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Biomedical subjects

C E Holt

Publications and source records attributed to C E Holt.

At least 55 records · Page 3Linked to original sources
PubMed1989

A single-cell analysis of early retinal ganglion cell differentiation in Xenopus: from soma to axon tip.

Intracellular injections of Lucifer yellow (LY) were made into the cell bodies of Xenopus retinal ganglion cells from the earliest stages of axonogenesis to the beginning of target innervation. Embryos were intact during the injection so that the entire cell (cell body, dendrites, axon, and growth cone) could be visualized. The purpose of the study was 3-fold: (1) to characterize the early steps in retinal ganglion cell differentiation before the axon reaches its target; (2) to determine whether guidepost cells exist as possible navigation cues in the vertebrate optic pathway; and (3) to investigate whether the morphology of early retinal ganglion cell growth cones varies in a position-dependent manner along the primordial optic pathway. Axons were generally initiated before dendrites and followed a well-defined course along the primordial optic pathway without branching. Surprisingly, at least 5% of the retinal ganglion cells sent more than one axon into the optic pathway. Sister axons from the same parent cell traveled separately in the pathway, indicating that their growth cones navigated independently. Examination of dendrite genesis showed that dendrites usually begin to emerge from the cell body well before the axon tip reaches the target. This observation argues against the possibility that target contact influences dendrite initiation. Nascent dendrites were commonly tipped with pronounced varicosities that did not resemble axon growth cones. Their number and branching correlated well with axon length, indicating that the age of the retinal ganglion cell itself, rather than the age of its presynaptic cells or local environment, is the strongest influence on dendrite genesis. Examination of LY-filled growth cones at varying points in the pathway showed no evidence of dye transfer to adjacent cells. This indicates that gap junctional contacts probably do not form during axonal pathfinding and suggests that direct intercellular communication between growing axons and other cells in the pathway does not play a major role in axon guidance. Growth cone morphology was analyzed quantitatively and found to vary at different positions along the pathway. Growth cones entering the optic nerve head were the largest and most complex; those on the retinal surface were the smallest and showed a simple morphology. Growth cones in the chiasm and optic tract showed a degree of complexity similar to those in the optic nerve head but were smaller.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Cellular determination in the Xenopus retina is independent of lineage and birth date.

Xenopus embryos injected with tritiated thymidine throughout the stages of embryonic retinal neurogenesis showed that more than 95% of the embryonic retinal cells are born within a 25 hr period. While there are shallow central to peripheral, dorsal to ventral, and interlaminar gradients of neurogenesis in these eyes, throughout most of this 25 hr period, postmitotic cells are being added to all sectors and layers. Small clones of differentiated retinal neurons and glia derived from single neuroepithelial cells injected with HRP. These clones were elongated radially. They were also composed of many different combinations of cell types, suggesting a mechanism whereby determination is arbitrarily and independently assigned to postmitotic cells. Such a model, when tested statistically, fits our data very well. We present a scheme for cellular determination in the Xenopus retina in which a coherent group of clonally related cells stretch out radially as lamination begins. This brings different cells into different microenvironments. Local interactions in these microenvironments then lead the cells toward specific fates.

Animals↗
PubMed1987

Retinal axons with and without their somata, growing to and arborizing in the tectum of Xenopus embryos: a time-lapse video study of single fibres in vivo.

Time-lapse video recordings were made of individual retinal ganglion cell fibres growing to and terminating in the optic tectum of Xenopus embryos. The fibres were stained by inserting a crystal of the lipophilic fluorescent dye, DiI, into the developing retina. Growth cones were observed in the optic tract and tectum using 20 ms flashes of light to induce fluorescence approximately once every minute. Fluorescent images were captured with a SIT camera, processed and saved on a time-lapse video recorder. The main conclusions from observing normal growing fibres are as follows. (1) Axons in the optic tract grow at a steady rate directly toward their targets without retracting or branching. (2) As axons approach the tectum they slow down and their growth cones become more complex. (3) Most terminal branches in the tectum are formed by back branching rather than by bifurcation of leading growth cones. In a second experiment, labelled growing axons were separated from their cell bodies by removing the retina. Such isolated axons continued to grow for up to 3 h in vivo and were capable of recognizing the tectum and arborizing there. This result shows that growth cones must contain the machinery needed to sense and respond to their specific pathways and targets.

Animals↗
PubMed1985

Growth cones of developing retinal cells in vivo, on culture surfaces, and in collagen matrices.

The outgrowth of axons from the early retina in vivo is compared with that from retinal explants in two types of culture systems. The normal time course of axonal growth along the primordial optic pathway to the optic tectum is characterized, using tritiated proline and horseradish peroxidase (HRP) as anterograde tracers. The rate of axonal elongation in vivo is estimated to be about 32 micron/hr at 22 degrees C. The HRP technique allows visualization of retinal growth cones in vivo. Observations can thus be made on their microanatomy and on the environment through which they navigate. The growth cones of retinal ganglion cells in the embryo have lamellipodia and fairly short filopodia (approximately 10 micron) which are directed forward. The growth cones are found near the pial surface of the brain but do not seem to maintain contact with it. Two culture systems were developed to investigate axonal pathways in vitro. In the first, different substrates and culture media were explored. Results indicate that growth cones prefer a polyornithine substrate over a collagen one. The media that promotes the best neurite outgrowth consists of L15 (60%), fetal calf serum (10%) and Xenopus embryo extract (1 mg/ml). Time-lapse video monitoring of substrate cultures reveals an average rate of outgrowth of about 18 micron/hr with great variability. The growth cones in these cultures are large, flattened, and complex compared to those in vivo, and their filopodia extend in many different directions. The second culture system is a collagen gel infiltrated with growth medium. In these conditions neurite outgrowth more closely mimics that in vivo. The rate is faster than on substrates, and the growth cones appear morphologically similar to those in the embryo. Preliminary experiments using the gel culture system to test for chemotaxis of retinal axons toward their targets failed to demonstrate such an effect.

Animals↗
PubMed1984

Analysis of an inducer of the amoebal-plasmodial transition in the myxomycetes Didymium iridis and Physarum polycephalum.

Recent studies with amoebae of the Myxomycetes Didymium iridis and Physarum polycephalum reveal that competence to undergo sexual fusion or to differentiate to plasmodia apogamically can be induced by a diffusible factor. Suspension cultures of Didymium amoebae have proven to be a good source of the inducer activity. Cell-free culture supernatants accelerated mating of Didymium and Physarum amoebae and also apogamic differentiation of the Physarum strain Colonia . Characterization and purification of the inducer activity is being conducted with a semiquantitative bioassay based on the induction of zygote formation in suspensions of mating-compatible Didymium amoebae.

Biological Assay↗
PubMed1984

Does timing of axon outgrowth influence initial retinotectal topography in Xenopus?

The question of whether timing is involved in generating the topographic organization of the earliest embryonic projection from the retina to the tectum has been examined in Xenopus laevis. First, the normal schedule of axonal outgrowth from the retina to the tectum was characterized. Groups of axons originating from either dorsal or ventral extremes of the retina were labeled by in vitro incubation of sectors (one-quarter to one-third) of eye primordia in [3H]proline and their time courses of outgrowth were determined using light microscope autoradiography. Comparisons of the growth of dorsal and ventral nerve fiber populations showed that those from the dorsal retina leave the eye first, grow along the optic pathway, and reach the tectum roughly 6 hr ahead of those from the ventral retina. This stereotyped sequence of outgrowth schedules the development of the initial retinotectal map: first the ventrolateral tectum receives input from the dorsal retina (stage 37/38), and then the dorsomedial part receives input from ventral retina (stage 40). Second, to test whether the accurate timing of axon outgrowth and target invasion defines the spatial ordering of the earliest connections, the normal schedule of retinal fiber outgrowth was altered by substituting dorsal halves of young stage 21 eye primordia, labeled with [3H]proline, for those in older stage 27 embryos. These heterochronic transplants resulted in retarded outgrowth from the dorsal retina such that the original pioneer fibers reached the tectum at least 9 hr later than normal, arriving after ventral retinal fibers, thereby reversing the normal sequence of tectal invasion. Despite this, the initial pattern of tectal innervation remained spatially normal. It is concluded that the retinotectal map is generated not by the temporal sequence of retinal axon outgrowth but, rather, by selective means of neuronal recognition.

Animals↗
PubMed1983

Order in the initial retinotectal map in Xenopus: a new technique for labelling growing nerve fibres.

Retinal nerve fibres form an orderly map of visual space in several centres in the vertebrate brain. Such topographic maps are a common feature of central nervous system organization, yet the way in which they develop is poorly understood. Early nerve projections in the fetal and neonatal mammalian brain have been found in several cases to be less restricted than those in the adult, suggesting that nerve fibres may initially form a diffuse set of connections in their target structure from which the adult map is sculpted by the elimination of terminals. Indeed, previous electrophysiological data indicate that the retinotectal map in Xenopus laevis might be initially disorganized. We report here, however, that the retinotectal projection is ordered from the beginning of tectal innervation (stage 39/40). We demonstrate this first autoradiographically by tracing groups of growing ganglion cell axons which we labelled by incubating sectors of eye rudiments, before axonal outgrowth, in 3H-proline and replacing them orthotopically. Separate labelling of dorsal and ventral parts of the initial projection showed that retinal fibres are organized topographically, as in the adult, in the tectal rudiment and throughout much of the pathway. Second, we show that visual responses are ordered in the tectum from the first stage that they can be mapped (stage 40). We conclude that the topographic ordering of retinotectal connections develops as a result of directed axonal outgrowth.

Animals↗
PubMed1983

Plasmodium formation in Physarum polycephalum: cytological events and their timing relative to commitment.

Amoebae of "selfing" strains of Physarum polycephalum transform into multinucleate plasmodia without change in ploidy. The transformation occurs when cultures of amoebae reach sufficiently high cell densities or when cultures of low densities are exposed through filters to dense "inducing" cultures of amoebae. We have induced selfing amoebae to transform individually and in populations in order to study cytological changes and their timing relative to the irreversible commitment of cells to plasmodium formation. An increase in cell size was observed to precede commitment, and the first nuclear mitosis, which gives rise to a binucleate cell, was found to follow commitment.

Cell Communication↗
PubMed1977

An extracellular inducer of asexual plasmodium formation in Physarum polycephalum.

Asexual conversion of amoebae to plasmodia was studied in the Colonia isolate of the myxomycete, Physarum polycephalum. When a culture of Colonia amoebae is grown on a bacterial lawn, a period of amoebic growth precedes the appearance of cells committed to the plasmodial state. The onset of plasmodium production appears to be related to amoebic nutrition since cultures supplied with fewer bacteria display earlier differentiation. For a period of time after differentiation is initiated, conversion of amoebae to plasmodia is rapid and proceeds as an exponential function of time. A filter-transmissible substance, apparently released by differentiating cells, is implicated in the control of this rapid conversion.

Cell Differentiation↗
PubMed1977

A mutation (gad) linked to mt and affecting asexual plasmodium formation in Physarum polycephalum.

Amoebae of the acellular slime mold Physarum polycephalum convert to plasmodia both asexually and sexually. Genetic analysis of a mutant that exhibits enhanced asexual plasmodium formation is reported. The mutant carries a single lesion (gad-11) located 12.3 map units from mt, a gene that controls mating specificity in sexual plasmodium formation. The mutation, which was isolated in an mt3 strain, is also expressed in mth and mt4 strains.

Chromosome Mapping↗