Retinal development in the lamprey (Petromyzon marinus L.): premetamorphic ammocoete eye

Unravelling the functional development of vertebrate pathways controlling gaze

Animals constantly redirect their gaze away or towards relevant targets and, besides these goal-oriented responses, stabilizing movements clamp the visual scene avoiding image blurring. The vestibulo-ocular (VOR) and the optokinetic reflexes are the main contributors to gaze stabilization, whereas the optic tectum integrates multisensory information and generates orienting/evasive gaze movements in all vertebrates. Lampreys show a unique stepwise development of the visual system whose understanding provides important insights into the evolution and development of vertebrate vision. Although the developmental emergence of the visual components, and the retinofugal pathways have been described, the functional development of the visual system and the development of the downstream pathways controlling gaze are still unknown. Here, we show that VOR followed by light-evoked eye movements are the first to appear already in larvae, despite their burrowed lifestyle. However, the circuits controlling goal-oriented responses emerge later, in larvae in non-parasitic lampreys but during late metamorphosis in parasitic lampreys. The appearance of stabilizing responses earlier than goal-oriented in the lamprey development shows a stepwise transition from simpler to more complex visual systems, offering a unique opportunity to isolate the functioning of their underlying circuits.

Evolutionary convergence of a neural mechanism in the cavefish lateral line system

Animals can evolve dramatic sensory functions in response to environmental constraints, but little is known about the neural mechanisms underlying these changes. The Mexican tetra, Astyanax mexicanus, is a leading model to study genetic, behavioral, and physiological evolution by comparing eyed surface populations and blind cave populations. We compared neurophysiological responses of posterior lateral line afferent neurons and motor neurons across A. mexicanus populations to reveal how shifts in sensory function may shape behavioral diversity. These studies indicate differences in intrinsic afferent signaling and gain control across populations. Elevated endogenous afferent activity identified a lower response threshold in the lateral line of blind cavefish relative to surface fish leading to increased evoked potentials during hair cell deflection in cavefish. We next measured the effect of inhibitory corollary discharges from hindbrain efferent neurons onto afferents during locomotion. We discovered that three independently derived cavefish populations have evolved persistent afferent activity during locomotion, suggesting for the first time that partial loss of function in the efferent system can be an evolutionary mechanism for neural adaptation of a vertebrate sensory system.

Hagfish to Illuminate the Developmental and Evolutionary Origins of the Vertebrate Retina

The vertebrate eye is a vital sensory organ that has long fascinated scientists, but the details of how this organ evolved are still unclear. The vertebrate eye is distinct from the simple photoreceptive organs of other non-vertebrate chordates and there are no clear transitional forms of the eye in the fossil record. To investigate the evolution of the eye we can examine the eyes of the most ancient extant vertebrates, the hagfish and lamprey. These jawless vertebrates are in an ideal phylogenetic position to study the origin of the vertebrate eye but data on eye/retina development in these organisms is limited. New genomic and gene expression data from hagfish and lamprey suggest they have many of the same genes for eye development and retinal neurogenesis as jawed vertebrates, but functional work to determine if these genes operate in retinogenesis similarly to other vertebrates is missing. In addition, hagfish express a marker of proliferative retinal cells (Pax6) near the margin of the retina, and adult retinal growth is apparent in some species. This finding of eye growth late into hagfish ontogeny is unexpected given the degenerate eye phenotype. Further studies dissecting retinal neurogenesis in jawless vertebrates would allow for comparison of the mechanisms of retinal development between cyclostome and gnathostome eyes and provide insight into the evolutionary origins of the vertebrate eye.

Evolution and development of complex eyes: a celebration of diversity

For centuries, the eye has fascinated scientists and philosophers alike, and as a result the visual system has always been at the forefront of integrating cutting-edge technology in research. We are again at a turning point at which technical advances have expanded the range of organisms we can study developmentally and deepened what we can learn. In this new era, we are finally able to understand eye development in animals across the phylogenetic tree. In this Review, we highlight six areas in comparative visual system development that address questions that are important for understanding the developmental basis of evolutionary change. We focus on the opportunities now available to biologists to study the developmental genetics, cell biology and morphogenesis that underlie the incredible variation of visual organs found across the Metazoa. Although decades of important work focused on gene expression has suggested homologies and potential evolutionary relationships between the eyes of diverse animals, it is time for developmental biologists to move away from this reductive approach. We now have the opportunity to celebrate the differences and diversity in visual organs found across animal development, and to learn what it can teach us about the fundamental principles of biological systems and how they are built.

The stepwise development of the lamprey visual system and its evolutionary implications

Lampreys, which represent the oldest group of living vertebrates (cyclostomes), show unique eye development. The lamprey larva has only eyespot-like immature eyes beneath a non-transparent skin, whereas after metamorphosis, the adult has well-developed image-forming camera eyes. To establish a functional visual system, well-organised visual centres as well as motor components (e.g. trunk muscles for locomotion) and interactions between them are needed. Here we review the available knowledge concerning the structure, function and development of the different parts of the lamprey visual system. The lamprey exhibits stepwise development of the visual system during its life cycle. In prolarvae and early larvae, the 'primary' retina does not have horizontal and amacrine cells, but does have photoreceptors, bipolar cells and ganglion cells. At this stage, the optic nerve projects mostly to the pretectum, where the dendrites of neurons in the nucleus of the medial longitudinal fasciculus (nMLF) appear to receive direct visual information and send motor outputs to the neck and trunk muscles. This simple neural circuit may generate negative phototaxis. Through the larval period, the lateral region of the retina grows again to form the 'secondary' retina and the topographic retinotectal projection of the optic nerve is formed, and at the same time, the extra-ocular muscles progressively develop. During metamorphosis, horizontal and amacrine cells differentiate for the first time, and the optic tectum expands and becomes laminated. The adult lamprey then has a sophisticated visual system for image-forming and visual decision-making. In the adult lamprey, the thalamic pathway (retina-thalamus-cortex/pallium) also transmits visual stimuli. Because the primary, simple light-detecting circuit in larval lamprey shares functional and developmental similarities with that of protochordates (amphioxus and tunicates), the visual development of the lamprey provides information regarding the evolutionary transition of the vertebrate visual system from the protochordate-type to the vertebrate-type.

The evolution of early vertebrate photoreceptors

Meeting the challenge of sampling an ancient aquatic landscape by the early vertebrates was crucial to their survival and would establish a retinal bauplan to be used by all subsequent vertebrate descendents. Image-forming eyes were under tremendous selection pressure and the ability to identify suitable prey and detect potential predators was thought to be one of the major drivers of speciation in the Early Cambrian. Based on the fossil record, we know that hagfishes, lampreys, holocephalans, elasmobranchs and lungfishes occupy critical stages in vertebrate evolution, having remained relatively unchanged over hundreds of millions of years. Now using extant representatives of these 'living fossils', we are able to piece together the evolution of vertebrate photoreception. While photoreception in hagfishes appears to be based on light detection and controlling circadian rhythms, rather than image formation, the photoreceptors of lampreys fall into five distinct classes and represent a critical stage in the dichotomy of rods and cones. At least four types of retinal cones sample the visual environment in lampreys mediating photopic (and potentially colour) vision, a sampling strategy retained by lungfishes, some modern teleosts, reptiles and birds. Trichromacy is retained in cartilaginous fishes (at least in batoids and holocephalans), where it is predicted that true scotopic (dim light) vision evolved in the common ancestor of all living gnathostomes. The capacity to discriminate colour and balance the tradeoff between resolution and sensitivity in the early vertebrates was an important driver of eye evolution, where many of the ocular features evolved were retained as vertebrates progressed on to land.

Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup

Charles Darwin appreciated the conceptual difficulty in accepting that an organ as wonderful as the vertebrate eye could have evolved through natural selection. He reasoned that if appropriate gradations could be found that were useful to the animal and were inherited, then the apparent difficulty would be overcome. Here, we review a wide range of findings that capture glimpses of the gradations that appear to have occurred during eye evolution, and provide a scenario for the unseen steps that have led to the emergence of the vertebrate eye.

Presence of glutamate, glycine, and gamma-aminobutyric acid in the retina of the larval sea lamprey: comparative immunohistochemical study of classical neurotransmitters in larval and postmetamorphic retinas

The neurochemistry of the retina of the larval and postmetamorphic sea lamprey was studied via immunocytochemistry using antibodies directed against the major candidate neurotransmitters [glutamate, glycine, gamma-aminobutyric acid (GABA), aspartate, dopamine, serotonin] and the neurotransmitter-synthesizing enzyme tyrosine hydroxylase. Immunoreactivity to rod opsin and calretinin was also used to distinguish some retinal cells. Two retinal regions are present in larvae: the central retina, with opsin-immunoreactive photoreceptors, and the lateral retina, which lacks photoreceptors and is mainly neuroblastic. We observed calretinin-immunostained ganglion cells in both retinal regions; immunolabeled bipolar cells were detected in the central retina only. Glutamate immunoreactivity was present in photoreceptors, ganglion cells, and bipolar cells. Faint to moderate glycine immunostaining was observed in photoreceptors and some cells of the ganglion cell/inner plexiform layer. No GABA-immunolabeled perikarya were observed. GABA-immunoreactive centrifugal fibers were present in the central and lateral retina. These centrifugal fibers contacted glutamate-immunostained ganglion cells. No aspartate, serotonin, dopamine, or TH immunoreactivity was observed in larvae, whereas these molecules, as well as GABA, glycine, and glutamate, were detected in neurons of the retina of recently transformed lamprey. Immunoreactivity to GABA was observed in outer horizontal cells, some bipolar cells, and numerous amacrine cells, whereas immunoreactivity to glycine was found in amacrine cells and interplexiform cells. Dopamine and serotonin immunoreactivity was found in scattered amacrine cells. Amacrine and horizontal cells did not express classical neurotransmitters (with the possible exception of glycine) during larval life, so transmitter-expressing cells of the larval retina appear to participate only in the vertical processing pathway.

Afferent Connections of the Optic Tectum in Lampreys: An Experimental Study

Tectal afferents were studied in adult lampreys of three species (Ichthyomyzon unicuspis, Lampetra fluviatilis, and Petromyzon marinus) following unilateral BDA injections into the optic tectum (OT). In the secondary prosencephalon, neurons projecting to the OT were observed in the pallium, the subhipoccampal lobe, the striatum, the preoptic area and the hypothalamus. Following tectal injections, backfilled diencephalic cells were found bilaterally in: prethalamic eminence, ventral geniculate nucleus, periventricular prethalamic nucleus, periventricular pretectal nucleus, precommissural nucleus, magnocellular and parvocellular nuclei of the posterior commissure and pretectal nucleus; and ipsilaterally in: nucleus of Bellonci, periventricular thalamic nucleus, nucleus of the tuberculum posterior, and the subpretectal tegmentum, as well as in the pineal organ. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus semicircularis, the contralateral OT, and bilaterally in the mesencephalic reticular formation and inside the limits of the retinopetal nuclei. In the hindbrain, tectal projecting cells were also bilaterally labeled in the dorsal and lateral isthmic nuclei, the octavolateral area, the sensory nucleus of the descending trigeminal tract, the dorsal column nucleus and the reticular formation. The rostral spinal cord also exhibited a few labeled cells. These results demonstrate a complex pattern of connections in the lamprey OT, most of which have been reported in other vertebrates. Hence, the lamprey OT receives a large number of nonvisual afferents from all major brain areas, and so is involved in information processing from different somatic sensory modalities.

Calbindin and calretinin immunoreactivity in the retina of adult and larval sea lamprey

The presence of calretinin and calbindin immunoreactivity is studied in the retina of larval and adult lamprey and their respective distributions are compared. Calretinin distribution is also studied in the retina of transforming stages. Western blot analysis in brain extracts showed a 29-kDa band with both polyclonal anti-calbindin and anti-calretinin antibodies. Calbindin and calretinin immunoreactivity has shown a partially different distribution. In the adult retina large and small bipolar cells, with respectively stratified or diffuse axons, the inner row of horizontal cells and ganglion cells and/or some amacrine cells were labeled with anti-calretinin antibody. The anti-calbindin antibody labels the same cell types except most of ganglion cells, but the label was less conspicuous. Therefore, the possible existence of these two calcium-binding proteins in the central nervous system of the sea lamprey could be discussed. In the differentiated central retina of larval lampreys, numerous calretinin immunoreactive bipolar and ganglion cells were observed, while, in the lateral retina, only ganglion cells were labeled, accordingly with the lack of differentiation of other neural cell types. CR-ir bipolar cells appeared in the retina by the stage 5 of transformation, i.e. about the time when differentiation of photoreceptors occurs. The comparison of the distribution of calretinin and calbindin between adult and larval central retina of lampreys shows striking differences that could be related to the different functionality of eyes in these two stages of the life cycle of lampreys. In addition, this is the first report on the presence of calcium-binding proteins in the larval and transforming lamprey retina, on the presence of calretinin- and calbindin-immunoreactive horizontal cells in adult lamprey retinas and on the differential stratification of bipolar cell terminals.

Early development of the retina and pineal complex in the sea lamprey: comparative immunocytochemical study

Lampreys have a complex life cycle, with largely differentiated larval and adult periods. Despite the considerable interest of lampreys for understanding vertebrate evolution, knowledge of the early development of their eye and pineal complex is very scarce. Here, the early immunocytochemical organization of the pineal complex and retina of the sea lamprey was studied by use of antibodies against proliferating cell nuclear antigen (PCNA), opsin, serotonin, and gamma-aminobutyric acid (GABA). Cell differentiation in the retina, pineal organ, and habenula begins in prolarvae, as shown by the appearance of PCNA-negative cells, whereas differentiation of the parapineal vesicle was delayed until the larval period. In medium-sized to large larvae, PCNA-immunoreactive (-ir) cells were numerous in regions of the lateral retina near the differentiated part of the larval retina (central retina). A late-proliferating region was observed in the right habenula. Opsin immunoreactivity appears in the pineal vesicle of early prolarvae and 3 or 4 days later in the retina. In the parapineal organ, opsin immunoreactivity was observed only in large larvae. In the pineal organ, serotonin immunoreactivity was first observed in late prolarvae in photoreceptive (photoneuroendocrine) cells, whereas only a few of these cells appeared in the parapineal organ of large larvae. No serotonin immunoreactivity was observed in the larval retina. GABA immunoreactivity appeared earlier in the retina than in the pineal complex. No GABA-ir perikaryon was observed in the retina of larval lampreys, although a few GABA-ir centrifugal fibers innervate the inner retina in late prolarvae. First GABA-ir ganglion cells occur in the pineal organ of 15-17 mm larvae, and their number increases during the larval period. The only GABA-ir structures observed in the parapineal ganglion of larvae were afferent fibers, which appeared rather late in development. The time sequence of development in these photoreceptive structures is rather different from that observed in teleosts and other vertebrates. This suggests that the unusual development of the three photoreceptive organs in lampreys reflects specialization for their different functions during the larval and adult periods.

Review of larval and postlarval eye ultrastructure in the lamprey (Cyclostomata) with special emphasis on Geotria australis (Gray)

The literature dealing with the lateral eye in lampreys is briefly reviewed here. While there appears to be no longer much doubt that the short and long photoreceptor cells in the lamprey eye correspond to rods and cones, questions of dark/light adaptational changes, the nature of visual pigments, and the roles of retinal serotonin and melatonin need to be re-addressed. Eyes of the larval and postlarval ("macrophthalmia") stages of the lamprey Geotria australis were examined by electron microscopy and it was found that the larval retina is largely undifferentiated except for a small central zone surrounding the optic nerve head. The retina of the postlarval stage is fully differentiated and the photoreceptor outer segments undergo renewal, which appears to involve the retinal pigment epithelium (RPE). The distribution of larval RPE and choroidal pigments, postlarval ganglion cells, and the orientation of scleral collagen are unusual for vertebrates. No obvious positional or size differences of any retinal cell type were apparent between day- and night-fixed specimens.

Development and organization of the ocular motor nuclei in the larval sea lamprey, Petromyzon marinus L.: an HRP study

Retrograde transport of horseradish peroxidase (HRP) after its application into the orbit was used to investigate the development of the different ocular motor nuclei in larvae of the sea lamprey (Petromyzon marinus) and to identify their regions of origin. In the smallest larvae studied (10-19 mm in length), the oculomotor and abducens neurons were ipsilateral to the site of HRP application, whilst trochlear neurons were contralateral. These motoneurons did not have dendritic processes. In larvae more than 19 mm in length, both ipsilateral and contralateral components were found in the oculomotor and trochlear nuclei; dendrites were present, and their length and branching increased with larval age. An adult-like pattern of topographic organization and dendritic arborization was reached in larvae of about 45-60 mm in length. In oculomotor neurons, medial dendrites appear first, then dorsolateral dendrites, and finally ventral dendrites. Similarly, in trochlear neurons ventral and ventrolateral dendrites develop first, followed by dorsal dendrites that course either to the caudal optic tectum or to the terminal fields of the octaval and lateral line nerves in the cerebellar plate. Dorsal and ventral dendrites of the abducens neurons arise at the same time, but dorsal dendrites attain an adult-like morphology earlier. A few motoneurons showed ventricular attachments in larvae longer than 40 mm. The significance of these processes and their possible usefulness as a marker for the regions of origin of the ocular motor nuclei are discussed. Finally, the results presented here indicate that differentiation of the ocular motor nuclei in larval lampreys precedes and is independent of the maturation of the eye at transformation.

Organization of the visual system in larval lampreys: an HRP study

The organization of the visual system of larval lampreys was studied by anterograde and retrograde transport of HRP injected into the eye. The retinofugal system has two different patterns of organization during the larval period. In small larvae (less than 60-70 mm in length) only a single contralateral tract, the axial optic tract, is differentiated. This tract projects to regions in the diencephalon, pretectum, and mesencephalic tegmentum. In larvae longer than 70-80 mm, there is an additional contralateral tract, the lateral optic tract, which extends to the whole tectal surface. In addition, ipsilateral retinal fibers are found in both small and large larvae. Initially, the ipsilateral projection is restricted to the thalamus-pretectum, but it reaches the optic tectum in late larvae. Changes in the organization of the optic tracts coincide with the formation of the late-developing retina and consequently, the origin of the optic tracts can be related to specific retinal regions. The retinopetal system is well developed in all larvae. Most retinopetal neurons are labeled contralaterally and are located in the M2-M5 nucleus of the mesencephalic tegmentum, in the caudolateral mesencephalic reticular area and adjacent ventrolateral portions of the optic tectum. Dendrites of these cells are apparent, especially those directed dorsally, which in large larvae extend to the optic tectum overlapping with the retino-tectal projection. These results indicate that in lampreys, visual projections organize mainly during the blind larval period before the metamorphosis, their development being largely independent of visual function.

The developing visual system and metamorphosis in the lamprey

Metamorphosis of the sea lamprey, Petromyzon marinus, is a true metamorphosis. The larval lamprey is a filter-feeder who dwells in the silt of freshwater streams and the adult is an active predator found in large lakes or the sea. The transformation usually occurs in the fifth or sixth year of life. Enlargement of the eye has been long accepted as a distinctive indication of metamorphosis in the sea lamprey, but it had been thought that this was because eye development in the larva was arrested after the formation of only the small central region. Recent studies indicate that all of the retina begins its development in the larva and that ganglion, amacrine, and horizontal cells differentiate in the peripheral retina of the larva. Retinal development is arrested during the premetamorphic period, to be resumed during metamorphosis. Metamorphic contributions include the differentiation of photoreceptor and bipolar cells. With the early appearance of ganglion cells, retinal pathways to the thalamus and tectum are established in larvae, as is a centripetal pathway. Tectal development spans the larval period but a spurt in tectal growth and differentiation is correlated with the completion of the retinal circuitry late in metamorphosis. The metamorphic changes in retina and tectum complete the functional development of the visual system and provide for the adult lamprey's predatory and reproductive behavior.

Ganglion cells and retinopetal fibers of the larval lamprey retina: an HRP ultrastructural study

The ultrastructure of ganglion cells and centrifugal fibers of the larval lamprey retinas were studied using horseradish peroxidase (HRP) as a marker. Larval ganglion cells were found both in the inner nuclear layer and the inner plexiform layer of the differentiated retina, and also were present in the undifferentiated retina. Direct photoreceptor-ganglion cell contacts and the presence of centrifugal fibers are described for the first time in the lamprey. The centrifugal fibers contact directly with ganglion cells in this species.

Neural differentiation in the retina of the larval sea lamprey (Petromyzon marinus)

The peripheral retina of the sea lamprey develops in a 5-year-long process in which only certain neurons differentiate each year. The growth of cell layers, the differentiation of the neurons, and the morphology of their dendrites and axons were studied with normal, HRP, and Golgi preparations. Ganglion cells are differentiated in 3-year-old larvae, amacrine and horizontal cells in 4-year-old larvae, photoreceptor cells in stage I transformers, and bipolar cells in stage III transformers. Each new development is expressed as a radial gradient of differentiation. As a result of this protracted and stepped process, lamprey retinal neurons, particularly ganglion cells, differentiate in the absence of other cells to which they will ultimately be connected and may express their individual genetic programs more fully than in other vertebrate retinas. This could account for the unusual relationship of the ganglion cell, inner plexiform, and optic nerve layers and for the very high ratio of displaced to orthotopic ganglion cells.

Centrifugal innervation of the lamprey retina. Light- and electron microscopic and electrophysiological investigations

Centrifugal fibers and their synaptic connections were studied in retinas of the lamprey Lampetra fluviatilis. The morphological analysis of retinofugal and retinopetal elements was performed after their horseradish peroxidase (HRP) filling through either the cut optic nerve in isolated retina preparations or after intracerebral HRP injections. In flat-mounted retinas, labeled ganglion cell bodies with their dendritic arborizations as well as centrifugal axons were found. The topography of labeled ganglion cell bodies and fibers in semi-thin plastic sections is described. The electron microscopic analysis revealed that the centrifugal terminals synapse either upon unlabeled somata or profiles containing synaptic vesicles (PCSVs). In more rare cases these boutons seem to establish synaptic contacts on ganglion cell dendrites. The target cell bodies were located within the inner part of the inner nuclear layer, whereas postsynaptic dendrites and PCSVs were mainly observed in the outer portion of the internal synaptic layer. Stimulation of the optic nerve in isolated retinas produced antidromic responses in 23 neurons and in 9 of these cells, an antidromic spike was followed by a postsynaptic potential (PSP). Ten cells yielded no antidromic response, but showed PSPs sometimes associated with spikes. The morphological and physiological evidence obtained indicate that these PSP-generating cells were activated synaptically by centrifugal fibers and that in the lamprey retina, these fibers make contacts either with dendrites or somata of amacrine cells and probably with ganglion cell dendrites.