ABSTRACT
The existence of well-defined boundaries to cells, and organelles within cells, is a fact known by microscopic anatomists for a very long time. The high resolution of the cell membrane structure achieved by electron microscopy of sections of a great variety of cells suggested the existence of a rather uniform type of structure, the “unit membrane”. The high potassium ion concentration of a mammalian cell in a low potassium environment and the low calcium ion concentration in a high calcium environment are examples of how the cell membrane guarantees a constant intracellular medium by actively conveying to its interior substances which the cell needs. Tumor cell membranes experimentally enriched with cholesterol have shown an improvement of tumor immunization after irradiation. The observation that difluoromethyl ornithine, a specific inhibitor of ornithine decarboxylase that causes a rapid depletion of intracellular polyamines and decreases the spread of metastasis, seems to indicate that polyamines play an important role in the process of tumor metastasis.
