Did You Know?Compared to compact bones, spongy bones have a higher surface area and increased vascularity, which allow better exchange of nutrients from the blood to the bone tissue.
Depending upon the structure of the bone, the human skeletal system consists of two types of bones: spongy and compact. The spongy bone is a tissue that lies in the interior of the bones. The compact bone acts like a protective shell, covering the lesser dense structure of spongy bones. Apart from the structure, there is noticeable difference in the weight, density, and shape of these two bones.
The compact bone is an elongated cylindrical structure that forms the tough, outer layer of the majority of bones. Also referred to as the cortical bone, it provides a framework and strong support to the human body. It is heavy, tensile, dense, and primarily consists of calcium. Its strength and hardness can be judged from the fact that a saw is required to break the bone into pieces.
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.
About 80% of the human skeleton comprises compact bone, while the rest is made up of spongy bone. This clearly indicates that majority of weight of the skeleton is concentrated in the compact bone. No wonder, it is considered to be the heaviest among the different types of bones. On the other hand, the porous nature of spongy bones makes them lightweight.
As compact bones have higher density, they have more mass than observed in the same amount of spongy bone. No wonder, the denser layers of compact bone are a major contributor to the weight of the body.
Calcium is the primary component of compact bones. A majority of calcium stored in the body lies within the compact bone. Its compressive strength and rigidity have been attributed to its calcium content.
Although calcium and other minerals are present in spongy bones, the quantity is very small.
A compact bone is cylindrical in shape and forms the outer, smooth white portion of all bones.
The shape of the spongy bone is very similar to that of a cuboid, and they are enclosed with a layer of compact bone.
Compact bones make up a major part of your long bones that are present in the legs and arms. These long bones have a thick-walled structure of compact bones that enclose the bone marrow.
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.
When observing the compact bone under the microscope, it seems to have a tightly packed cell structure. A compact bone may appear like a solid, smooth structure, but when examined under a microscope, it is made up of canals (tubular pathways) that store the nerves and blood vessels.
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.