Vertebrate ␣-bungarotoxin-like molecules of the Ly-6 superfamily have been implicated as balancers of activity and survival in the adult nervous system. To determine whether a member of this family could be involved in the development of the avian ciliary ganglion, we identified 6 Gallus genes by their homology in structure to mouse lynx1 and lynx2. One of these genes, an ortholog of prostate stem cell antigen (psca), is barely detectable at embryonic day (E) 8, before neuronal cell loss in the ciliary ganglion, but increases Ͼ100-fold as the number of neurons begins to decline between E9 and E14. PSCA is highly expressed in chicken and mouse telencephalon and peripheral ganglia and correlates with expression of ␣7-containing nicotinic acetylcholine receptors (␣7-nAChRs). Misexpressing PSCA before cell death in the ciliary ganglion blocks ␣7-nAChR activation by nicotine and rescues the choroid subpopulation from dying. Thus, PSCA, a molecule previously identified as a marker of prostate cancer, is a member of the Ly-6 neurotoxin-like family in the nervous system, and is likely to play a role as a modulator of ␣7 signaling-induced cell death during development.

This investigation was undertaken to establish the gross and ultrastructural organization of the photoreceptors and retina in the Malayan tree shrew (Tupaia glis). Photographs of the fundus revealed no specialization or differentiation of a central foveal region. Histologic sections revealed a single row of relatively short and thick cones distributed uniformly throughout the retina. Electron micrographs of the retina indicated that the receptor outer segments are closely invested by pigment-filled epithelial processes and an amorphous interstitial material. The internal fine structure of the receptor outer segments revealed the characteristic stacks or arrays of bimembranous discs. The ellipsoid portions of the cone inner segments include tightly packed and extraordinarily large mitochondria. These mitochondria consist of unique patterns of concentric cristae arranged in highly ordered whorls of lamellar configurations. The cone synaptic pedicles contain a unique system of tubules not previously described in synaptic endings. Histologic sections indicated that only cone populations are located in the central region of the retina, whereas histologic, histochemical, and ultrastructural comparisons suggested that photoreceptors with some "rodtype" features are located more peripherally. The relatively small proportion of these rodtype receptors among the great preponderance of cone populations is in general accord with the tree shrew's diurnal habits as well as its great reliance on photopic vision and its visually guided behavior.

An examination of the topographic distribution of lipofuscin pigment granules with the light and electron microscope revcalcd either smaller and randomly "dispersed" or largcr and morc complex "clustered" pigment configurations in the cytoplasm of neurons in the dorsal ganglia and ventral spinal cord of 24-month old male mice. Qualitative comparisons revealcd no major diffcrenccs in shapc, size, complexity, dcnsity, orientation, and cytologic distribution of the pigment bodies in motor and sensory neurons. In gcncral, whcn the pigment granulcs werc quite numerous within the 2 types of cclls, they were smaller in size (~I/~), had a dense homogeneous matrix with few bands or lamcllae, and werc uniformly distributed throughout the cytoplasm. In contrast, when the pigment configurations were less in number, they were usually larger in size (~3/z), had a more complex internal banded structure, and appeared more localized within the cell. Examination of the bands revealed a repeating pattern of ~70 A. The bands appeared to fuse, forming hexagonal arrays of linear densities intersecting at an angle of approximately 120 ° in some regions of the pigment bodies. Structural similarities suggested that the striated membranous bands may be composed of phospholipids.

The process of retinal degeneration and regeneration in retinectomized or enucleated and reimplanted eyes of adult Notophthalmus uiridescens has been studied following staged administrations of tritiated thymidine or colchicine. In control left eyes, no labeled cells were observed in either the retina, pigment epithelium, or ora serrata. In retinectomized or enucleated eyes, retinal degeneration proceeded from the posterior central pole circumferentially to the ora serrata, with complete retinal degeneration being consistently observed. Retinal replacement originated from two cellular sources: the posterior pigment epithelium and the anterior complex, consisting of cells in the ora serrata and pars ciliaris retina.The posterior pigment epithelium gave rise to an initial central retinal portion which differentiated in an annular pattern which extended from the optic nerve to the vortex veins. Cellular addition from the pigment epithelium became restricted to these two regions by day 25. The anterior retinal mass was regenerated from the anterior elements (ora serratalpars ciliaris retina) with a minor contribution from the underlying anterior pigment epithelium. Data concerning the relative numbers of cellular types involved in the regenerative sequence and the times of their function are presented. Peak labeling occurred in the posterior pigment epithelium at day 20, the pars ciliaris retina at days 18 and 40, and the new retina at days 28 and 41. By 30 days regional sites of cellular proliferation became established that coincided with the pattern of ocular blood vessels. Retinal differentiation followed the pattern commonly observed during retinal development.The significance of these observations with respect to earlier work demonstrating a different sequence for retinal restoration is discussed.The retina is a compound neural tissue specialized for the reception of light energy from the environment and for the generation and initial integration of the neural response. The complex nature of these functions reflects the dependency of the organism upon the retina for its environmental adaptation. Similarly, the functional complexity of the retina is reflected by the nature of its cellular interactions within a precisely oriented and spatially distributed framework and the rigid specificity observed in its central connections.One of the more intriguing of the cellular interactions occurs between elements of the neural retina and the pigment epithelium. Although the cells of these two layers are derived embryologically from a single .J. EXP. ZOOL., 284: 185-206. layer, they have, under differing inductive systems, undergone considerable morphological divergence during development (Coulombre, '65), although they remain intimately dependent upon one another. If these two layers of the vertebrate eye become separated, the neural retina cannot survive and degenerates. This has been frequently cited as evidence that the pigment epithelium plays an important role in nourishing the neural retina or providing s...

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