Embryonic development of the nucleus isthmo-opticus in the chick: a Golgi and electron microscopic study

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Fast retrograde effects on neuronal death and dendritic organization in development: the role of calcium influx

Retrograde signals from axon terminal to cell body are known to regulate neuronal survival and differentiation during development. They are generally attributed to the uptake and transport of trophic factors, but there is recent evidence in the isthmo-optic nucleus for a remarkably fast-acting retrograde signal from the contralateral retina that is not mediated by the conventional trophic route. The isthmo-optic nucleus undergoes 55% neuron death between embryonic days 12 and 17, and becomes laminated at embryonic day 14 owing to dendritic re-organization. Blockade of retinal electrical activity just before day 14 reduces neuronal death and lamination in the isthmo-optic nucleus within as little as 6 h. We here investigate how action potentials initiate the fast-acting retrograde signal, and we provide evidence that the first step is calcium entry into the isthmo-optic axon terminals. Neuronal death and lamination are rapidly reduced in the isthmo-optic nucleus by intraocularly injected omega-conotoxin, a blocker of N-type calcium channels known to be located mainly on axon terminal. Similar effects occurred with two other calcium channel blockers (cadmium and alpha-bungarotoxin) believed to act on both the isthmo-optic terminals and their target cells, but not with nifedipine, a blocker of L-type (mainly somatic) channels, supporting a presynaptic initiation of the fast signal. Nevertheless postsynaptic events may also be involved because pharmacological destruction of the amacrine targets cells of the isthmo-optic nucleus reduced its cell death and lamination 9-12 h later.

Presynaptic initiation by action potentials of retrograde signals in developing neurons

Until recently, the only means by which electrical activity was believed to initiate retrograde signals was via postsynaptic events: modulated synthesis or release of trophic factors. We have evidence in chick embryos for a presynaptic initiation of retrograde signals from the retina to the isthmo-optic nucleus, which is known to undergo 55% neuron death between embryonic days 12 and 17 and to become laminated during this period. Intraocular injections of saxitoxin just before embryonic day 14 reduce neuron death and prevent lamination in the isthmo-optic nucleus within as few as 6 hr. We show that these rapid effects are attributable to the direct action of saxitoxin on the isthmo-optic terminals. Alternative possibilities, such as an indirect effect via the target cells, are ruled out by control experiments. Normally, action potentials may lead to a chain of second messenger events in the axon terminal that is signaled retrogradely via the transport of a long-lived second messenger.

Dendritic reorientation and cytolamination during the development of the isthmo-optic nucleus in chick embryos

In the mature isthmo-optic nucleus (ION, source of efferents to the contralateral retina), the neuronal perikarya are generally described as being arranged in a single convoluted lamina surrounding a U-shaped region of neuropil, into which their highly polarized (unidirectional) dendritic arbors project perpendicularly. We find, however, that the details are more complicated than this description suggests, and are variable, as might be expected if the ION is self-organized through neuron-to-neuron interactions in development. The laminated conformation of the ION first appears at embryonic day (E) 14. Our previous experiments indicate that this involves the displacement of perikarya and is not due to sculpting by neuronal death. We here present a quantitative demonstration that the dendritic arbors reorient during the period of lamination. At E11, they are already highly polarized, but their directions are different from those in the adult, being mostly medio-rostro-ventral. Then, between E11 and E13, the arbors in the border region of the ION undergo major changes in their direction of polarization, projecting towards the center of the ION. The arbors within the core of the ION make more subtle changes. The dendritic reorganization seems to be intrinsically linked to the process of cytolamination, since the two events occur synchronously and disruption of either affects the other. Mechanisms are discussed; interaction with afferents is not responsible for lamination.

Neuron death in the developing avian isthmo-optic nucleus, and its relation to the establishment of functional circuitry

The present review covers all the published data on neuron death in the developing avian isthmo-optic nucleus (ION), which provides a particularly convenient situation for studying the causes and consequences of neuron death in the development of the vertebrate central nervous system. The main conclusions are as follows: The naturally occurring neuron death in the ION is related both temporally and causally to the ION's formation of afferent and efferent connections. The ION neurons need to obtain both anterograde and retrograde survival signals in order to survive during a critical period in embryogenesis. They may compete, at least for the retrograde signals, but the nature of the competition is still unclear. The retrograde signals are modified by action potentials. Neurons dying from a lack of anterograde survival signals can be distinguished morphologically from ones dying from a lack of retrograde signals. The neuron death refines circuitry by selectively eliminating neurons with "aberrant" axons projecting to the "wrong" (i.e., ipsilateral) retina or to the "wrong" (topographically inappropriate) part of the contralateral retina.

Changes in lamination and neuronal survival in the isthmo-optic nucleus following the intraocular injection of tetrodotoxin in chick embryos

We have studied how the development of the isthmo-optic nucleus (ION) is affected by electrical activity in the ION's axonal target territory, the contralateral retina. Electrical activity was blocked or reduced in the retina for various periods by tetrodotoxin injected intraocularly in different doses. The effects on the morphology of the retina appear to have been minor. During the ION's period of naturally occurring neuronal death (embryonic days 12 to 17), the injections substantially reduced this neuronal death and disrupted the development of lamination in the contralateral ION; there was also a lesser reduction in neuronal death in the ipsilateral ION. The dose of tetrodotoxin required to affect lamination was lower than that affecting neuronal death. Thus, the effects on neuronal death and on lamination were independent, since either could occur without the other. These effects were mediated by retrograde signals (probably two or more) from the eye; they occurred too early for the alternative anterograde route via the optic tectum (which projects to the ION) to be responsible. After embryonic day 17, the ION's response to intraocular tetrodotoxin changes abruptly from increased survival to total and rapid degeneration.

Ultrastructural study of the avian ventral lateral geniculate nucleus

The ultrastructure of the pigeon and quail ventral lateral geniculate nucleus was analyzed with standard electron microscopy and horseradish peroxidase tracing of its retinal and tectal afferents. Six types of neurons were distinguished: two large, two medium-sized, and two small types. The latter do not project to the optic tectum and appear to be interneurons. Large and medium-sized neurons project to the optic tectum and are thus relay neurons. Profiles with round, large synaptic vesicles were identified as retinal axon terminal afferents and those with pleomorphic, loosely grouped synaptic vesicles as tectal afferents. Gap junctions were seen between perikarya of small neurons and also with unidentified profiles.

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.

Endocytosis and autophagy in dying neurons: an ultrastructural study in chick embryos

In an effort to understand naturally occurring neuronal death in the developing isthmo-optic nucleus, we have accentuated one of its most probably causes, failure to receive adequate trophic maintenance from the axonal terminal zone in the retina, and have studied the dying neurons ultrastructurally. Retrograde trophic maintenance was blocked by means of intraocularly injected colchicine, which caused all the isthmo-optic neurons to die by just one of the two or more kinds of cell death that they undergo during normal development. The present paper deals with the very prominent cytoplasmic aspects of this kind of cell death, notably the uptake of exogeneous horseradish peroxidase and autophagy. There were also nuclear changes, which are dealt with mainly in the accompanying paper (Clarke and Hornung, J. Comp. Neurol. 283:438-449,'89). Numerous cytoplasmic vacuoles occurred in both soma and dendrites, and they were of three main kinds, of which the smallest (less than 0.5 microns diameter) had unstructured contents, whereas the larger two (1-2 microns and 2-7 microns) were secondary lysosomes (mostly residual bodies). Intravascularly injected horseradish peroxidase labeled all three kinds of vacuole but not the free cytoplasm, indicating that the uptake was by endocytosis rather than by leakage through holes in the membrane, as is confirmed by our failure to detect any such holes. We suspect that the smallest vacuoles are the primary endosomes, that these subsequently fuse with vacuoles of the intermediate kind, and that the largest vacuoles are formed by the fusion of these latter. The purpose of the endocytosis may be to channel the plasma membrane piecemeal into the lysosomes for destruction.

Combined effects of deafferentation and de-efferentation on isthmo-optic neurons during the period of their naturally occurring cell death

We have studied the effects on the chick embryo's isthmo-optic nucleus of de-efferentation alone or in combination with deafferentation. De-efferentation was achieved by pharmacological destruction of the axonal target cells in the retina at E13, or by colchicine-blockade of axoplasmic transport in the intraocular parts of the isthmo-optic axons at E13; deafferentation was by a tectal lesion at E11 or E12. De-efferentation alone causes all the isthmo-optic neurons to die, and mostly by the "endocytic-autophagic" mode of cell death, which is characterized by pronounced endocytosis (of an intravascularly injected label) and by intense, clumped activity of two lysosomal enzymes (acid phosphatase and N-acetyl-beta-glucosaminidase). Deafferentation plus de-efferentation caused there to be less endocytic-autophagic dying cells in the isthmo-optic nucleus than after de-efferentation alone, but all the neurons still died. Our interpretation is that deafferentation switched many of the isthmo-optic neurons to a completely different (nonendocytic, nonautophagic) mode of cell death.

Abrupt loss of dependence of retinopetal neurons on their target cells, as shown by intraocular injections of kainate in chick embryos

We have examined the capacity of neurons in the chick isthmo-optic nucleus (ION) to survive when their target neurons in the contralateral retinal are destroyed by intraocular injections of kainate (KA) at different stages in development. The retinal vulnerability to KA builds up progressively from embryonic day 10 (E10) until a plateau is reached at E15 (see accompanying paper); and the effects on the ION increase in parallel, almost all the ION neurons being rapidly lost after the E15 injections. KA injection before E15 lesioned only part of the retina and caused degeneration only in the topographically corresponding region of the ION. Near the end of the natural cell death period in the ION (E17), this initial dependence on the target cells is rapidly lost. Already at E16 the injections kill less ION neurons, and by E19 they kill none of them. The ION neurons have become completely insensitive to the KA injections and appear normal more than 4 months later, although axotomy (by eye removal) at a similar age would by then have killed them. The ectopic ION neurons, scattered outside the ION but projecting to the retina, are never affected by KA injections at any age.

A study of the centrifugal projections to the pigeon retina using two fluorescent markers

The centrifugal projection to the retina of the adult pigeon was studied by the retrograde transport of Diamidino yellow and Fast blue. On the contralateral side in the nucleus isthmo-opticus 8400-10,900 cells were stained while in the surrounding area 1500-2800 marked ectopic cells were counted. Up to 94 neurones, which are nearly all ectopic, project to the ipsilateral eye. Using double-labelling we concluded that there are no cells projecting to both eyes. This study shows that in the adult pigeon there exist more ectopic cells projecting to the retina than previously reported in horseradish peroxidase experiments.

Development of the avian accessory optic system: effects of embryonic optic lesions

Representative cross-sections of the nuclei ectomammillaris (EM) from both normal and optically lesioned chick embryos (45 h of incubation, stage 12), were analyzed and compared on days 6, 8, 10, 12, 14 and 16 of incubation. An identifiable EM is clearly present at 8 days, in both normal and lesioned embryos, and increases in cell number and area up to embryonic day 12. However, embryos with partial or complete unilateral optic ablations demonstrate an apparent acceleration in cell death rate when compared with normals, from days 12-16, when a relatively mature and stable form of EM is apparently reached. Thus, early optic lesions do not affect the morphology of EM until day 12. These data also indicate that embryonic ipsilateral pathways to EM may persist and even expand when one eye primordium is removed or partially lesioned.

Neuronal death during development in the isthmo-optic nucleus of the chick: sustaining role of afferents from the tectum

Neurons have been counted in the isthmo-optic nucleus following lesions of the optic tectum, its main source of afferents. Late lesions, made at 10.8-12.2 days of incubation, were employed as they cause the fewest non-specific side effects. The lesions spared the isthmo-optic tract, and although they caused many retinal ganglion cells to die, the degeneration did not spread to the inner nuclear layer, which contains the target cells of the isthmo-optic fibers. Hence the effects on the isthmo-optic nucleus were due to its being deprived of afferents. Even in unoperated embryos, 60% of the isthmo-optic neurons are known to die between embryonic days 12 and 17. The tectal lesions greatly increased the cell loss ipsilaterally; this was due to cell death, since other explanations such as migration away or differential cellular shrinkage have been ruled out. The fact that additional neuronal death occurred mainly during the latter half of the period of natural cell death implies that the tectal afferents are important for the survival of the isthmo-optic neurons during this latter half, but not before.

The genuineness of isthmo-optic neuronal death in chick embryos

It has previously been estimated that about 60% of the neurons in the chick's isthmo-optic nucleus die during development, since its total number of neurons decreases by this percentage. Theoretically, however, the decrease need not have been due to cell death, but could have been caused by either of two alternative possibilities: neurons might have migrated out of the nucleus, or they might have shrunk and therefore been misidentified as glial cells at later developmental stages. These possibilities have been tested, using horseradish peroxidase and tritiated thymidine as tracers, and both have been disproved. Hence, the 60% neuronal death is genuine.