Amoeboid Types

All eucaryotic cells contain various proteins in their cytoplasm that interact to form mechanically stabilizing structures. The amounts of these proteins differ with cell type, and the structural elements - collectively referred to as the cytoskeleton -can be very labile. Labile transformations of cytoskeletal networks are involved in such essential biological phenomena as chromosome movement and cell division, intracellular material transport, shape changes relating to tissue development, and amoeboid-like locomotion (1-3). A great deal of work in recent years has led to the biochemical characterization of numerous cytoskeletal proteins(A) and the elucidation of their spatial localization within a cell(2). However, few quantifiable models yet exist that are appropriate for incorporating that information into notions of shape transformation and cell movement(5-8). 

Figure 9 Combined elemental maps of (a) the C03.1 carbonaceous chondrite Kainsaz and (b) ungrouped CO/CM-like carbonaceous chondrite Acfer 094. Kainsaz contains abundant small chondrules, CAIs, and AOAs. Acfer 094 is texturally and mineralogically similar to CO chondrites, but contains higher abundance of matrix. AOA = amoeboid ohvine aggregate BO = barred olivine chondrule PO(P)i n = type I (II) porphyritic olivine (pyroxene) chondrule.

There are two types of refractory inclusions calcium- and aluminum-rich inclusions (this section) and amoeboid olivine aggregates (Section 1.07.5.3). Since the mineralogy, chemistry and isotope chemistry of refractory inclusions were reviewed by MacPherson et al. (1988), many new analyses have been made of CAIs in CV, CM, CO, CR, CH, CB, ordinary and enstatite chondrites that provide important constraints on physicochemical conditions, time, and place of CAI formation. CAIs are addressed in detail in Chapter 1.08, the role of condensation and evaporation in their formation in Chapter 1.15, and their clues to early solar system chronology in Chapter 1.16. 

Many different processes were involved in making each chondritic component. Unaltered chondrite matrices may contain at least six different types of micrometer-to-nanometer-sized components, which formed in diverse environments amorphous FeO-rich silicate, forsterite and enstatite grains, refractory grains, presolar grains, carbonaceous material, and iron-rich olivine. Chondrules formed by several nebular processes (closed-system melting, condensation, and possibly evaporation) and at least one asteroidal process (impact melting in regoliths). CAIs may be condensates, residues or processed versions of both. An exception to this preference for complexity is provided by the amoeboid olivine inclusions all AO As could have formed by the same basic process nebular condensation. Aluminum-rich chondrules may provide a second exception, at least within carbonaceous chondrites. 

Contraction in amoeboid cells makes use of nonmuscle forms of myosin type II whieh form bipolar thiek filaments in the cytoplasm and in association with the actin filaments [23, 54]. In Dictyostelium, actin filaments form a eortieal shell directly under the plasma membrane, with random orientation of the filaments [132]. Activation of myosin contractile activity by phosphorylation of the myosin light ehain protein results in contraction of the cortical network [54, 110], It is unclear whether this contraetion is uniform throughout the eell or whether there is spatial regulation of the activity. In polarized motile Dictyostelium cells, myosin is concentrated at the rear of the cell [31], which could maintain the polarization, while a myosin heavy chain kinase (which phosphorylates the myosin heavy chain protein and inhibits thick filament formation), is localized at the front of the cell. 

As noted previously, the evidence for the contribution of microtubules to amoeboid cell motility and chemotaxis is mixed [242]. The microtubule organizing center has been reported to be localized to either the front or rear side of the nucleus, depending upon the cell type [167, 187, 188]. Alterations in microtubules can affect fibroblast lamellipod extension and motility [150], but in some assays, chemotactic responses may be unaffected [202]. Alterations in acetylation enhance chemotactic ability [98]. Microtubules have been proposed to alter the stability of adhesion sites, enhancing their disassembly [II9]. In sum, microtubules are likely to be permissive for amoeboid motility and chemotaxis, and respond to polarization of the cell generated by the actin system with polarization of the microtubule system. This may in turn stabilize cell polarity and enhance overall chemotactic efficiency. In the absence of a strong external stimulus, or in cases in which autocrine secretion influences cell polarization, the microtubule apparatus may provide critical signals for cell polarity [164]. 

The classification can be extended to amoeboid, dendritic, or dotlike types, but these terms are self-descriptive and need for alphabetic designations appears unnecessary. Thus one might symbolize a belite grain as Type AE, meaning that it has normal lamellae and they extend into the matrix or Type CF, indicating a complex internal structure and satellite crystals, etc. Designations of polymorph variety in the writer s classification are not intended, but polymorph variety and abundance are of major significance. 

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