Structural Analysis of Cell Membrane Complex of Hair Fiber by Micro X-ray Beam

A mammalian hair fiber consists of the medulla, cortex and cuticle from the center to the outside in this order [1]. The major mass of hair fiber is located in the cortical region, which mainly consists of keratin filaments and matrix proteins. On the outside of the cortical region, there is cuticle which has about 5 μm in thickness and consists of about 10 keratinized sheet-like-cells. There are pathways for water molecules through the cuticle into the central cortex, one of which is the cell membrane complex (CMC) in the cuticle (Fig. 1). Recently, on the cuticle surface, covalently bound lipids mainly consisting of 18methyleicosanoic acid have been found, which contribute to the hair surface property. Therefore, it is important to obtain detailed structures of the cortex and cuticle including water molecules to understand the penetration of water. However, since the cuticle is too thin and scatters X-rays only weakly, small-angle X-ray diffraction (SAXD) had been less applicable to the cuticle than the cortex. On the basis of a pioneering SAXD experiment using a micro X-ray beam [2], we have recently developed a SAXD technique to characterize CMC and cortex structures in a mammalian hair [3]. We performed SAXD experiments using a microbeam at beamline BL40XU. A high-flux X-ray beam (λ = 0.083 nm, wavelength) from a helical undulator was focused with two mirrors. In the experimental hutch, an X-ray beam of 5 μm in diameter was obtained behind two pinholes, the first 5 μm and the second 100 μm in diameter. The sample-to-detector distance was about 2.4 m. The reciprocal spacing S (= 1/d = (2/λ)sin(2θ /2), 2θ is the scattering angle and d is the repeat distance) was calibrated by the spacing of standard materials at room temperature. From the SAXD profile of CMC (Fig. 2(a)), the equator of CMC is tilted by about 3o with respect to the equatorial direction of the fiber axis, in accordance with the structural evidence from an electron microscopic observation. Figure 2(b) is similar to the SAXD profile of the bundle of mammalian fibers. Thus, the present experiment provides structural information on CMC without interference from other structures. CMC in the cuticle of a mammalian hair fiber is composed of three layers, which are the β, δ and β layers. It has been assumed that the electron density profile of CMC can be expressed by box-type wells. The δ and β layers are composed of proteins with flexible shapes and l ipid molecules of various hydrocarbon-chain lengths, respectively. When we take into account the variation of thickness in the δ and β layers, overall features of the SAXD profile (Fig. 3) can be explained satisfactorily [3]. We focused our attention on an influence of humidity on the cortex and cuticle structures in a rat whisker. A rat whisker sample was incubated at a relative humidity (RH) between 3% and 100% which was controlled by a humidity generator (Model HUM-1, Rigaku).