Two adhesive systems cooperatively regulate axon ensheathment and myelin growth in the CNS

Central nervous system myelin is a multilayered membrane produced by oligodendrocytes to increase neural processing speed and efficiency, but the molecular mechanisms underlying axonal selection and myelin wrapping are unknown. Here, using combined morphological and molecular analyses in mice and zebrafish, we show that adhesion molecules of the paranodal and the internodal segment work synergistically using overlapping functions to regulate axonal interaction and myelin wrapping. In the absence of these adhesive systems, axonal recognition by myelin is impaired with myelin growing on top of previously myelinated fibers, around neuronal cell bodies and above nodes of Ranvier. In addition, myelin wrapping is disturbed with the leading edge moving away from the axon and in between previously formed layers. These data show how two adhesive systems function together to guide axonal ensheathment and myelin wrapping, and provide a mechanistic understanding of how the spatial organization of myelin is achieved.

Regeneration of Plants from Nodal and Internodal Segment Cultures of Ephedra gerardiana using Thidiazuron

Ephedra gerardiana is an important medicinal plant used as Soma in Indian Ayurvedic system of medicine and as Traditional Chinese Medicine since several thousand years. Excessive use of this plant has led to decline in its natural population. Present report describes the use of TDZ in MS for induction of somatic embryos and shoot buds in internodal and nodal segment cultures of E. gerardiana. Somatic embryos were obtained from internodal segment culture onto MS + 0.5 to 1 ?M TDZ. In case of nodal segment culture, lower concentrations of TDZ induced only shoot buds in 63.33 - 88.88 per cent cultures. Onto higher concentrations, all cultures showed callus induction with shoot bud formation. Shoot buds elongated and rooted onto one fourth strength of MS having 20 ?M IBA. Rooted shoots were transferred to plastic pots. DOI: http://dx.doi.org/10.3329/ptcb.v22i2.14205 Plant Tissue Cult. & Biotech. 22(2): 153-161, 2012 (December)

Which stem parts of Slender speedwell (Veronica filiformis) are the most successful in plant regeneration?

Regeneration of Slender speedwell (Veronica filiformis) from small parts of stem without using stimulative agents was the focus of the investigation. Four different short fragments of shoot (main terminal, secondary terminal, nodal segment and internodal segment) were cultivated under greenhouse and natural conditions. All tested vegetative segments induced roots, rooted in a soil substrate and in a semi-natural lawn, survived winter, and flowered. Multiplication of clonal plants was confirmed for both the main and secondary terminal segments. These terminal segments had the best response in number of growing individuals, flowering stems, and weight of dried biomass. The manipulative experiment revealed that clonal success of speedwell is connected with possibilities to hive off of all tested above-ground segments. Establishment of plantlets from the segment in natural condition was not successful when accompanied by certain grass species.

Formation and propagation of cell division-centers in the epidermal layer of internodal segments of Torenia fournieri grown in vitro. Simultaneous surface observations of all the epidermal cells

A cytological study of bud neoformation was done using a technique of direct observation of whole epidermal strips excised and stained at different stages of development from one side of an internodal segment of Torenia fournieri grown in vitro. It was shown that cell divisions start at random points over the epidermis and spread out progressively and apparently by contagion, forming cell division-centers. These centers are each formed of a central, highly divided cell surrounded by several less divided cells. The number of divisions per original cell decreases with the distance from the highly divided central cell.The probable origin of bud meristems was shown to be either one part of an original cell, one cell, several parts of adjacent cells, or several entire adjacent cells.A competition phenomenon was shown to exist between cell division-centers; few centers form meristematic primordia and only a few of these last form buds (about 20). Thus, on an epidermal strip 10 × 2.5 mm containing about 5000–6000 cells, fewer than 100 cells are implicated in the formation of buds observed morphologically.

THE FINE STRUCTURE OF NERVE CELL BODIES AND THEIR MYELIN SHEATHS IN THE EIGHTH NERVE GANGLION OF THE GOLDFISH

The eighth cranial nerve ganglion consists of bipolar nerve cell bodies each occupying part of an internodal segment. The perikaryal sheaths range from a single layer of Schwann cell cytoplasm on the smallest cells to typical thick compact myelin on the largest. On most perikarya, the sheath displays an intermediate form, consisting of multiple layers of Schwann cell cytoplasm (loose myelin), or of loose and compact myelin continuous with each other. Internodes beyond the one containing the cell body bear only compact myelin. In loose myelin the thickness of each layer of Schwann cell cytoplasm is about 100 A. It may be much greater (∼ 3000 A) particularly in the outermost layers of the sheath, or the cytoplasm may thin and even disappear with formation of a major dense line. The cytoplasmic layers are separated from each other by a light zone, 40 to 200 A wide, which in its broader portions may contain an intermediate line. Desmosomes sometimes occur between lamellae. In addition to the usual organelles, the perikaryal cytoplasm contains granular and membranous inclusions. Large cells covered by compact myelin have a consistently higher concentration of neurofilaments, and some of the largest cells, in addition, show a reduced concentration of ribosomes. The functional significance and possible origins of perikaryal myelin sheaths are discussed.

CONTINUOUS CONDUCTION OF IMPULSES IN PERIPHERAL MYELINATED NERVE FIBERS

1. Conduction of impulses in peripheral myelinated fibers of a nerve trunk is a continuous process, since with uninjured nerve fibers: (a) within each internodal segment the conduction time increases continuously and linearly with increasing conduction distance; (b) the presence of nodes of Ranvier does not result in any detectable discontinuity in the conduction of the impulse; (c) the ascending phase of the spike always has an S shape and never presents signs of fractionation; (d) the shape and magnitude of the spike are constant at all points of each internodal segment. 2. Records have been presented of the external logitudinal current that flows during propagation of an impulse in undissected single nerve fiber (Fig. 6). 3. Propagation of impulses across a conduction block occurs with a readily demonstrable discontinuity.