now let's get a closer look bones are composed primarily of an extracellular calcified material called the bone matrix or collagen matrix it has three main types of cells osteocytes osteoblasts and osteoclasts osteocytes are found in cavities or lacunae between the layers of the bone matrix and assists with the nutrition of the bone osteoblasts are found mostly along the surface of bones but also within the bone matrix itself they are responsible for both synthesizing and mediating the mineralization of the bone matrix osteoclasts are large multinucleated cells that are responsible for removing calcified bone matrix and allowing for the constant turnover and remodeling of bones morphologically there are two main types of bones the compact or cortical bone is the dense portion that's found closer to the surface of bones the deeper portion of bones have interconnecting cavities and are called the cancellous or trabecular bone in this electron microscopy or em image of the tibia the compact bone is also blue and the more central trabecular bone is yellow microscopically both compact and trabecular bones will be organized or arranged in two forms layered or lamellar bone has a bone matrix that's arranged in sheets eighty percent of lamellar bone is found within the compact bone and only twenty percent is found within the trabecular bone the more immature woven bone has collagen fibers that are arranged randomly this form of bone is mostly found within developing and growing bones as well as bones that have healed after being fractured alright let's compare sections of a long bone and a flat bone this long bone is a partial cross-section of the head of a long bone and the flat bone is a cross-section from the skull both sections have been decalcified in order to make it easier to cut the bone into thin sections but the collagen is still present in the slides since the denser compact bone is typically found closer to the surface it's seen in the long bone along the outer edge whereas the flat bone has compact bone on both of its outer flat surfaces trabeculae means little beam in latin which is why the trabecular bone has small beam-like structures that are also called spicules the spicules connect with each other to form a network of interconnecting spaces that contain bone marrow the vertebrae is another example of a bone that has both types of morphology but its compact bone is only a very thin outer layer with a majority of the bone consisting of trabecular bone in this image we can also see that the bone marrow is still present within the spaces formed by the spicules the osteoclast can be found within the bone marrow in this image the osteoclast is sitting within a resorption cavity if formed on the surface of the bone matrix identifying the multiple nuclei within the osteoclast can help differentiate it from other similar looking cells like megakaryocytes which only have a single multi-lobed nucleus okay let's take a closer look at compact bones which have structural units called the heversion system or osteons an osteon consists of concentric lamellae or layers with a diameter of about 100 to 250 micrometers and a central canal called the haversian canal this canal contains small blood vessels and nerves as we can see in this image we can see a small blood vessel and remnants of nerve fibers as well the concentric layers of mineralized bone matrix radiate out from the canal and are called the heversion lamellae along the border of each osteon is a thin purple or basophilic line that's called the cement line the white spaces throughout the osteon are the lacunae which contain remnants of osteocytes in this image of a perversion canal the osteoblasts can be seen lining the inner surface of the heversion canal osteoblasts are large spindle-shaped or cuboidal cells that have a basophilic cytoplasm alright as a quick recap morphologically bones have two main types the denser compact or cortical bone which is found closer to the surface and the cancellous or trabecular bone which is the spongy bone that consists of trabeculae or spicules that form interconnecting cavities microscopically the lamellar bone is arranged in sheets of collagen matrix and woven bone has collagen fibers that are arranged randomly there are also three main cell types that are part of bone osteocytes are found in cavities or lacunae between the layers of collagen matrix osteoblasts are found along the surface of bones and also lining the heversion canal within the bone and finally osteoclasts are large multinucleated cells that can be found on the surface of the bone matrix helping current and future clinicians focus learn retain and thrive learn more you