Cell types animal

Bcl-2. Human oncoprotein that plays a role in tissue development and maintenance by preventing apoptosis of specific cell types. Animal models suggest that failure to induce normal levels of apoptosis due to overexpression of Bcl-2 may contribute to the development of lymphoproliferative disorders and acceleration of autoimmunity. The role in human autoimmunity is not clear at this time. 

In summary, a pyrimidine phosphoribo ltransferase activity is present in many cell types, animal and microbial uracil and 5-fiuorouracil, but not cytosine, are substrates for this enzyme. 

Compared to bacterial cells, which are identical within a given cell type (except for O antigen variations), animal cells display a wondrous diversity of structure, constitution, and function. Although each animal cell contains, in its genetic material, the instructions to replicate the entire organism, each differentiated... 

ITowever, most normal somatic cells lack telomerase. Consequently, upon every cycle of cell division when the cell replicates its DNA, about 50-nucleotide portions are lost from the end of each telomere. Thus, over time, the telomeres of somatic cells in animals become shorter and shorter, eventually leading to chromosome instability and cell death. This phenomenon has led some scientists to espouse a telomere theory of aging that implicates telomere shortening as the principal factor in cell, tissue, and even organism aging. Interestingly, cancer cells appear immortal because they continue to reproduce indefinitely. A survey of 20 different tumor types by Geron Corporation of Menlo Park, California, revealed that all contained telomerase activity. 

Intermediate filaments are present in most animal cells. They are composed of more than 50 proteins which are expressed in a cell-type specific manner. Their diameter is about 10 nm and thus between those of the larger microtubules and the smaller F-actin. They form scaffolds and networks in the cyto- and nucleoplasm. 

Microtubules are universally present in eukaryotes from protozoa to the cells of higher animals and plants (Porter, 1966 Hardham and Gunning, 1978 Lloyd, 1987), but they are absent in mammalian erythrocytes and in prokaryotes. Microtubules participate in a number of cellular functions including the maintenance of cell shape and polarity, mitosis, cytokinesis, the positioning of organelles, intracellular transport to specific domains, axoplasmic transport, and cell locomotion. The diversity of microtubule fimctions suggests that not all microtubules are identical and that different classes of microtubules are present in different cell types or are localized in distinct domains in the same cell type (Ginzburg et al., 1989). 

Occasionally, some of the animal cells in short-term cultures do not die, but instead survive indefinitely. These types of animal cell cultures, which can divide indefinitely, are called established cell lines. Established animal cell lines have been obtained from both normal and tumorigenic cells. Immortalized animal cell lines have also been successfully obtained from short-term cultures following their transformation with appropriate oncogenes. 

TGFs are secreted by animal cells following their biosynthesis. Subsequently, these TGFs may either stimulate the growth of the very cells that have produced them (in this case, the TGFs act as autocrine factors), or may stimulate the growth of other adjacent cell types (in this case, the TGFs act as paracrine factors). 

In normal human subjects, some 25 % of total body iron (800-1000 mg) is present in the storage forms, mostly as ferritin. Whereas it is likely that all mammalian cell types contain some ferritin, haemosiderin in normal subjects is essentially restricted to cells of the reticuloendothelial system. Ferritin turns out to be almost universal in its distribution ferritin and ferritin-like proteins have been found in all organisms except for one or two archaebacteria. In contrast, haemosiderin has not been found to any extent outside of iron-loaded animals, except for a brief report of a phytosiderin in pea seeds (Laulhere et ah, 1989). 

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