PubMed · 858249
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C R Taylor. 1977. Why big animals?. https://pubmed.ncbi.nlm.nih.gov/858249/
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The fate of environmental pollutants--the various isotopes of elements, and inorganic or organic compounds--is a fundamental aspect of ecology and ecotoxicology, and bioaccumulation is a phenomenon often discussed in this context. Human activities have drastically altered natural concentrations of many substances in the environment and added numerous new chemicals. An understanding of the processes of bioaccumulation is important for several reasons. 1) Bioaccumulation in organisms may enhance the persistence of industrial chemicals in the ecosystem as a whole, since they can be fixed in the tissues of organisms. 2) Stored chemicals are not exposed to direct physical, chemical, or biochemical degradation. 3) Stored chemicals can directly affect an individual's health. 4) Predators of those organisms that have bioaccumulated harmful substances may be endangered by food chain effects. While former theories on the processes of bioaccumulation focused on single aspects that affect the extent of accumulation (such as the trophic level within the food chain or the lipophilicity of the chemical), modern theories are based on compartmental kinetics and the integration of various environmental interactions. Concepts include results from quantitative structure-activity relationships (QSAR), pharmacokinetics, ecophysiology and general biology, molecular genetic aspects and selection, and finally the structure of communities and man-made alterations in them.
The use of porous microcarriers has given manufacturers the option of having a high cell density/medium volume (10-100 L)/high specific productivity/long-term process. The advantages of the system are listed in Table 7 and many of the criteria listed in Table 1 are met. The question of being able to license products from continuous processes has been addressed and guidelines are being prepared. Methods are available to compare the master and end-of-process (extended) cell banks to ensure that no change has occurred to the cell or product. Rigid attention to process definition and validation, as well as to the definition of production lots, overcomes many of the other reservations. In conclusion, the method has been demonstrated as technically feasible, economically advantageous, and licensable and the many advantages of a high density perfusion process (listed in Table 8) make porous microcarrier technology an ideal scaleup process because it is a simple, reliable, and scalable system.
An early embryo becomes a blastula at the moment that its constituent cells become organised into a simple epithelium. Epithelial folding and compartmentation are essential elements of animal development. All the different cell types--epithelial and other ones--of which a differentiated organism consists differ in their plasmamembrane-cytoskeletal complex but they are assumed to have an identical genome. The hypothesis is put forward that, perhaps, the basic mechanism underlying differentiation can be defined as the generation of cells which have an identical genome but which differ in their plasmamembrane-cytoskeletal complex and which, because of these differences, can engage in differential protein synthesis-physiology.