Contradictory to its name, spongy bones are not soft. They are named so because they closely resemble a kitchen sponge. They are solid and hard, but when viewed under a microscope, are very similar to that of honeycomb. In short, these bones are porous, but that doesn't mean they can be squished like a sponge. Their porous nature helps to hold the bone marrow―soft tissue that produces red blood cells. The tiny spaces or cavities of the bone also hold the nerves and capillaries that exchange nutrients in and out of the spongy bone.
Although calcium and other minerals are present in spongy bones, the quantity is very small.
The shape of the spongy bone is very similar to that of a cuboid, and they are enclosed with a layer of compact bone.
Short bones, on the other hand, found in the ankles and wrists are mainly constructed of spongy bones. The spongy bone is also present at the end of long bones.
The compact bone primarily consists of star-shaped cells, referred to as osteocytes. These cells are stacked one above the other, encircling the canals. The combined structure of osteocytes forming a ring-like pattern around the canals is referred to as osteons. An osteon, under the microscope, looks like a long cylindrical tube with the central section of the structure containing blood vessels, veins, and nerves. Osteons together look like a bundle of straws that are oriented in the same direction.
The spongy bone is made up of trabeculae―microscopic pillar-shaped units that form a mesh-like network of connective tissue. Unlike the compact bone that forms a well-organized pattern of osteons, the spongy bone has a lamellar or layered pattern, meaning it forms a sheet-like mass of connective tissue. It is an interconnected network of flat pieces of bone with open spaces that are occupied by the bone marrow.