organ in micro- and megacrochiroptean bats. We found that there is a high similarity between the retina and pineal organ in megachiropterans when compared to other species investigated so far. Besides of photoreceptor derived pinealocytes, the pineal organ of both micro- and megachiropterans contain intrapineal neurons and/or ganglionic cells as well as glial cells. Like spherules and pedicles of retinal photoreceptors, axon-type processes of pinealocytes form synaptic ribbon containig terminals. Similar to retinal photoreceptors and neurons, pinealocytes and pineal neurons contain immunoreactive glutamate and aspartate. In addition, excitatory amino acids accumulate in the pineal neurohormonal endings and might have a role in the hormonal (serotonin?) release of the organ. Concerning the structure of the retina the highest similarity to the organization of the pineal organ was found in the megachiroptean fruit eating bats Cynopterus sphinx and Rusettus niloticus. The retina of these species forms folds and crypts in its photoreceptor layer. This organization is similar to the folds of the pineal wall successively developed during evolution. Since a folded photoreceptor layer is not viable for a photolocator screen in decoding two-dimensional images, we suppose that this peculiar organization of the megachiropteran retina is connected to a "pineal-like" photometer task of the eye needed by these species active at night.     

Xenopus Pax-6 and retinal development

We have cloned a homolog of Pax-6 in Xenopus laevis. Its deduced amino acid sequence has a 95% overall identity with Pax-6 homologs in other vertebrates. It is expressed early in development in cells fated to form the eye and parts of the forebrain, hindbrain, and spinal cord. It has two phases of expression in the eye. In the early phase, from stage 12.5 to stage 33/34, Xenopus Pax-6 is expressed throughout the developing retina. In the late phase, after stage 33/34, it is excluded from mature cells in the outer half of the retina and from cells in the ciliary marginal zone, remaining only in amacrine and ganglion cells. Misexpression of Pax-6 early in development results in axial defects, but no specific eye phenotype is observed. Targeted misexpression in the retina at later stages does not result in any significant bias toward formation of amacrine or ganglion cells or away from photoreceptors. Ectopic expression of the proneural gene NeuroD alters the pattern of Pax-6, substantially reducing its expression in the eye field and later reducing or eliminating the eye itself. Our results show that Pax-6 expression appears to be necessary, but not sufficient, for eye formation in Xenopus. © 1997 John Wiley & Sons, Inc. J Neurobiol 32, 45–61, 1997.     

Avian Melatonin Synthesis: Photic and Circadian Regulation of Serotonin N-Acetyltransferase mRNA in the Chicken Pineal Gland and Retina

Abstract: The circadian rhythms in melatonin production in the chicken pineal gland and retina reflect changes in the activity of serotonin N -acetyltransferase (arylalkylamine N -acetyltransferase; AA-NAT; EC 2.3.1.87). Here we determined that the chicken AA-NAT mRNA is detectable in follicular pineal cells and retinal photoreceptors and that it exhibits a circadian rhythm, with peak levels at night. AA-NAT mRNA was not detected in other tissues. The AA-NAT mRNA rhythm in the pineal gland and retina persists in constant darkness (DD) and constant lighting (LL). The amplitude of the pineal mRNA rhythm is not decreased in LL. Light appears to influence the phase of the clock driving the rhythm in pineal AA-NAT mRNA in two ways: The peak is delayed by ∼6 h in LL, and it is advanced by >4 h by a 6-h light pulse late in subjective night in DD. Nocturnal AA-NAT mRNA levels do not change during a 20-min exposure to light, whereas this treatment dramatically decreases AA-NAT activity. These observations suggest that the rhythmic changes in chicken pineal AA-NAT activity reflect, at least in part, clock-generated changes in mRNA levels. In contrast, changes in mRNA content are not involved in the rapid light-induced decrease in AA-NAT activity.     

Nerve-independent DNA synthesis and mitosis in regenerating hindlimbs of larval Xenopus laevis

Summary Xenopus laevis larvae at stage 52–53 (according to Nieuwkoop and Faber 1956) were subjected to amputation of both limbs at the thigh level as well as to repeated denervations of the right limb. Results obtained in larvae sacrificed during wound healing (1 after amputation), blastema formation (3 days) and blastema growth (5 and 7 days) showed that denervated right limbs have undergone the same histological modifications observed in innervated left limbs and have formed a regeneration blastema consisting of mesenchymal cells with a pattern of DNA synthesis and mitosis very similar to that in presence of nerves. Also, the patterns of cellular density in regenerating right and left limbs were very similar. On the whole, the data here reported show a highly remarkable degree of nerve-independence for regeneration in hindlimbs of larval Xenopus laevis at stage 52–53 and lend some substance to the hypothesis that, in early limbs, there would exist trophic factors capable of replacing those released by nerves, promoting DNA synthesis and mitosis in blastemal cells. Offprint requests to: S. Filoni