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G Vrensen

Publications and source records attributed to G Vrensen.

At least 37 records · Page 2Linked to original sources
PubMed1984

Development of synapses on pyramidal and multipolar non-pyramidal neurons in the visual cortex of rabbits. A combined Golgi-electron microscope study.

A combined Golgi-electron microscope method was used to study the ultrastructural maturation of synapses on identified pyramidal and multipolar non-pyramidal neurons in the visual cortex of young and adult rabbits. In samples of 10 (time of eye opening), 14, 20 day old and 7 month old animals, fully impregnated pyramidal neurons within the layers II-V and multipolar non-pyramidal neurons mainly located in lower layer III and layer IV was studied. We found that synapses in 10 and 14 day old animals were occasionally immature in appearance. They were characterized by either a poorly defined postsynaptic band or equal rims of pre- and postsynaptic electron-dense material and could therefore not be classified as Gray type I or II. The distinction between both types of synapses was easier at day 20 and in the adults when the postsynaptic band of the asymmetrical (type I) synapses had become remarkably thicker. In pyramidal neurons the cytoplasmic organelles increased in number during development. Although a few symmetrical synapses were present on dendritic spines of pyramidal neurons in 14 and 20 day old animals, all pyramidal neurons exhibited the same types of synapses on specific sites of their neuronal surface. They received exclusively type II synapses on their somata, type I synapses on their dendritic spines and both types of synapses on their dendritic shafts. However, in the adult animals the frequency of occurrence of type II synapses, especially on basal dendritic shafts, had increased. In some cases only type II and no type I synapses were present. A striking finding in all young and adult animals was that synapses at the borderline between somata and apical dendritic shafts as well as on dendritic spines were frequently complex or interrupted. The characteristic ultrastructural features of adult spine-free and sparsely spiny multipolar non-pyramidal neurons e.g. the many cytoplasmic organelles and type I and II synapses on somata and on dendrites were already present at day 10. After day 10 the number of organelles and synapses increased prominently and in adult animals the different types of synapses on dendrites were located at relatively short intervals of about 4 microns. In contrast with the dendritic shafts of pyramidal neurons many asymmetrical synapses were observed on dendritic shafts of the non-pyramidal neurons analysed in the adult animals. Furthermore, it appeared that the number of synapses on these non-pyramidal neurons is about twice that on pyramidal neurons in day 20 old animals and about four times in adult animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Synaptic characteristics of identified pyramidal and multipolar non-pyramidal neurons in the visual cortex of young and adult rabbits. A quantitative Golgi-electron microscope study.

The visual cortex of 20 day old rats and rabbits has been considered as mature on the basis of the observations that the dendritic arborization and the overall synaptic population have almost reached their adult stage in these animals. In the present study we have investigated the visual cortex of 20 day and 7 month old (adult) rabbits in order to determine whether this apparent adult appearance also holds for the synaptic organization of individual neurons. Neurons mainly located in layers III and IV of the primary visual cortex (area 17) were Golgi-impregnated, gold toned and deimpregnated and were then, after embedding in plastic, sectioned serially. The number and length of synaptic profiles, and the length of the neuronal boundaries were analysed in every tenth section. From these counts and measurements the size distribution of the synaptic discs, the number of synapses per 100 micron2 neuronal surface and the receptive surface expressed as the percentage of the total neuronal surface covered with synaptic contacts were estimated using stereological methods. At both ages studied, the density of synapses was significantly higher for the non-pyramidal neurons than for the pyramidal neurons. Differences in the amount of receptive surface were parallel to the differences observed for the number of synapses per 100 micron2. At day 20 the receptive surface of the non-pyramidal neurons was significantly larger than that of the pyramidal neurons. The receptive surface of the non-pyramidal neurons in the adult stage was not only larger than that of the pyramidal neurons in the adults, but also larger than that of the day 20 non-pyramidal neurons. From our results the following conclusions can be drawn: (1) The synaptic input received by the pyramidal neurons is mainly established at day 20 of postnatal life, i.e. prior to the establishment of adult visual behaviour. (2) The non-pyramidal neurons complete their maturation in a later stage than the pyramidal neurons. (3) Medium to large sized synaptic contacts are newly formed after day 20 and are mainly added to the synaptic population on dendrites of non-pyramidal neurons. (4) The specific increase in the number of synapses on non-pyramidal neurons is discussed in relation to intracortical inhibition which is thought to be important for the fine regulation of visual function during development.

Animals↗
PubMed1983

Retinopretectal projections in albino and pigmented rabbits: an autoradiographic study.

The ipsilateral and contralateral retinal projection was studied in pigmented rabbits and in 3 strains of albino rabbits by anterograde transport of [3H]proline and [3H]fucose combined with autoradiographic techniques. Special attention was paid to the terminals in the pretectal area of both the pigmented and albino strains. On the contralateral side terminal labeling was found in both pigmented and albino rabbits in the nucleus of the optic tract (NOT), the anterior pretectal nucleus (PA), the posterior pretectal nucleus (PP) and the pretectal olivary nucleus (PO). Ipsilaterally labeling was found only in the pigmented strain in small patches in the PP. Ipsilateral projection was not found in the albinos in the pretectal area. The results are in agreement with the findings of Scalia in pigmented rabbits. The absence of ipsilateral labeling in the pretectal region in albinos is in contrast with earlier findings of Giolli and Takahashi et al., in pigmented rabbits but is in agreement with the observations of Takahashi and Oyster. Since no radioactively labeled fibers were found to project to the NOT in either pigmented or albino rabbits, these results do not support the hypothesis of Collewijn that the inverted optokinetic nystagmus in albinos is due to misrouting of the ipsilateral retinal fibers to the NOT.

Animals↗
PubMed1983

Termination of geniculocortical afferents on bipolar neurons in area 17 of the albino rat: a Golgi/EM study.

Degenerating terminals of specific afferents to the primary visual cortex of the albino rat were identified by electron microscopy after electrolytic lesions of the lateral geniculate nucleus. Bipolar neurons were marked by Golgi impregnation. We observed smooth dendrites of two bipolar cells postsynaptic to degenerating geniculate terminals. We conclude that this cortical cell type is directly contacted by thalamic afferents.

Animals↗
PubMed1983

Geniculocortical synapses involving apical dendrites of layer V pyramidal cells in area 17 of the albino rat visual cortex: a combined Golgi/EM study.

Using the combined Golgi/EM technique (Fairén et al. 1977) we have investigated whether apical dendrites of layer V pyramidal cells with different diameters receive different numbers of geniculocortical synapses. An electrolytic lesion was made in the lateral geniculate nucleus (LGN) of albino rats. After a survival time of two days degenerating boutons of LGN afferents onto visual cortex (area 17) were identified by electron microscopy. Postsynaptic layer V pyramidal cells marked by Golgi impregnation were studied. The location of nine identified pyramids was determined by light microscopy. The diameter of the apical dendrites - ranging from 0.9 to 2.5 mum - was measured on electron micrographs. The dendrites of eight out of the nine pyramids make synaptic contacts with geniculocortical afferents in layer IV and lower layer III. The degenerative geniculocortical terminals were mostly of the electron dense type; one times the electron lucent type was observed. These axon terminals synapse mainly with asymmetrical synaptic junctions on spines. Occasionally synaptic contacts were found on dendritic shafts. Most degenerating synapses are localized on the pyramidal cell with the thinnest apical dendrite. There was also an other thin apical dendrite, which did not receive any degenerating bouton. All out of one apical dendrites of different size are involved in geniculocortical synapses without giving any preference to dendrites of a defined diameter.

Animals↗
PubMed1982

The three-dimensional organization of lens fibers in the rhesus monkey.

The three-dimensional organization of lens fibers in the Rhesus monkey was studied by means of scanning electron microscopy. The mutual anchoring of lens fibers is brought about by three types of interlocking devices: (1) interlocking protrusions on the apical and lateral edges, (2) ball-and-socket junctions on the apical and lateral surfaces, and (3) microplicae or tongues and grooves on the apical and lateral surfaces. Interlocking protrusions are present all over the lens, whereas ball-and-socket junctions and microplicae are restricted to cortical and nuclear regions, respectively. The distinction between interlocking protrusions and ball-and-socket junctions is discussed in detail.

Animals↗
PubMed1982

Argon laser lesions of the rabbit iris: quantitative aspects.

The destructive effect of nonperforating argon laser coagulations in the brown irides of rabbits was studied with quantitative histological methods. Power, spot size, and exposure time were systematically varied. The lateral and axial extensions of the crater and the denaturation of the adjacent tissue were studied. The main conclusions are: (1) the extent of tissue damage is significantly correlated with power and spot size, though the correlation is not proportional; (2) the lateral extension of the tissue reaction is far more pronounced than the axial extension; (3) increase in exposure time is effective up to 0.2 s, though a further increase up to 0.5 s does not enhance the tissue reaction. It is discussed that for conventionally switched lasers the main determinant for the final extension of the lesion is the density and distribution of the pigment granules within the iris. The radial distribution of the protrusions of the melanocytes in brown irides of the rabbits used in this study favors the lateralization of the tissue reaction. The nonlinearity of the tissue reaction and exposure time might be due to the fact that after initial damage in early phases of irradiation (e.g., up to 50 or 100 ms, the absorbed laser energy is dissipated in already destroyed tissue. Some practical aspects for iridectomy are discussed.

Animals↗
PubMed1981

Rejected human corneal grafts. I. Clinical study.

As introduction to a light- and electron-microscopial examination of rejected human corneal grafts a summary is given of the clinical signs of rejection. It is extremely important to recognize these signs in time and to initiate the correct therapy. Most failures in corneal transplantation are due to rejection. Rejection reactions are often not recognized early enough. The differential diagnosis is difficult because so many factors can trigger off a rejection. As the diagnosis is so often missed it is a good thing to consider the clinical picture of graft rejection once more. Maumenee: 'most ophthalmologists do not recognize the signs of early graft failure and usually do not refer the patient back for therapy until considerable oedema has developed from endothelial destruction' (1962). In the cases with a good prognosis the rejection percentage is plus or minus 12%; in vascularized corneas and complicated corneal transplantations as high as 75% (Polack, 1977).

Adult↗
PubMed1981

Rejected human corneal grafts. II. Light and electron microscopic study.

Six rejected human corneal grafts out of a group of 40 'bad-risk corneas' were systematically studied by light and electron microscopy. Remarkable fine structural similarities in this sample of corneas were observed. Granulocytes, lymphocytes and plasma cells are common in all rejected grafts. Furthermore, the epithelial and endothelial linings and the stroma reveal the characteristic alterations. The observations indicate an immunological reaction of the recipient to the donor corneas.

Adult↗
PubMed1981

The three-dimensional organization of lens fibers in the rabbit. A scanning electron microscopic reinvestigation.

The importance of the present reinvestigation of the three-dimensional structure of lens fibers in the rabbit is that it has shown the presence of all types of membrane specializations described before in one single lens of one species, the restricted distribution of some specializations, and the conformational changes within one fiber. Interlocking protrusions are present throughout the whole lens and must be considered as the main anchoring processes in the rabbit. Ball-and-socket junctions are present in restricted regions and can be considered an additional anchoring processes. Undulations of the apical and lateral edges of the hexagonal fibers and folds and bends of the lateral surface have a limited distribution. It is postulated that they may play a role in lens changes during accommodation.

Animals↗
PubMed1980

The presynaptic grid: a new approach.

A new electron microscopic facility is described which enables systematic visualization of E-PTA-stained presynaptic grids in full en face position. This EM-facility is used to analyze the size and the number of dense projections of synaptic grids in different brain areas of the rabbit. The observations support the view that dense projections form an intrinsic part of all central synapses and are organized in a hexagonal or triangular pattern. The observation of annulate and horseshoe-shaped synaptic grids is in agreement with previous observations on synaptic connections with subsynaptic plate perforations. A non-normal frequency distribution of dense projections per synaptic grid with distinct peaks is suggestive for the existence of distinct size classes of synaptic contacts. Analyses of the frequency distribution of dense projections in different areas and at different levels below the pial surface in adult animals point to a specificity of the distinct size classes related to the afferent origin or the target cell they are projecting on. Investigation of grid size and number of dense projections during a period of rapid synaptogenesis reveals that newly formed synapses also have a specific size. The complementarity of dense projections and vesicle attachment sites implicit in the model of Akert et al. has been used to calculate the number of vesicle attachment sites per sq. micrometer on account of the density of projections per grid. The agreement between our values and those of Akert and Peper, based on the analyses of vesicle attachment sites in freeze-etch specimen, is striking. The possible implications of these observations in relation to synaptic efficacy and neuronal microcicuitry are discussed.

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