E A Schmitt

Harvard University, Cambridge, MA, United States

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Publications (12)60.33 Total impact

  • E A Schmitt, J E Dowling
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    ABSTRACT: The morphological differentiation of the zebrafish retina was analyzed by using light (LM) and transmission electron (TEM) microscopy between the time of initial ganglion cell differentiation (approximately 32 hours postfertilization; hpf) and shortly after the point when the retina appears functional (approximately 74 hpf), i.e., when all major cell types and basic synaptic connections are in place. The results show that the inner retinal neurons, like the photoreceptor and ganglion cells, differentiate first within the ventronasal region, and differentiation subsequently spreads asymmetrically into the nasal and dorsal regions before reaching the ventrotemporal retina. In addition, we show that the attenuation of the optic stalk occurs in parallel with ganglion cell differentiation between 32 and 40 hpf. The first conventional synapses appear within the inner plexiform layer simultaneously with the first photoreceptor outer segment discs at 60 hpf; functional ribbon triads arise within photoreceptor synaptic terminals at 65 hpf; and synaptic ribbons occur within bipolar cell axon terminals at the time larvae exhibit their first visual responses (approximately 70 hpf). Although development is initially more advanced within the ventronasal region between 50 and 60 hpf, development across the retina rapidly equilibrates such that it is relatively comparable within all quadrants of the central retina by 70 hpf. An area within the temporal retina characterized by tightly packed and highly tiered cones emerges with subsequent development. Retinal differentiation in the zebrafish corresponds with that generally described in other vertebrates and can be correlated with the development of visual and electroretinographic responses in the animal.
    The Journal of Comparative Neurology 03/1999; 404(4):515-36. · 3.66 Impact Factor
  • Ellen A. Schmitt, John E. Dowling
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    ABSTRACT: The morphological differentiation of the zebrafish retina was analyzed by using light (LM) and transmission electron (TEM) microscopy between the time of initial ganglion cell differentiation (≈32 hours postfertilization; hpf) and shortly after the point when the retina appears functional (≈74 hpf), i.e., when all major cell types and basic synaptic connections are in place. The results show that the inner retinal neurons, like the photoreceptor and ganglion cells, differentiate first within the ventronasal region, and differentiation subsequently spreads asymmetrically into the nasal and dorsal regions before reaching the ventrotemporal retina. In addition, we show that the attenuation of the optic stalk occurs in parallel with ganglion cell differentiation between 32 and 40 hpf. The first conventional synapses appear within the inner plexiform layer simultaneously with the first photoreceptor outer segment discs at 60 hpf; functional ribbon triads arise within photoreceptor synaptic terminals at 65 hpf; and synaptic ribbons occur within bipolar cell axon terminals at the time larvae exhibit their first visual responses (≈70 hpf). Although development is initially more advanced within the ventronasal region between 50 and 60 hpf, development across the retina rapidly equilibrates such that it is relatively comparable within all quadrants of the central retina by 70 hpf. An area within the temporal retina characterized by tightly packed and highly tiered cones emerges with subsequent development. Retinal differentiation in the zebrafish corresponds with that generally described in other vertebrates and can be correlated with the development of visual and electroretinographic responses in the animal. J. Comp. Neurol. 404:515–536, 1999. © 1999 Wiley-Liss, Inc.
    The Journal of Comparative Neurology 02/1999; 404(4):515 - 536. · 3.66 Impact Factor
  • E A Schmitt, G A Hyatt, J E Dowling
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    ABSTRACT: We report here a reexamination of the developmental expression of cone opsins in the zebrafish retina. The red- and blue-sensitive opsins appear at 51 h postfertilization (hpf) whereas ultraviolet (UV) opsin is not seen until after 55 hpf. More cells show red cone opsin expression than blue at 51 and 55 hpf, indicating the sequence of cone opsin expression in zebrafish is first red, then blue, and finally UV. Curiously, morphological development of the cones is in reverse order; UV cones appear quite mature by day 6-7 postfertilization (pf), but morphologically, red cones do not appear adult-like until 15-20 days pf.
    Visual Neuroscience 01/1999; 16(3):601-5. · 1.48 Impact Factor
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    ABSTRACT: Determination of cell fate in the vertebrate retina has been shown to be largely independent of lineage. After cell fates are determined, retinal neurons become organized in a precise laminar pattern. The mechanisms for this patterning could involve morphogens distributed in gradients or, alternatively, direct cell-cell interactions. In the zebrafish mutant cyclops (Cyc(b16)), most embryos have two partial retinas joined in the ventral midline. This presents developing retinal cells near the midline with abnormal cellular environments, whereas laterally the pattern of developing cells is normal. We examined the consequences of this for patterning in the mutant's retina. We found that the retinas are joined in the midline at the apical surfaces of the photoreceptor layers. A laminar pattern emerges in the midline that preserves normal positional relationships between retinal cell types locally but is abnormal with respect to patterning over the entire retina. Lateral to the midline, retinal patterning appears normal. Metabolic labeling experiments showed that late rounds of DNA synthesis precede the emergence of the novel pattern in this midline region. We conclude that these observations in the cyclops mutant are compatible with mechanisms of pattern formation in the retina involving local cell interactions.
    The Journal of Comparative Neurology 06/1997; · 3.66 Impact Factor
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    ABSTRACT: Application of exogenous retinoic acid (RA) to zebrafish during the initial stages of photoreceptor differentiation results in a precocious development of rod photoreceptors and an inhibition of cone photoreceptor maturation. The acceleration of rod differentiation is observed initially within the ventral retina 3 days after fertilization, following 24 hr of RA application, and within the dorsal retina 4 days after fertilization, following 48 hr of RA application. The differentiation of rods was impeded significantly when the synthesis of endogenous retinoic acid was inhibited by citral prior to the initial stage of rod differentiation. RA-treated embryos labeled for bromodeoxyuridine (BrdU) uptake revealed that RA exerts its effect on a postmitotic cell population within the developing retina. During normal development in zebrafish, rod differentiation is most robust within the ventral retina, a region previously shown to be rich in RA. Our data suggest that the RA signaling pathway is involved in the differentiation and maturation of both the rod and cone photoreceptors within the developing zebrafish retina.
    Proceedings of the National Academy of Sciences 12/1996; 93(23):13298-303. · 9.81 Impact Factor
  • E A Schmitt, J E Dowling
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    ABSTRACT: Earlier studies suggested retinal differentiation in the zebrafish commences ventrally rather than centrally as is the case in other vertebrates. Here we describe the topographical spread of cell differentiation for ganglion cells, double cones and rods in the zebrafish retina between 36 and 72 hours postfertilization (hpf), by using immunohistochemical markers in retinal wholemounts. Staining for all three cell types commenced within the ventral retina on the nasal side of the optic nerve and choroid fissure, at 38 hpf for ganglion cells and 50 hpf for double cones and rods. Within 3 to 4 hours, the staining of ganglion cells and double cones spread in a continuous wave-like fashion into the nasal region of the ventral retina. After this time, the staining patterns for ganglion cells and double cones progressed dorsally into the central and temporal retina. Finally, stained somata of ganglion cells were observed within the temporal-ventral region by approximately 48 hpf, more than 8 hours later than the first ganglion cells within the nasal retina. The topographical spread of double cone staining was slightly less orderly. After staining had extended into the nasal retina between 50 and 54 hpf, a small group of stained double cones often appeared at the temporal edge of the choroid fissure by 56 hpf, simultaneously with initial staining observed dorsal and temporal to the optic nerve. The topographical spread of rod staining in the ventral retina was more symmetrical. After rod staining appeared near the nasal edge of the choroid fissure at 50 hpf, rods accumulated within a localized patch nasal to the fissure. Approximately 5 hours after initial rod staining, scattered rod staining appeared on the temporal side of the choroid fissure (approximately 55-57 hpf). Rods increased rapidly within the ventral retina, and a dense symmetrical patch extended out from the choroid fissure into the nasal and temporal regions of the ventral retina by 70 hpf. A scattered pattern of rod staining also occurred within the dorsal retina at this time.
    The Journal of Comparative Neurology 08/1996; 371(2):222-34. · 3.66 Impact Factor
  • E A Schmitt, J M Fadool, J E Dowling
    Investigative Ophthalmology &amp Visual Science 05/1996; 37(5):695. · 3.44 Impact Factor
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    ABSTRACT: The developing eye is known to be rich in retinoic acid (RA), and perturbations in RA levels during formation of the optic primordia, as well as RA receptor mutations, cause retinal malformations, especially in ventral eye regions. To test the hypothesis that RA plays a role in the establishment of ventral retinal characteristics, we examined several dorsal and ventral ocular markers in RA-treated zebrafish. The optic stalk represents the ventral-most region of the early eye field. During normal development, the optic stalks constrict, decreasing in width and are gradually replaced by the optic nerve. Systemic high RA levels cause an expansion in the optic stalk with an increased cell content and a patent lumen. In addition, the stalks do not constrict and persist into later stages of development indicating an enhancement of early ventral eye characteristics. Expression of the transcription factor pax[b], normally confined to the ventral retina, expands into the dorsal retina following RA treatment, whereas msh[c], normally expressed in the dorsal retinal pole, disappears. Activity of an aldehyde dehydrogenase that normally occupies the dorsal third of the retina is reduced or abolished following high systemic RA. When a localized RA source, an RA-soaked bead, is placed next to the developing eye, a fissure resembling the choroid fissure appears in the eye facing the bead. Taken together, these observations suggest that RA is involved in the determination of the ventral retina.
    Development 02/1996; 122(1):195-204. · 6.21 Impact Factor
  • J Robinson, E A Schmitt, J E Dowling
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    ABSTRACT: In zebrafish, the first class of cone photoreceptor to become morphologically distinct is the ultraviolet-sensitive short single cone, at 4 days postfertilization, whereas the last class, the red- and green-sensitive double cone, becomes distinct at 10 days postfertilization. We have examined the time course of visual pigment gene expression in zebrafish using whole-mount in situ hybridization. Within the retina, opsins may be detected as early as 40 h postfertilization with the ultraviolet and rod visual pigments being expressed before the blue- (48 h) and red- (60 h) sensitive pigments. In the pineal, red-sensitive opsin is expressed at 48 h postfertilization. Visual pigment expression provides a useful tool for investigations of early cell fate in zebrafish.
    Visual Neuroscience 01/1995; 12(5):895-906. · 1.48 Impact Factor
  • E A Schmitt, J E Dowling
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    ABSTRACT: Early eye morphogenesis in the zebrafish between 12 and 36 hours postfertilization was studied by light- and scanning electron microscopy. Overall, early eye morphogenesis in the zebrafish is similar to that of other vertebrates even though the optic primordia evaginate from the forebrain as solid masses of cells. After initial evagination (6-7 somite stage [SS]), the optic primordia take on a wing-like shape (8-9 SS). Subsequently, they bend ventrally and rotate slightly in an anterior direction (10-12 SS). These changes serve to bring the primordia from a horizontal to a more vertical orientation in relation to the embryonic neural axis. Invagination commences from the center of each primordium (14 SS) and progresses symmetrically out towards the periphery (14-20 SS). The choroid fissure forms by an involution along the anterior region of the eyecup (18-20 SS). By 24 hours postfertilization (pf), the eyecups are well formed. Between 24 and 36 hours pf, the eyes rotate further in relation to the axis of the embryo, and this repositions the choroid fissue to a typical ventral location by 36 hours pf. Because of the two rotations of the eye during early morphogenesis, particularly the later one, the anterior-posterior orientation of the emerging optic primordium ultimately becomes the ventral-dorsal axis of the completed eyecup.
    The Journal of Comparative Neurology 07/1994; 344(4):532-42. · 3.66 Impact Factor
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    ABSTRACT: In many vertebrates, UV-sensitive photoreceptors have been identified by microspectrophotometry and UV-visual sensitivity has been identified by behavioral studies, but as yet no vertebrate UV-sensitive pigment gene has been isolated. We have sequenced a cDNA clone that hybridizes to short single cone cells in the zebrafish (Brachydanio rerio). These cells, which make up 25% of the cone population in zebrafish retinae, are UV-sensitive (lambda max approximately 360 nm). The visual pigment encoded by this gene is unusual in that its amino acid sequence is more homologous to the rod pigment rhodopsin (up to 89%) than to other cone pigments (35-83%). Like all other vertebrate visual pigments, it contains a lysine residue at position 296, the presumptive retinal binding site, and a glutamate residue at position 113. However, it is unique in possessing a lysine residue at position 126, which may account for the UV-sensitivity of the pigment.
    Proceedings of the National Academy of Sciences 08/1993; 90(13):6009-12. · 9.81 Impact Factor
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    ABSTRACT: Exogenous treatment of zebrafish embryos with retinoic acid induces a duplication of the retinas during development. These effects occur only when retinoic acid is applied within a 2-hr period prior to and during the initial formation of the optic primordia, and they are concentration-dependent. Light microscopic examination reveals that the second retina derives from cells in the ventral region of the developing eyecup that normally become pigment epithelial cells. Two distinct ganglion cell fields are usually observed in eyes with duplicated retinas. Bundles of axons from each ganglion cell field join as they leave the eye and innervate the contralateral tectum.
    Proceedings of the National Academy of Sciences 10/1992; 89(17):8293-7. · 9.81 Impact Factor