Pyramid growth

Figure 9.10. Phase contrast photomicrograph of a (111) face ofan octahedral diamond crystal from Siberia. Note the triangular pyramidal growth hillocks and small trigons at the summits of the respective growth hillocks. Photographed by K. Tsukamoto [12].

Figure 7.6 Schematic representation of various growth forms observed in metal deposition (a) layer growth, (b) ridge growth, (c) block growth, (d) pyramidal growth, and (e) dendritic growth.

Even in the case of spinal cord injury where application of anti-Nogo antibodies results in regeneration of the cut axons, an additional important element for functional recovery is enhanced fiber growth from the unlesioned fibers, i.e. compensatory plasticity, as discussed above. After high corticospinal tract injury in the rat at the level of the medullary pyramid and treatment with anti-Nogo antibodies, rubrospinal pathways were shown to sprout into deafferented areas of the spinal cord, resulting in high levels of functional recovery, i.e. a functional switch in the remodeled pathway [42]. 

When the charge-transfer step in an electrodeposition reaction is fast, the rate of growth of nuclei (crystallites) is determined by either of two steps (I) the lattice incorporation step or (2) the diffusion of electrodepositing ions into the nucleus (diffusion in the solution). We start with the first case. Four simple models of nuclei are usually considered (a) a two-dimensional (2D) cylinder, (b) a three-dimensional (3D) hemisphere, (c) a right-circular cone, and (d) a truncated four-sided pyramid (Fig. 7.2). 

When the step separation is wide enough, typical spiral step patterns observable by optical microscopy may appear, but if the separation becomes narrower than the resolution power of the optical microscope, the spirals appear in the forms of polygonal pyramids or conical growth hillocks. Even if spiral patterns are not directly observable, we may assume that these growth hillocks are formed by the spiral growth mechanism. Examples representing the two cases are compared in Fig. 5.8. 

Figure 5.8. (a) Typical spiral pattern (phase contrast photomicrograph of (0001) face of Sic grown from the vapor phase), and spiral growth hillocks which appear as (b) polygonal and (c) conical pyramids due to narrow step separation. Part (b) is a differential interference photomicrograph, (1010), and part (c) is a reflection photomicrograph, (1011), of hydrothermally synthesized quartz. 

Anyone who has seen the well-formed crystals of minerals in our museums must have been impressed by the great variety of shapes exhibited cubes and octahedra, prisms of various kinds, pyramids and double pyramids, flat plates of various shapes, rhombohedra and other less symmetrical parallelepipeda, and many other shapes less easy to describe in a word or two. These crystal shapes are extremely fascinating in themselves artists (notably Durer) have used crystal shapes for formal or symbolic purposes, while many a natural philosopher has been drawn to the attempt to understand first of all the geometry of crystal shapes considered simply as solid figures, and then the manner in which these shapes are formed by the anisotropic growth of atomic and molecular space-patterns. 

Figure 6.2. The crystalline habit of lactose a-hydrate. (A) Prism, formed when velocity of growth is very high. (B) Prism, formed more slowly than prism A. (C) Diamond-shaped plates transition between prism and pyramid. (D) Pyramids resulting from an increase in the thickness of the diamond. (E) Tomahawk, a tall pyramid with bevel faces at the base. (F) Tomahawk, showing another face which sometimes appears. (G) The form most commonly decribed as fully developed. (H) A crystal having 13 faces. The face shown in F is not present. (I) A profile view of H with the tomahawk blade sharpened. (From van Krevald and Michaels 1965. Reprinted with permission of the Journal of Dairy Science 48(3), 259-265.)...

Less frequently observed growth forms are pyramids, spirals, whiskers, and dendrites. The structure of deposits is discussed further in Chapter 16. 

Similarly, apolipoprotein E expression increases in neurotoxicity mediated by KA (Table 6.3) (Boschert et al., 1999). Apolipoprotein E is a major lipoprotein in the brain. It is involved in the transport, distribution, and other aspects of cholesterol homeostasis. Apolipoprotein E also plays a dominant role in the mobilization and redistribution of brain lipids in repair, growth, and maintenance of nerve cells (Mahley, 1988). The secretion of apolipoproteins E and D may be differentially regulated in cultured astrocytes. In cell culture systems this depends upon the extracellular lipid milieu (Patel et al., 1995). During neurotoxicity mediated by KA, apolipoprotein E levels increase moderately in astrocytes and apolipoprotein E mRNA increases very strongly in clusters of CA1 and CA3 pyramidal neurons. Based on hybridization in situ and immunohistochemical studies, expression of apolipoprotein E in neurons may be a part of a rescue program to counteract neurodegeneration mediated by KA (Boschert et al., 1999). 

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