At the end of this survey of MT research, which is of necessity incomplete—more than one hundred interesting papers are published each year and only a fraction of the literature has been mentioned—,the task of covering the whole field with the purpose of indicating the well-established facts and the fields in which new research is needed, is by no means easy. MT were born at the time of the great change from pro- to eukaryotes, and for eons have been put to use by cells of all types, motile or immotile, undifferentiated or highly specialized, resting or dividing. It is not surprizing that under the apparent unity of their shape and chemistry a far greater complexity than was imagined ten years ago is becoming evident. The formation of more or less helical tubules is a quite general property of proteins, as exemplified by viruses, bacterical flagella, and some abnormal hemoglobins. The tubulins have maintained through evolution the property of becoming assembled into MT, which are in many conditions labile structures which can be disassembled when no longer required by the cell. These tubulins are also capable of assembling into other geometrical structures—rings, helices, sheets. Moreover, at a higher level, this polymorphism is maintained as indicated by the multiple patterns made of grouped MT. The typical cilia and flagella structure of 9 + 2 has played and is playing such an important role in cell motility that some authors have imagined that it may have preceded, as a symbiotic structure, that of single MT.
KeywordsPermeability Estrogen Fractionation Melatonin Alkaloid
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