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Cancellous (spongy) bone tissue, coloured scanning electron micrograph (SEM). Bone tissue is divided into compact (cortical) and cancellous (spongy). Cortical bone makes up the exterior of the bone. Cancellous or spongy bone fills the interior of the bone. Supporting trabeculae are thin rods or bars composed of irregular lamellae of bone that forms the honeycomb arrangement of cancellous bone. The cavities between the trabeculae are occupied by bone marrow (a blood and stem cell forming substance - not seen). Spongy bone tissue possesses lacunae that contain osteocytes. Osteoblasts are the cells responsible for bone formation. Once they cease activity and become embedded in the bone matrix they are called osteocytes. Magnification: x12 when shortest axis printed at 25 millimetres.(KEYSTONE/SCIENCE PHOTO LIBRARY SPL/DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY)