EXAMINATION OF PARTICULATE INCLUSIONS IN TOOTHPASTE 483 measurable, but detrimental, factor. The pastes examined in this paper are, nevertheless, ranked in order of decreasing abrasivity in Table I.* Conclusions The freeze fracture and replication method can be used for a wide variety of toothpastes, and gives, in less than 1 h, a realistic representation of the morphology of solid and liquid phases present in a strip of toothpaste as extruded from the tube. With practice such micrographs can be used to categorize pastes into classes of likely performance. REFERENCES (1) Bull, W. H., Callender, R. M., Pugh, B. R. and Wood, G.D. The abrasion and cleaning properties of dentifrices. Brit. Dent. J. 125 331 (1968). (2) Wilkinson, J. B. and Pugh, B. R. Toothpastes--cleaning and abrasion. J. $oc. Cosmet. Chem. 21 595 (1970). (3) Ashmore, H., van Abbe, N.J. and Wilson, S. J. The measurement in vitro of dentine abra- sion by toothpaste. Brit. Dent. J. 133 60 (1972). * The technique used to measure the abrasivities with respect to dentine is fully described in reference 3.

J. Soc. Cosmet. Chem. 26 485--496 (1975) ¸ 1975 SocieO, of Cosmetic Chemists of Great Britain The role of essential fatty acids (EFA) in the regulatory processes of malpighian cell membranes M. CAMBRAI* Presented at the IFSCC VIIIth International Congress on 'Cosmetics--Quality and Safety', organized by the Society of Cosmetic Chemists of Great Britain at London on 26-30th August 1974. Synopsis--Living cells possess the ability to adjust their MEMBRANE PERMEABILITY in response to various stimulations. The electrical resistance of membranes as a function of current intensity proved a convenient means for studying this property. The non-linearity of membranes, i.e. the sudden change in electrical resistance which occurs beyond a critical current intensity, has been the subject of extensive studies. Evidence is presented showing that, in rats deficient in ESSENTIAL FATTY ACIDS, the membranes of living EPIDERMAL CELLS do not retain their normal, non-linear behaviour. Others have established that, under similar conditions, membrane-bound enzymes lose their ALLOSTERIC PROPERTIES. These findings lead to the conclusion that essential fatty acids are a fundamental requirement for the manifestation of regulatory processes in cell membranes. Possible mechanisms and the influences on hormonal regulation at the level of the skin are discussed. INTRODUCTION The disorders arising from lack of essential fatty acids (EFA) have been extensively studied. As far as the skin is concerned, EFA-deficiency is known to cause dermatosis, hyperkeratosis with epithelial h-yperplasia, decreased sebum secretion and increased water permeability. These effects have been observed in animals, mainly rats and mice. The present paper is * Unilever Research, Saint-Denis Laboratory, 8 impasse de la Montjoie, La Plaine Saint-Denis, 93212, France. 485