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Biomedical subjects

R D Fernald

Publications and source records attributed to R D Fernald.

At least 19 recordsLinked to original sources
PubMed1992

Retinal growth and cell addition during embryogenesis in the teleost, Haplochromis burtoni.

Neurogenesis of the developing embryonic retina is described for the African cichlid fish, Haplochromis burtoni, from 4 days post fertilization until all cell phenotypes are generated (day 7). Cell addition and differentiation both begin at the same absolute location which later becomes the central retina. As observed in most other vertebrates, cones and ganglion cells differentiate first, followed by amacrine and bipolar cells. Rod photoreceptors, which are added late, differentiate last. Changes in retinal thickness, retinal stretching, cell size, and cell density were measured during development. From day 4 through 7, there is an increase in retinal thickness largely due to the expansion of the inner plexiform layer (IPL) and outer nuclear layer (ONL). The inner nuclear layer (INL) decreases in thickness and there is a transient decrease in the density of cells in the scleral portion of the INL. Cells increase in size in the ganglion cell layer (GCL) and the vitread INL, decrease in size in the sclerad INL, and remain the same in the ONL. Changes in the density of the cell layers were observed: the density of ONL cells increased, the density of GCL cells decreased, and INL cells increased then decreased. From day 4 to day 6, eye growth is entirely due to cell addition because no retinal stretching was observed in the ONL or the horizontal layer. During this same developmental period, the pattern and rate of neurogenesis were measured in the differentiated portion of the retina by means of 3H-thymidine labeling. A small number of cell divisions within the differentiated INL precede the onset of cell divisions in the ONL. The number of 3H-thymidine labeled cells within the INL increases at a low rate consistent with an asymmetric pattern of cell division characteristic of stem cells. In contrast, cell divisions in the ONL increase exponentially, consistent with a symmetric pattern of cell division characteristic of progenitor cells. Double-label experiments (3H-thymidine and a rod specific opsin antibody) show that some of the symmetrically dividing cells in the ONL express the rod specific opsin within 2 days, suggesting that these dividing cells are rod progenitors. Although we do not hae conclusive evidence, these developmental processes support the hypothesis that stem cells within the INL could be the source of rod precursors in the embryonic teleost retina.

Animals↗
PubMed1992

Hypertrophy of gonadotropin releasing hormone-containing neurons after castration in the teleost, Haplochromis burtoni.

In the African cichlid fish, Haplochromis burtoni, males are either territorial or nonterritorial. Territorial males suppress reproductive function in the nonterritorial males, and have larger gonads and larger gonadotropin-releasing hormone- (GnRH) containing neurons in the preoptic area (POA). We describe an experiment designed to establish the causal relationship between large GnRH neurons and large testes in these males by determining the feedback effects of gonadal sex steroids on the GnRH neurons. Territorial males were either castrated or sham-operated, 4 weeks after which they were sacrificed. Circulating steroid levels were measured, and the GnRH-containing neurons were visualized by staining sagittal sections of the brains with an antibody to salmon GnRH. The soma areas of antibody-stained neurons were measured with a computer-aided imaging system. Completely castrated males had markedly reduced levels of circulating sex steroids [11-ketotestosterone (11KT) and testosterone (T)], as well as 17 beta-estradiol (E2). POA GnRH neurons in castrates showed a significant increase in mean soma size relative to the intact territorial males. Hence, in mature animals, gonadal steroids act as a brake on the growth of GnRH-containing neurons, and gonadal products are not responsible for the large GnRH neurons characteristic of territorial males.

Animals↗
PubMed1992

Control of vertebrate retinal cell production.

Regeneration of vertebrate sensory cells can be seen as an extension and elaboration of the process of cellular repair and to understand repair requires knowledge of how cell division and cell fate are determined. To approach these problems, we have developed a slice culture for the teleost retina. Cells continue to divide in the same pattern in this slice culture as they do in vivo as demonstrated with [3H]thymidine labeling. Moreover, cells which divided in culture became retinal cell phenotypes as identified with monoclonal antibodies. Some presumptive rod progenitors in the outer nuclear layer in the center of the retina were also labeled cone-specific, possibly as a regeneration response. These data add to the evidence that cell fate is determined by the environment. This slice preparation will be a useful model system for analyzing putative environmental cues responsible for guiding cell proliferation and differentiation in the fish retina.

Animals↗
PubMed1991

Thin slices of teleost retina continue to grow in culture.

Thin slices of differentiated fish retinas were maintained up to 5 days in culture conditions where they exhibited properties essentially identical to those found in retinas of intact animals. Retinal slices were prepared by embedding eyecups from young fish in agarose and sectioning them on a vibratome. Phenotypic integrity of specific cell types was maintained, as demonstrated by specific antibody staining patterns. Stem cells in the retinal margin and presumptive rod progenitor cells in the outer nuclear layer continued to proliferate in vitro, just as they do in vivo. Some of these cells differentiated in vitro as demonstrated by labelling both cell division and cell phenotype. After several days in culture, some regeneration-like responses were observed, such as growth of neurites and swelling of cell bodies in the ganglion cell layer. This retinal slice preparation appears to offer a unique opportunity for studying the interactions among developing retinal cells.

Animals↗
PubMed1991

Characterization of complementary DNA encoding the precursor for gonadotropin-releasing hormone and its associated peptide from a teleost fish.

Reproductive maturity among male African cichlids Haplochromis burtoni is cued by a series of environmental and social interactions and is mediated physiologically by GnRH. A cDNA clone encoding the precursor for GnRH was isolated from this teleost. The molecular architecture of the predicted prohormone is analogous to that of the previously characterized mammalian forms; however, the predicted sequence of the associated peptide is strikingly different. Attempts to isolate a putative second precursor using low stringency hybridization were not successful despite evidence that a second related decapeptide exists in at least some teleost species.

Amino Acid Sequence↗
PubMed1990

Teleost vision: seeing while growing.

Teleost fish eyes grow throughout life without compromising visual performance of the animal. This is made possible by a set of novel adaptations in the growth and development of the eye. Increased retinal area is achieved both by stretching the existing retina and by generation of new tissue at the retinal germinal zone at the margin of the eye. Rods are added in a fundamentally different fashion than are all other retinal cell types: they appear last as new retina is produced at the margin and they are inserted throughout the functional retina as it stretches. In this way, the animal maintains a constant rod density to preserve vision in low light level. Because the larger eye produces a larger image, visual acuity improves slightly as the animal grows. Adaptations responsible for regulation of retinal growth are analyzed and discussed.

Accommodation, Ocular↗
PubMed1990

Metamorphosis and fish vision.

Many species of fish exhibit metamorphosis in which dramatic external transformations occur as a consequence of coordinated changes in gene expression within an organism. Because postembryonic development and change appears to be the rule rather than the exception in teleost fish species, we view metamorphosis as one of many developmental strategies in fish which have continued plasticity as a common theme. Metamorphic changes are manifested in the visual system by modification of photoreceptor peak sensitivity, rod photoreceptor cell addition, and retinal reorganization. These changes correspond to significant changes in the natural habitat of the animal and in its visual capabilities as demonstrated behaviorally. Thyroxine is the main metamorphic hormone as has also been found in amphibia. The sequence of metamorphic events occur in all teleosts, but they are compressed in time in direct developing animals suggesting that such animals might prove useful for understanding the evolution of metamorphosis in fish. It seems likely that rod photoreceptors may have evolved in conjunction with the change from larval to juvenile stage through metamorphosis in indirect developing fishes. During evolution, the contraction and/or loss of the larval stage has resulted in earlier appearance of rod photoreceptors during development although they always arise later than cone photoreceptors. This ontogenetic developmental sequence supports Walls's (1942) proposal that cones are phylogenetically older than rods and suggests that rods may have evolved several times.

Animals↗
PubMed1990

Social control of neuronal soma size.

Factors responsible for sexual maturation differ significantly among vertebrate species. In many, age is most important, whereas in others seasonal or social cues play a central role. Here we report that maturation in the African cichlid fish Haplochromis burtoni is socially controlled, and that this control includes regulation of soma growth in a population of preoptic neurons immunoreactive to gonadotropin-releasing hormone (irGnRH). Males reared in aquaria among fish of the same age mature sexually in about 3 months, whereas males reared in the presence of older, more aggressive conspecifics remain immature even at 5 months of age. Immature males display hypogonadism and have conspicuously undersized preoptic irGnRH neurons in comparison to those of mature siblings of the same age. This variable maturation rate increases the likelihood that individual males will survive to an age when they can successfully reproduce.

Aggression↗
PubMed1989

Circadian rhythm and light regulate opsin mRNA in rod photoreceptors.

Disk membranes in the outer segment of rod photoreceptors are continuously renewed, being assembled at the outer segment base, displaced outward by new disks and eventually shed at the tip. In lower vertebrates, disk assembly occurs with a diurnal rhythm with 2-4% of the outer segment length produced daily. We have discovered that in toad and fish retinas the level of mRNA for opsin, the most abundant protein in rod disks, fluctuates with a daily rhythm and is regulated both by light and by a circadian oscillator. The mRNA level rises before light onset, remains high during the light phase of a diurnal cycle and decreases four to tenfold during the dark phase. In constant darkness, mRNA elevation occurs during subjective daytime. At night, rod opsin mRNA can be elevated by exposure to light.

Animals↗
PubMed1987

Control of a teleost social signal. I. Neural basis for differential expression of a color pattern.

Territorial male Haplochromis burtoni (Teleostei; Cichlidae) have a dark facial stripe, the 'eyebar', which can appear and disappear within seconds, independently of other coloration patterns. It is used to signal territory ownership and aggressive intent. Some males, called 'barless', have functional melanophores in the eyebar region but never display this pattern, because melanin in eyebar pigment cells is never dispersed. The eyebar melanophores are controlled by a specialized branch of the maxillary nerve. Lesioning the 'eyebar nerve' resulted in immediate melanin dispersion and consequent darkening of the eyebar pattern, and it abolished the normal paling response in all behavioral situations. Nerve lesion produced similar results in both barred and barless males, except that the coloration of the denervated eyebar in barless males was more similar to camouflage markings than to the conspicuous black eyebar used as a social signal. Electrical stimulation of the maxillary nerve produced melanin aggregation. Photoelectric recordings of this paling response revealed no differences between barred and barless males, or between the eyebar and other facial chromatophores that do not function as visual displays. Thus, the difference in the physiological state of eyebar melanophores in intact barred and barless males cannot be explained by differences in peripheral nerve anatomy or physiology.

Aggression↗
PubMed1987

Control of a teleost social signal. II. Anatomical and physiological specializations of chromatophores.

A prominent dark facial stripe, the 'eyebar', is an important component of the reproductive coloration and dominance displays of 'barred' territorial male Haplochromis burtoni (Teleostei; Cichlidae). 'Barless' territorial males are identical to barred, behaviorally and morphologically, except they completely lack an eyebar during agonistic encounters with conspecifics. Both anatomical and physiological differences characterize eyebar pigment cells of barred and barless males. Melanophores and iridophores, which form a single structural and functional unit in the eyebar, contain less pigment in barless males. Physiologically, eyebar melanophores are tonically expanded in barred males, contracted in barless males. This physiological difference is correlated with a difference in the alpha adrenoceptor-mediated aggregation response of the cells in vitro: eyebar melanophores of barless males are significantly more responsive to physiological concentrations of the sympathetic neurotransmitter norepinephrine than those of barred males, and this intermorph difference appears to be unique to the eyebar color pattern. Physiological and morphological characteristics of eyebar pigment cells are functionally related. When barless males first begin to use the eyebar as a social display, the physiological state of melanophores changes from aggregation to dispersion. In subsequent weeks, additional pigment develops, which enhances conspicuousness of the display.

Animals↗
PubMed1986

Development of the optic tract in the cichlid fish Haplochromis burtoni.

In the teleost fish, Haplochromis burtoni, the optic tract is composed of 3 distinct components: the marginal tract, which projects to the optic tectum and is by far the largest, and the axial and medial tracts which project to diencephalic targets. In this paper we report on the normal development of these pathways in larval H. burtoni, an African cichlid fish. The earliest optic tract fibers are found in what will become the marginal optic tract. These fibers hug the wall of the diencephalon in a cohesive bundle. The first fibers in the axial tract location appear on day 5, increasing in number between days 6 and 18. Like marginal tract fibers, axial tract fibers form a cohesive bundle. It is not clear from these experiments whether the first axial tract fibers actually arrive at this location at day 5, or whether they are fibers arriving earlier that were physically displaced from the marginal tract at day 5. Medial tract fibers are not evident until day 6 of development and the number of medial tract fibers also increases as the animal gets older. Unlike fibers in the other two pathways, medial tract fibers do not travel together in a bundle. Rather, each one follows an independent trajectory to its target site. Comparison of this larval development with the adult optic tract organization which we have studied earlier suggests constraints on the mechanisms of axon guidance.

Animals↗
PubMed1985

The organization of the diencephalon and the pretectum in the cichlid fish, Haplochromis burtoni.

Although teleost fish comprise the largest vertebrate radiation, surprisingly little is known about the structure and development of their central nervous systems. Since teleosts are being used much more frequently as model systems in neurobiological research, particularly in understanding visual function, detailed information is needed about central nervous system structures and interconnections. By using the Bodian method with Nissl counterstaining we have analyzed the major nuclei in the diencephalon and pretectum of a cichlid fish, Haplochromis burtoni, which is dependent on vision for its survival. Although our results are broadly comparable with those from other teleost species, there are clear differences, particularly among the visual nuclei. By using animals of a range of sizes to confirm our descriptions we were able to identify possible developmental relationships among several nuclei.

Animals↗
PubMed1985

The organization of retinal projections to the diencephalon and pretectum in the cichlid fish, Haplochromis burtoni.

The organization of retinofugal projections was studied in a cichlid fish by labelling small groups of retinal ganglion cell axons with either horseradish peroxidase or cobaltous lysine. Two major findings resulted from these experiments. First, optic tract axons show a greater degree of pathway diversity than was previously appreciated, and this pathway diversity is related to the target nuclei of groups of axons. The most striking example is the formation of the medial optic tract. Fibers that will become the medial optic tract move abruptly away from their neighbors, at about the level of the optic chiasm, and coalesce at the dorsomedial edge of the marginal optic tract. The medial optic tract projects to the thalamus, the dorsal pretectum, and the deep layer of the optic tectum. The axial optic tract is a group of fibers which segregates from the most medial portion of the marginal optic tract, at about the level of the optic chiasm. The axial tract stays medial to the marginal optic tract for a few hundred microns and then curves laterally to rejoin the marginal optic tract. At least some axial trat axons terminate in the suprachiasmatic nucleus. Within the marginal optic tract, retinal ganglion cell axons from a given retinal quadrant are always segregated into at least two groups. The smaller group projects to the superficial pretectal nucleus. The larger group projects to the superficial layer of the optic tectum. Second, each nontectal retinal termination site receives a unique pattern of retinal input. Within the pretectum the parvocellular superficial pretectal nucleus receives a highly retinotopically organized input from all retinal regions; the basal optic nucleus receives a roughly retinotopically organized input from all retinal regions; the dorsal pretectum receives an input from all retinal regions; and the central pretectal nucleus receives input only from the ventral hemiretina. Within the diencephalon the thalamus receives an input from all retinal regions, but this input is not retinotopically organized; the suprachiasmatic nucleus receives input from the region of central retina that lies just dorsal to the optic nerve head, via the axial optic tract. The accessory optic nucleus receives input from the dorsal hemiretina.

Animals↗
PubMed1985

Spectral sensitivity of the African cichlid fish, Haplochromis burtoni.

Spectral sensitivity of the cichlid fish Haplochromis burtoni was measured under both scotopic and photopic conditions using a two-choice, food reward, operant conditioning paradigm. The highest absolute sensitivity (scotopic) is one quantum for every 5 to 50 rods measured at 475 nm (equivalent to a corneal irradiance of 3.8 x 10(6) Q s-1 cm-2). A P500(1) photopigment apparently mediates spectral sensitivity over most of the visible spectrum; microspectrophotometric studies of rods had previously shown them to contain this photopigment. However, the scotopic behavioral action spectrum shows a sensitivity to short wavelength light higher than is consistent with a P500(1) photopigment alone mediating the scotopic visual process. Determinations made under photopic conditions reveal a behavioral action spectrum broader than that found under scotopic conditions and consistent with mediation by interaction of the three known cone types in an opponent processing manner. The calculated photopic threshold value of approximately 10(4) Q s-1 (receptor)-1 is in agreement with results from other species and corresponds to a corneal irradiance of about 7 x 10(10) Q s-1 cm-2.

Animals↗
PubMed1985

Growth of the visual system in the African cichlid fish, Haplochromis burtoni. Optics.

We have measured the chromatic aberration and optical resolution of the lens of the African cichlid fish, Haplochromis burtoni as a function of growth. We found that in H. burtoni, lenses of all sizes have a longitudinal chromatic aberration of ca 1.9% of the focal length. The resolution of the lens increases as the lens grows larger, presumably because diffraction effects decrease and any imperfections in the lens become relatively less significant. We found the lens in H. burtoni is very nearly diffraction limited so that a small lens (0.59 mm) has a resolution of 289 sec of arc and a large lens (2.90 mm) has a resolution of 66 sec of arc in red light (604 nm). This resolution is nearly ten times greater than can be resolved by the cone matrix.

Animals↗
PubMed1985

Growth of the visual system in the African cichlid fish, Haplochromis burtoni. Accommodation.

We have both measured directly and computed the amplitude of accommodation as a function of fish size to discover how the accommodative process changes during growth in the African cichlid fish, Haplochromis burtoni. We found that accommodation is achieved by lens movement in the pupillary plane along the naso-temporal axis. Accommodative lens movement is thereby directed toward a region of retinal specialization characterized by higher density of all cell types (except rods) in H. burtoni. When relaxed, the temporal retina is focused for near vision and active accommodation through lens movement adjusts the focus in the temporal retina for far vision. As the animal grows, the near and far focus points change appropriately for the fish size.

Accommodation, Ocular↗