Precocious invasion of the optic stalk by transient retinopetal axons

Signaling - transcription interactions in mouse retinal ganglion cells early axon pathfinding -a literature review

Sending an axon out of the eye and into the target brain nuclei is the defining feature of retinal ganglion cells (RGCs). The literature on RGC axon pathfinding is vast, but it focuses mostly on decision making events such as midline crossing at the optic chiasm or retinotopic mapping at the target nuclei. In comparison, the exit of RGC axons out of the eye is much less explored. The first checkpoint on the RGC axons' path is the optic cup - optic stalk junction (OC-OS). OC-OS development and the exit of the RGC pioneer axons out of the eye are coordinated spatially and temporally. By the time the optic nerve head domain is specified, the optic fissure margins are in contact and the fusion process is ongoing, the first RGCs are born in its proximity and send pioneer axons in the optic stalk. RGC differentiation continues in centrifugal waves. Later born RGC axons fasciculate with the more mature axons. Growth cones at the end of the axons respond to guidance cues to adopt a centripetal direction, maintain nerve fiber layer restriction and to leave the optic cup. Although there is extensive information on OC-OS development, we still have important unanswered questions regarding its contribution to the exit of the RGC axons out of the eye. We are still to distinguish the morphogens of the OC-OS from the axon guidance molecules which are expressed in the same place at the same time. The early RGC transcription programs responsible for axon emergence and pathfinding are also unknown. This review summarizes the molecular mechanisms for early RGC axon guidance by contextualizing mouse knock-out studies on OC-OS development with the recent transcriptomic studies on developing RGCs in an attempt to contribute to the understanding of human optic nerve developmental anomalies. The published data summarized here suggests that the developing optic nerve head provides a physical channel (the closing optic fissure) as well as molecular guidance cues for the pioneer RGC axons to exit the eye.

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.

Characteristics of cellular proliferation in the developing human retina

The expression of proliferation-associated proteins Ki67 and PCNA was studied in the retinal rudiments of human embryos at 5-8 weeks of development; studies also addressed the numbers of nucleoli in the nuclei of neuroepithelial cells (with consideration of their distances to the apical surface) and DNA-synthesizing cells after transient (20 min) in vitro incubation in serum-free medium containing BrdU. The retinal rudiment of embryos at five weeks of development had neuroepithelium of the typical structure. BrdU-positive nuclei and nuclei with small numbers of nucleoli were located in the basal part of the ventricular zone. However, this organization was disrupted during the initial period of formation of the inner nuclear layer (six weeks). At this time, DNA-synthesizing cells were found even at the apical surface. Retinal rudiments of embryos at 6-7 weeks of development contained an additional area of cell proliferation in the Chievitz layer and the inner nuclear layer. In eight-week embryos, dividing cells were located in the outer nuclear layer, which again acquired the organization typical of neuroepithelium.

Maturational gradients in the retina of the ferret

In the present set of studies, we have examined the site for the initiation of retinal maturation in the ferret. A variety of maturational features across the developing inner and outer retina were examined by using standard immunohistochemical, carbocyanine dye labelling, and Nissl-staining techniques, including 1) two indices of early differentiation of the first-born retinal ganglion cells, the presence of beta-tubulin and of neuron-specific enolase; 2) the receding distribution of chondroitin sulfate proteoglycans within the inner retina; 3) the distribution of the first ganglion cells to grow axons along the optic nerve; 4) the emergence of the inner plexiform layer; 5) the emergence of the outer plexiform layer and 6) the onset of synaptophysin immunoreactivity within it; 7) the differentiation of calbindin-immunoreactive horizontal cells; and 8) the cessation of proliferative activity at the ventricular surface. Although we were able to define distinct maturational gradients that are associated with many of these features of inner and outer retinal development (each considered in detail in this report), with dorsal retina maturing before ventral retina, and with peripheral retina maturing last, none showed a clear initiation in the region of the developing area centralis. Rather, maturation began in the peripapillary retina dorsal to the optic nerve head, which is consistent with previous studies on the topography of ganglion cell genesis in the ferret. These results make clear that the order of retinal maturation and the formation of the area centralis are not linked, at least not in the ferret.

Centrifugal innervation of the rat retina

Centrifugal fibers innervating the retina have been shown in all classes of vertebrate, except for mammals where conventional tract-tracing methods have not been able to unmistakably demonstrate their existence. In a previous study, a unilateral, intravitreal injection of 5,7-dihydroxytryptamine was used to reveal indoleamine-accumulating centrifugal fibers which were visualized by an immunoreaction against serotonin. In the present study, I employed a modification of this method to stain retinopetal neurons in the rat. Terminals were located preferentially in the outer retina; labeled fibers could be traced back along an ipsilateral pathway to somata in the dorso-caudal portions of the chiasm or the medio-lateral preoptic area, and thence towards the suprachiasmatic nuclei. The unique beaded appearance of the fibers distinguishes them from retinal ganglion cell axons. The labeling of central cell bodies strongly suggests that they possess terminals in the retina. Thus, at least some mammalian retinas receive centrifugal innervation. This indoleamine-accumulating retinopetal pathway may be involved in retinal melatonin synthesis, coordination of circadian rhythms, and interocular phenomena.