Search PubMed⌕ Search

Biomedical subjects

K Akert

Publications and source records attributed to K Akert.

At least 55 records · Page 3Linked to original sources
PubMed1978

Intramembranous particles at the nodes of Ranvier of the cat spinal cord: a morphometric study.

Size and distribution of intramembranous particles at nodes of Ranvier of the cat spinal cord were investigated by the freeze-etching technique and compared with those at the internodal axon. The particles are larger (up to 20 nm) at the nodal than at the internodal segment (up to 13 nm), and these large particles are more densely packed in the nodal (400 per sq micrometer) than in the internodal E (external) face (4 per sq micrometer). The nodal E face reveals a much denser overall population (1200--1300 per sq micrometer) of particles than the internodal E face (100--200 per sq micrometer), while at the P (protoplasmic) face the particle density is similar in nodal and internodal segments (1200--1600 per sq micrometer). It is suggested that the large nodal particles may be related to the mechanism of nerve excitation.

Animals↗
PubMed1977

Neuritic growth cone and ependymal gap junctions in the feline subfornical organ during early development.

Intercellular contacts in the subfornical organ (SFO) of kittens 3, 16, and 29 days old were studied in thin sections and by the freeze-etch method. Gap junctions appeared between growing nerve processes and target cells. The junctions were interspersed between immature synapses lacking mitochondria as well as full pre- and postsynaptic membrane specializations. Gap junctions were seen on filopodia as well as on more mature processes. The morphology of these junctions was typical of those described earlier but they were of small size (0.2-0.3 micron). Gap junctions of peculiar form were also seen between ependymal elements in the SFO at 16 days. These were of large size (0.5-0.8 micron) and were often of segmented character. This segmentation consisted of bands 3-4 particles in width with a center-to-center spacing of 90 nm with particle free corridors between corresponding to the width of about two rows of particles. The margin of the group might be circumscribed by a row of particles. Although gap junctions of large size were seen between ependymal cells in thin section, features corresponding to the particle free corridors have not been observed to date.

Aging↗
PubMed1977

Efferent connections of cortical, area 8 (frontal eye field) in Macaca fascicularis. A reinvestigation using the autoradiographic technique.

The efferent projections from the cortical area 8 (frontal eye field) have been re-examined in four adult monkeys (Macaca fascicularis) by injecting small amounts of H3-proline into the rostral bank of sulcus arcuatus and using the autoradiographic tracing technique. Ipsilateral cortical projections could be traced into specific areas of the depths of sulcus principalis, sulcus temporalis superior and sulcus intraparietalis. Label was found contralaterally in area 8. Subcortical connections were observed ipsilaterally to n. caudatus, putamen, claustrum, n. ventralis anterior pars magnocellularis, n. medialis dorsalis pars multiformis and pars densocellularis, n. centralis lateralis and paracentralis, n. parafascicularis, n. pulvinar oralis, zone incerta, n. subthalamicus, pretectal area, colliculus superior and griseum pontis as well as the ipsi- and contralateral n. reticularis tegmenti pontis. Negative results were obtained with respect to the oculomotor nuclei, n. interstitials and Darkschewitsch as well as to the paramedian pontine reticular formation.

Animals↗
PubMed1977

The Ranvier nodes in the neurogenic electric organ of the knifefish Sternarchus: a freeze-etching study on the distribution of membrane-associated particles.

The two types of Ranvier nodes (type I with narrow gap, type II with giant gap) and internodes in nerve fibers composing the Sternarchus electric organ have been studied by means of freeze-etching electron microscopy. Numerical analysis of the distribution of membrane-associated particles revealed the following features: (1) the P-faces of both types of nodes and of the internodal axon bear a similarly high density of particles (1000-1200 particles/sq. micron on the average). (2) particle density is differential in E-faces: the histogram for type I nodes has a wider range of particle concentrations (114-1522 particles/sq. micron) than that for type II nodes (45-576 particles/sq. micron) whose density values are in the same range as those of the internodal axon. At least some type I nodes (narrow gaps) generate spikes and probably have a low resistivity; these nodes may be those with high particle density on E-faces. The low particle density on E-faces of type II nodes may be associated with high resistivity and absence of excitability. Similarly, the low particle density in internodes may reflect inexcitability. There is evidence that the transition from one nodal type to the next is gradual: as the gap width of type I nodes increases, there is an occurrence of surface elaborations and the density of E-face particles tends to drop towards the range of type II nodes.

Animals↗
PubMed1976

The fine structure of the perineural endothelium.

Fine strands of motor nerves were examined with the electron microscope using thin section as well as freeze-etching techniques. The specimens were taken from frog cutaneous pectoris nerve, rat sciatic nerve, mouse and shrew phrenic nerves and from human skin nerves. The perineural sheath (Henle, Ranvier, Key and Retzius) consists of one to several concentric laminae of endothelial cells; it encases nerve fascicles and eventually individual nerve fibers and terminals. The endothelial cells are extremely thin and fitted togeether smoothly by overlap and dove-tailing of their border zones. The cell contacts are formed by continuous zonulae occludentes, often reinforced by maculae adhaerentes, and in depth they comprise 3-15 strands with an average of 5-6 strands per junction. The membranes of endothelial cells are studded with attachment sites and stomata of plasmalemmal vesicles suggesting a high level of pinocytotic activity. This phenomenon is by no means restricted to the external laminae of the endothelial sheath. Each endothelial lamina is vested with basement membranes on both (epineural and endoneural) sides, and the spaces between laminae contain a few collagen fibers and fibroblasts. Occasionally, punctate tight junctions are seen between laminae. Cytological evidence supports the hypothesis that the perineural endothelium provides a relatively tight and highly selective barrier separating the peripheral nerves from surrounding tissue and its extracellular fluid spaces. This effect is achieved on the one hand by the sealing of pericellular spaces and on the other hand by a membrane controlled transcellular transport mechanism (pinocytosis), both of which are enhanced by their serial arrangement.

Animals↗
PubMed1976

Specific arrangements of membrane particles at sites of exo-endocytosis in the freeze-etched neurohypophysis.

Images have been obtained from freeze-etch replicas of neurohypophyses which are consistent with the view that orderly arranged aggregates of membrane particles occur in regions where fragments of membrane are being added to and taken away from the plasma membrane during secretion. Aggregates of particles included rosette-like and necklace-like patterns similar to those described by other authors at sites of exocytosis and endocytosis.

Animals↗
PubMed1975

Ultrastructural evidence for sinusoid spaces and coupling between pituicytes in the rat.

The pericapillary palisade of the rat neurohypophysis was examined by means of thin-section and freeze-etch electron-microscopy. Special attention was given to pituicyte processes intermingled with neurosecretory terminals. These processes are identified by the presence of lipid droplets and ribosomes. Extracellular spaces are conspicuously enlarged in circumscribed regions between fingerlike protrusions of pituicyte processes. Neurosecretory axons seem to have free access to these enlarged spaces. Zonulae occludentes often combined with small gap junctions are found at the border of these sinusoid spaces. Gap junctions and occasionally intermediate junctions are seen between pituicyte processes. The topographic relationship and the functional significance of these structural features remain to be further elucidated.

Animals↗