Cell Survival in the Uncrossed Projection of the Mammalian Retina is Independent of Birthdate

Serendipity and the Siamese cat: the discovery that genes for coat and eye pigment affect the brain

One day in the late 1960s, Ray Guillery was examining brain sections through the visual thalamus of cats, and he recognized that the arrangement of layers in the lateral geniculate nucleus (LGN) of one cat was strangely abnormal. The cat was identified as a Siamese cat, one of a breed selected for its unusual coat color, with reduced pigment over much of the body and eyes. This chance observation and the recognition of its significance led to a broad-ranging series of investigations. These experiments showed that the lack of normal levels of pigment in the retina in Siamese cats (and other hypopigmented mammals) was the critical factor in the misdirection of many of the projections of the retina to the brain, the nature of the projection error, and the developmental consequences of the relay of the misdirected retinal inputs to visual cortex. As a result, we have a better understanding of how the brain forms proper connections and of the neural basis of visual problems in albino humans.

Postnatal changes in the uncrossed retinal projection of pigmented and albino Syrian hamsters and the effects of monocular enucleation

Anterograde and retrograde tracing techniques have been used to study the uncrossed retinal projection in neonatal pigmented and albino Syrian hamsters. The total number of retinal ganglion cells projecting ipsilaterally peaks at postnatal days 2-4 (P2-P4) and declines to adult values by P12. The change in cell numbers has a similar time course in albino and pigmented animals. Although the population of uncrossed cells in the temporal retina of albino hamsters is always less than that in pigmented hamsters, no difference between the colour phases was found for the population of uncrossed cells in nasal retina. Differential cell death also contributes to the adult albino decussation pattern in hamsters: The relative loss of cells from temporal retina in albinos (72%) is greater than that in pigmented animals (56%). The additional loss in albinos does not appear to depend on binocular interactions: The same proportion (30%) of uncrossed cells is "rescued" from death by neonatal monocular enucleation in both colour phases. Flat-mount preparations showing the distribution of uncrossed fibres reveal that a distinct focus of terminals emerges in rostral superior colliculus, which is topographically appropriate for a binocular mapping, at the peak of uncrossed ganglion cell numbers (P4). Comparison of uncrossed terminal distributions and ganglion cell death reveals considerable refinement of the terminals prior to the main phase of cell death. Monocular enucleations performed some time after birth have a greater effect on uncrossed terminal distributions than on cell death. These observations suggest that independent mechanisms may be involved in the regulation of terminal distributions and of cell numbers in the developing uncrossed retinal pathways.

Differential action of the albino mutation on two components of the rat's uncrossed retinofugal pathway

The development of the uncrossed retinofugal pathways in normally pigmented and albino rats, aged from embryonic day (E) 14.5 to E18.5, was investigated. DiI was placed into one optic tract and the retinal origin of the uncrossed component, as well as its course in the optic stalk, was studied. The results show that, as in the mouse, the uncrossed retinal projection has two components. The first component is seen at E15.5 in normally pigmented animals. It develops exclusively in the central parts of the retina and is normal in albino littermates. The second component, which arises from the peripheral parts of the ventrotemporal retina, is seen two days later at E17.5 in all animals but is significantly smaller in albinos than in their pigmented littermates. Studies of axons in the optic stalk labelled retrogradely with DiI placed in the optic tract indicate that the uncrossed axons have no preference for any position in the stalk except when they approach the chiasm, where they tend to accumulate at the caudal region of the stalk. The uncrossed axons intermingle with the crossed axons along the entire length of the stalk. In albino embryos, no obvious difference in the prechiasmatic course of uncrossed axons was seen at any age examined. It is concluded that the albino mutation in rats affects the late ventrotemporal component of the uncrossed pathway selectively. It does not act on the early central component. Further, the intermingling of crossed and uncrossed axons in the stalk and the apparently unaffected prechiasmatic course of uncrossed axons in albinos indicate that the albino gene has its primary action in the retina.

Development of the mesencephalic nucleus of the trigeminal nerve in chick embryos: target innervation, neurotrophin receptors, and cell death

The goal of this study was to determine whether processes of neurons in the mesencephalic nucleus of the trigeminal nerve (Mes V) of chick embryos arrive in their peripheral target prior to the period of developmental cell death, and to determine whether neurons with early target contact survive to a greater extent than neurons with processes that reach their peripheral target later. The arrival of Mes V nerve fibers in the masticatory muscles was determined by injecting the fluorescent tracer DiI, and the position of labeled and unlabeled neurons was mapped in subdivisions of the Mes V nucleus. Developmental changes in the numerical configuration of Mes V subdivisions were studied in DiI-labeled as well as Nissl-stained material. The expression of low-affinity (p75) neurotrophin receptors was investigated throughout development of the Mes V nucleus with in situ hybridization to assess whether and how levels of expression of this trophic receptor may relate to target innervation and cell death. The extent of cell death was evaluated by counting pyknotic nuclei. Processes of Mes V neurons invade their peripheral target between 5 and 7 days of incubation (E5-7). At E7-12, between 800 and 1,400 labeled Mes V neurons were distributed throughout the two main subdivisions of the Mes V nucleus, the tectal commissure and the optic tectum. Only few Mes V neurons were labeled in the posterior commissure or outside the brain. Cell counts in Nissl-stained material from E7-13 revealed that the numbers of Mes V neurons in the optic tectum decreased to about 40-60%, and in the tectal commissure to 20-25%, whereas Mes V neurons in the posterior commissure disappeared almost entirely. Few Mes V neurons remained in the leptomeninges at E8-10, but a considerable number was found outside the midbrain at E11, indicating ongoing migration of some Mes V neurons. Neurotrophin receptors were differentially expressed in the Mes V nucleus: Before and after the period of cell death, 90-100% of Mes V neurons expressed neurotrophin receptors, whereas during, and immediately preceding the period of developmental cell death (E9-E13), merely 70% of Mes V neurons expressed this receptor. These findings are consistent with the hypothesis that early target contact may provide an advantage for the survival of Mes V neurons and that competition for trophic factors may occur in the peripheral target of this nucleus prior to the period of cell death.

Changes in fiber organization within the chiasmatic region of mammals

Introduction

Classical views of the optic chiasm maintain four propositions about the retinofugal pathways: (1) each optic nerve contains a retinotopic representation of its respective retinal surface; (2) this retinotopic map in the nerve is the basis for the subsequent segregation of the decussating from the non-decussating fibers; (3) this retinotopy in the nerve is also the basis for the presence of retinotopy found within the half-retinal maps in the optic tracts; and (4) the half-retinal maps from each optic nerve are brought together within the chiasm to yield a unified, binocularly congruent, map in the optic tract (Brodal, 1969; DukeElder, 1961; Polyak, 1957; Wolff, 1940). The appeal of this classical view is in its simplicity, based on the assumption that the retinofugal pathway should replicate the sensory surface along its course. We now know that each of these four propositions is incorrect, and that the error is not one simply of degree or extent (Guillery, 1982, 1991). Rather, the above description of the visual pathway is fundamentally flawed because it has failed to take into account the constraints under which the pathway develops. We shall first consider the evidence for rejecting the classical view, from recent studies on the organization of the retinofugal pathway in adult animals and on the development of that organization. We shall then describe three transformations in the fiber order which all occur in the chiasmatic region, two of which were only recently recognized, and for which we must account.

Observations from adult organization

The difference in the fiber order in the optic nerve and tract