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J. Cosmet. Sci. ! 58, 359-368 Quly/August 2007) Hair medulla morphology and mechanical properties RITA WAGNER and INES JOEKES, Departamento de Ffsico Qufmica! lnstituto de Qufmica! Universidade Estadual de Campinas! UN/CAMP! CP 6154, 13083-970! Campinas, SP, Brazil. Synopsis The morphology of human hair was extensively discussed in the last century, except for hair medulla, mainly because it was believed to have little or no influence on any useful hair property. Early SEM results showed that medulla is formed by unorganized fibrilar material that could be macrofibrils randomly located in the fiber center. The present paper aims to correlate the fibrilar structures with the macrofibrils using trans- mission electron microscopy (TEM) and to evaluate the influence of medulla on the mechanical properties of hair. TEM micrographs show that the interface between cortex and medulla is surrounded by a CMC layer and that there is less electronically dense material between cortical cells. Cortical cells in medulla give the usual microfibril crystalline arrangement. The cells become scarce and less organized in the center of the medulla, which also shows air filled granules. Average values of the mechanical properties are similar for unmedullated and medullated fibers. However higher dispersion in data for medullated fibers is observed. Unmedulated fibers are more uniform and show smaller diameters. These data indicate that the air cavities in medulla do not interfere with the mechanical properties, but leave hair strength less uniform. INTRODUCTION Human hair morphology has been extensively discussed in recent decades due to the interest of the cosmetic industry and the dermatological and forensic sciences (1). With advances in microscopic techniques, it is possible to obtain additional physical and chemical information about morphological hair structures, using atomic force micros- copy (AFM) or image mass spectrometry, for example (2,3). Despite more difficult sample preparation methods, the classical electron microscopy techniques (TEM and SEM) are still very much used to study human hair morphology (4,5) mainly because of their resolutions. Human hair is composed mainly of a-keratin (- 80% w/w), which is an a-helical protein with high cystine content (6,7). Morphologically, human hair is divided into four units: cortex, cuticle, intercellular material and medulla the medulla is not always present. The cuticle has an amorphous character. Each cuticle cell is formed by 4 subunits that have distinct chemical compositions and reactivities. Their cross-linked structure de- pends on the cystine content. (8,9). The cell membrane complex (CMC), which is found between the cuticle cells, is a hydrophilic material rich in polar amino acids and lysine 359