Leech neurogenesis. I. Positional commitment of neural precursor cells.

This paper reports analyses of the differentiation and distribution of identified peripheral neurons and central 5-HT-containing neurons in embryos of the glossiphoniid leech Theromyzon rude that have been deprived of one of the bilaterally paired major ectodermal cell lines called the n bandlets. Cells descended from a lone surviving n bandlet were abnormally distributed across both sides of the ventral midline. Nevertheless, they produced the complement of identified neurons that they would have produced in a normal embryo. Neurons produced by cells that crossed the midline occupied the normal positions of their absent homologs, as demonstrated by morphometric analysis of normal and n-bandlet-deprived ganglia. Ablations of ectodermal cell lines other than the n bandlets (o and p, or q) allowed the formation of normal distributions of neurons descended from the n bandlets. These results are interpreted as showing that neural precursor cells are committed to occupy particular positions before reaching those positions and probably use positional cues of predominantly nonectodermal origin to recognize those positions. Together, the results reported here and in the accompanying paper (S. Torrence, M. Law, and D. Stuart, 1989, Dev. Biol. 136, 40-60) suggest that ectodermal cells that are committed to give rise to specific neurons use cues provided by the mesoderm to find positions appropriate to their fates.

Leech neurogenesis. II. Mesodermal control of neuronal patterns.

This paper reports analyses of the effects of eliminating mesoderm from one or both sides of embryos of the glossiphoniid leech Theromyzon rude on the differentiation and distribution of ectodermal cells, especially identified peripheral neurons and central 5-hydroxytryptamine (5-HT)-containing neurons arising from the bilateral pair of cell lines called the n bandlets (n-kinship cells). In mesoderm-deprived regions, no segmental hemiganglia formed, and identified neurons were not organized into recognizable patterns, although 5-HT neurons underwent neurochemical differentiation and grew axons. In unilaterally mesoderm-deprived embryos, segmental hemiganglia were formed in a midbody experimental zone, and cells that had abnormally crossed the ventral midline from the deprived side gave rise to identified neurons that were incorporated as supernumeraries into the normal organization of hemiganglia on the nondeprived side. In a posterior experimental zone, ganglionic morphology was disrupted on both sides. We conclude that precursor cells are committed to specific neuronal fates regardless of whether they occupy normal positions and that mesodermal tissues provide positional cues necessary for such precursor cells to find positions appropriate to their fates.

Organization of primary visual cortex (area 17) in the ferret.

Anatomical and electrophysiological mapping techniques were used to determine topographic organization and arrangement of ocular dominance columns in the primary visual cortex of ferrets. From its border with area 18 on the posterior lateral gyrus, area 17 extends around the caudal pole of the hemisphere and over the splenial gyrus to the caudal bank of the splenial sulcus. The visuotopic map is oriented with the isoazimuth lines approximately parallel to the long axis of the posterior lateral gyrus and the isoelevation lines approximately perpendicular to the isoazimuths. Central azimuths are represented on the posterior lateral gyrus and peripheral azimuths are represented on the splenial gyrus; the inferior visual field maps medially and the superior visual field maps laterally. As in other species, the representation of the central visual field is expanded. The ferret has a considerable degree of binocular vision. Receptive fields driven through the ipsilateral eye extended more than 20 degrees into the contralateral visual field. Within the region of area 17 corresponding to the binocular portion of the visual field, tritiated proline injected into one eye transneuronally labelled an ipsilateral projection as a series of patchy bands roughly complementary to gaps in the labelled contralateral projection. Physiological ocular dominance columns were evident as well in that neurons and groups of neurons recorded in this region showed clustered ocular dominance preferences. Most single neurons studied were binocularly driven.

A banded distribution of retinal afferents within layer 9A of the normal frog optic tectum.

A banded distribution of retinal ganglion cell axons within layer 9A of the superficial tectal neuropil in Rana pipiens was revealed through anterograde labeling with horseradish peroxidase. Layer 9A previously has been demonstrated to mediate binocular vision through a polysynaptic pathway by way of the nucleus isthmi. This nucleus interconnects analogous regions of the two tectal lobes such that isthmic axons retinotopically map the visual world of the ipsilateral eye within tectal layers 9A and 8. Thus, we have found that a pattern of retinal ganglion cell bands occurs in binocular regions of normal frogs. This pattern is similar, but not identical, to the experimentally produced stripes previously observed in the doubly innervated tecta of 3-eyed and single tecta frogs. Qualitative and quantitative comparisons of these two types of afferent segregation patterns have implicated several structural and functional parameters which might be involved in band formation.

Anatomy and physiology of experimentally produced striped tecta.

Transplantation of a third eye primordium to the forebrain region of a frog (Rana pipiens) embryo causes two retinal projections to converge on a single tectal lobe. These projections form stereotyped eye-specific termination bands (Constantine-Paton, M., and M. I. Law (1978) Science 202: 639-641) that are similar to the source-specific stripes found normally in many regions of the mammalian brain. In the present study, we use quantitative analyses of anatomical data and double labeling techniques to demonstrate that induced bands of three-eyed frogs are approximately 200 micrometers wide, that they invariably run in a rostrolateral to caudomedial direction, and that they represent interdigitating synaptic zones which fill the entire superficial neuropil of the dually innervated tectal lobe. This periodic segregation pattern is not seen in the superinnervated diencephalon or in the optic tracts. Morphometric measurements on animals with dually innervated tectal lobes revealed an approximate 30% hyperplasia that was restricted largely to the deeper predominantly cellular tectal layers. Each of the banded retinal projections occupied neuropil volumes that were roughly 50% smaller than the volume occupied by the noncompeting retina of the same animal. Eye-specific segregation was detected with extracellular recording techniques as an ability of one of the two eyes to elicit consistently more and larger action potentials throughout a radial penetration of the superficial tectal neuropil. In several preparations, electrolytic lesions confirmed that physiologically defined positions of eye dominance corresponded to a band from the same eye. Maps of the visual field projections within dually innervated tecta were relatively normal and their orientation was consistent with the initial embryonic orientation of the retinas. Eye laterality, time of arrival, or fasciculation during growth are not responsible for this induced banding. Instead, our results are attributed to two mechanisms that are probably fundamental to neural mapping. These are: differential affinities between retinal and tecta loci which normally align the projection by bringing together appropriate pre- and postsynaptic areas and interactions among retinal ganglion cell fibers. The latter causes axons from physically neighboring retinal ganglion cell bodies to terminate together within the tectum and consequently increases the internal order of map. We propose that these same two mapping mechanisms may be responsible for the afferent segregation found in diverse regions of many vertebrate brains.

Right and left eye bands in frogs with unilateral tectal ablations.

Surgical ablation of a single tectal lobe in Rana pipiens can cause regenerating retinal ganglion cell axons to cross to the remaining tectum. These synaptically deprived fibers can obtain termination space in a retinotopic and highly stereotyped manner. Each of the two eyes can share the undisturbed tectum by terminating in mutually exclusive, eye-specific stripes that alternate across the medial-lateral extent of the tectal lobe. Invading axons from the ipsilateral eye must actively displace established synapses from the contralateral eye in order to form these exclusive termination zones because the normal projection to the intact tectum is not severed in these experiments. In animals in which a large proportion of anomalous fibers do not reach the undisturbed tectum, only a few ipsilateral eye bands are observed. Nevertheless, these bands have the same width, periodicity, and orientation as those observed in fully banded preparations. When ipsilateral eye terminal density is extremely low, banding is absent. The completely striped termination pattern of unitectal animals is identical to the pattern previously reported in the dually innervated tecta of three-eyed R. pipiens. We theorize that this pattern results from a compromise between two synaptogenic forces that are active in regeneration as well as in development.

Eye-specific termination bands in tecta of three-eyed frogs.

An extra eye primordium was implanted into the forebrain region of embryonic Rana pipiens. During development both normal and supernumerary optic tracts terminated within a single, previously uninnervated tectal lobe. Autoradiographic tracing of either the normal or supernumerary eye's projection revealed distinct, eye-specific bands of radioactivity running rostrocaudally through the dually innervated tectum. Interactions among axons of retinal ganglion cells, possibly mediated through tectal neurons, must be invoked to explain this stereotyped disruption of the normally continuous retinal termination pattern.

Familial occurrence of multiple myeloma.

Two brothers with multiple myeloma, both with immunoglobulin IgGK, are described. Although the cause and pathogenesis of this disease in instances of familial occurrence are uncertain, genetic predisposition and environmental factors may play a significant role.