Biomedical subjects
Publications and source records attributed to G Rager.
The semiautomatic image analysis system A.S.M. of Leitz was connected to a PDP-11 network. Thus, the x-y-z coordinates of serial sections of brains and of brain structures could be stored. These coordinates could then be used for three-dimensional reconstruction.
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Retinal ganglion cells and retinotectal synapses of chick embryos can be activated by electrical stimulation at early stages of development (Rager, '76a,b), whereas light evoked responses occur only towards the end of the incubation period. Thus, photoreceptors seem to be the last cells to mature in the chain of elements necessary to enable transmission of visual information to tectal neurons. In the present study the development of light evoked activity in the retina was investigated and compared with the structural maturation of retinal cells. This ontogenetic approach offers a solution to the problem of the cellular origin of the b-wave called in question by recent records of the potassioretinogram (KRG). Lammellar structures in the developing outer segments of photoreceptors can first be observed on incubation day 17. Late on the same day a corneal electroretinogram (ERG) and a visual evoked response on the optic tectum (VER) can be recorded. The response properties of the developing b-wave and VER were tested using various stimulus parameters. From the latencies of the b-wave and of the VER it is concluded that the b-wave is not generated directly by the activity of neurons involved in intraretinal signal transmission. Thus it is necessary to consider secondary processes triggered by neuronal activity such as depolarization of glial cells. In the chick retina, Müller cells are virtually the only glial cells. They fulfill all structural requirements necessary to explain the current which spreads through the retina during the b-wave. Electronmicroscopic analysis reveals that Müller cells undergo drastic changes during the early phase of b-wave development (incubation day 18). In particular, the number of microtubules per unit volume and the surface area of Müller cell processes in the outer plexiform layer increase considerably. It is, therefore, suggested that the b-wave originates in the depolarization of Müller cells secondary to synaptic activity in the outer plexiform layer.i
Onset, temporal sequence, and pattern of ingrowth of retinal fibers into the developing optic tectum of the chicken were investigated with histological procedures including the Golgi technique. Invading fibers could first be detected by stage 34 (eight days of incubation) at a specific locus which is the central area of the optic tectum. Compared to other tectal regions the central area is distinguished at this time by its advanced cytoarchitectural development and by the maturation of dendrites of radial cells located within superficial laminae. Immediately after their arrival at the central area some fibers can be observed invading the outer tectal layers and forming side branches. These observations permit the conclusion that fibers do not wait at their termination site for several days, as has been suggested earlier. Retinal axons start to invade the tectum at the site which is most advanced in its structural development. This early maturation of neurons in a specific tectal region might be a sufficient explanation for the central retinal fibers connecting to neurons of this area, which, propter hoc, is called the central tectal area.
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A reduced silver technique using physical development to stain embryonic nervous tissue is described. Brains are fixed in Bodian's fixative. Paraffin sections are pretreated with 1% chromic acid or 5% formol. They are impregnated with 0.01% silver nitrate dissolved in 0.1 M boric acid/sodium tetraborate buffer of pH 8 or with silver proteinate. Finally they are developed in a special physical developer which contains 0.1% silver nitrate, 0.01-0.1% formol as reducing agent, 2.5% sodium carbonate to buffer the solution at pH 10.3, 0.1% ammonium nitrate to prevent precipitation of silver hydroxide, and 5% tungstosilicic acid as a protective colloid. The development takes several minutes in this solution, thus the intensity of staining can be controlled easily. The method yields uniform, complete and reproducible staining of axons at all developmental stages of the nervous tissue and is easy to handle.
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The total number of optic nerve fibers of the chicken was determined at twenty sequential developmental stages from incubation day 5 to 104 days after hatching. It was found that the total number of optic nerve fibers increases from 4400 on incubation day 5 to about 4.0 million on incubation days 10 and 11. Thereafter, it decreases to a final value of about 2.4 million by incubation day 18 and remains constant from that time on until adulthood. Thus, 40% of optic fibers degenerate. Degenerating ganglion cells in the retina are first detectable by incubation day 9. Initially degenerating cells are located mainly in the central retina, but on subsequent days they can be found predominantly in peripheral zones. It is postulated that cell death occurs because of competition for adequate arborization space. If more retinal afferent fibers arrive than tectal termination sites are available, supernumerary fibers may degenerate. By degeneration the two systems retina and optic tectum, are matched in size.
Quantitative data on generation and degeneration of retinal ganglion cells during development (Rager and Rager, 1978) are interpreted in terms of a mathematical model which consists of a system of differential equations. By these equations we attempt to describe the formation of retinal ganglion cells and their termination domains in the tectum. Since ganglion cells seem not to degenerate before their axons have arrived at their termination site and start branching, from the arrival time on they may become competent either to continue to mature or to die. Therefore, to find the actual number of competent cells the extension of the fiber pathway between the retina and the optic tectum had also to be measured and computed. The differential equations are united by the principle that at any given time cells in excess of the number of termination domains have to die. By this model the mathematical function was determined. Several parameter values of this function were optimized with the Gauss-Newton method by which the curve was fitted to the measured values. The high correlation obtained by this method allows to conclude that, to a first approximation, the model may be satisfactory. The evidence of competition for termination sites and of systems-matching by cell death is discussed.
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