Cancellous bone tissue, polarised light micrograph. The trabeculae show the alternation of bright lines with dark lines, demonstrating the lamellar organization of the bone matrix. The alternation of bright bands with other dark ones is due to the order of the collagen fibres in each of the lamellae. In bright ones, they are arranged in such a way that when the polarised light falls, they deflect it and increase its brightness. In the adjoining lamella, the collagen fibres change their orientation (forming an angle of approximately 90 degrees), which is why they do not deflect the polarised light and it are observed to be dark.