Short communication Collagen fibril diameter in relation to bone site. A quantitative ultrastructural study Margaret Tzaphlidou * , Panagiotis Berillis Address: Laboratory of Medical Physics, Medical School, Ioannina University, P.O. Box 1186, 45110 Ioannina, Greece Abstract The collagen fibril diameter was measured in cortical bone samples from the femoral neck, rear and front tibia of rats using electron microscopy analysis. Statistically significant differences (0.001!p!0.04) in mean fibril diameter values between different bo detected demonstrating a dependence upon bone use. q 2005 Elsevier Ltd. All rights reserved. Keywords: Collagen fibrils; Diameter; Electron microscopy; Cortical bone 1. Introduction In bone, collagen represents more than 90% ofthe organic bone matrix. It confers resistance to the structure and establishes the biomechanical properties of the tissue (Moro et al., 2000).The collagen fibril diameter has been regarded as the most important factor related to biomecha- nical strength of tissues (Parry et al., 1978; Baek et al., 1998; Ottaniet al., 1998).To understand the biomechanical strength of various bone sites, therefore, it is important to study this structural parameter of collagen fibrils in these sites. A number of factorshave been implicated in the regulation of tissue collagen fibril diameter (Tzaphlidou, 2001). In heterotypic fibrils, composed of different fibrillar collagens, minor collagens may act as determinants of fibril diameters (Birk et al., 1990).In bone,although type I collagen is the major type, representing over 95% of the total, the remaining 5% is made up by minor collagen types playing an important role in fibril formation ( Keene et al., 1991;Prockop and Kivirikko, 1995;Kadler etal.,1996). Among other candidate factors, the developmental stage and location within the tissue ( Keene et al., 1995) seem to affect the fibril diameter. Linesof evidence(Fung et al., 2003)suggesthat thereis a significant variation in mean fibril diameter distributionamongsixanatomical sitesof rat medial callateraligament.Thesedifferences in collagensize wereattributed to different mechanical strength in the examined regions. It is known that the mechanical strengthof bone depends primarily on the condition of the cortical bone (Stein and Granik, 1976; Stenstrom etal., 2000). Mechanical testing ofexcised femoral necks has shown thatthe cortex contributes 40-60% of the overall strength of the femur(Werneret al., 1988).In addition,finite element modeling has suggested that corticalbone in the femoralneck region may support 50% of the stresses associated with normal gait (Lotz et al., 1995).Hence, the present study uses only cortical bone. The aim of this study was to investigate and compare the collagen fibril profile in ratcorticalbone from three differentbone sites, femoralneck aswell as rearand fronttibia. 2. Materials and methods 2.1. Animals Male Wistar rats, 18 months of age, were used. Animals were breed and housed in natural conditionsand killed underlightetheranesthesia. Throughout the experiments care was taken to minimize pain or discomfort. All studies Micron 36 (2005) 703–705 www.elsevier.com/locate/micron 0968-4328/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2005.05.012 * Corresponding author. Tel.: C30 2651 097595; fax: C30 2651 097854. E-mail address: mtzaphli@cc.uoi.gr (M. Tzaphlidou).