Difference Between Compact and Spongy Bones

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Bones are connective tissues that differ in shapes and functions. They form a skeletal system that provides a body framework. It supports the body and allows locomotion. They are present in various shapes such as flat or wide plates with large muscle and hollow tubes with thick walls. Bones are also differentiated based on their structure and functions. This classification brings us to the compact and spongy bone structures. Both compact bone and spongy bone are parts of the bone tissue. Compact bones are the hard exterior, while spongy bones are the porous interior structures of bone tissue.

What is Compact Bone?

The external cortical layers of all bone tissues are heavy, hard, and have a smooth surface. This layer of bone tissue is called compact bone. A compact bone example would be the significant cortical bones of all long bones, such as those on arms and legs. They are hard and contain yellow bone marrow. These cylindrical bones constitute 80% of the skeletal system's weight.

What is the Spongy Bone Structure?

The interior layers of bones, which are also called cancellous or trabecular bones, make up the spongy bone structure. These bones are also found in long bone tissues, but they have a lesser amount of calcium and are made of trabeculae. The name ‘spongy bone’ comes from the several lamellar spaces in these bones, which gives it a porous structure.

What is the difference between Compact Bone and Spongy Bone?

Compact Bone Structure

Spongy Bone Structure

They form the exterior of bone tissue and are called cortical bones.

They are found in the interior of bone tissue and are also called cancellous bones.

They also form the diaphysis of long bones, which is the midsection. 

They constitute the epiphysis or the enlarged ends of long bones.

They are tough and heavy with a smooth exterior.

They are light and spongy with several lamellar spaces.

They are made of closely packed osteons.

They are composed of minerals containing trabeculae.

Compact bone’s function is to provide structural support to the body.

They function as a buffer system to the compact bone and support its function. 

They are cylindrical shaped.

They have a cuboidal structure.

Compact bones have high calcium content.

They have low calcium content.

They contain a yellow bone marrow in the bone marrow cavity.

They don’t have a bone marrow cavity as such but do contain red bone marrow.

Their bone marrows store fat.

Their bone marrows produce red blood cells and white blood cells.

They constitute 80% of the skeletal systems’ total weight. 

They account for 20% of the total skeletal system's weight.

They are found in long bone tissues such as in the arms and legs.

They are found mostly in short bones such as those in the wrist and ankles.

They are designed to withstand heavy weight.

They are not designed to buffer the shock produced from any movement.

They do not transform into spongy bones.

The action of osteoblasts deposits a new bone matrix between trabeculae and eventually transforms spongy bones to compact bones. 

Some interesting similarities between Compact and Spongy Bones are given below.

  • They are the bone tissues in animals that provide shape and support to the body. 

  • Both types of bones contain osteoblasts and osteoclasts that are necessary for creating bones.

  • Both compact and spongy bones contain proteins like collagens and osteoids, which mineralize to help in bone formation.

  • There are inorganic mineral salts deposited in the matrix of both spongy and compact bones. 

  • There are blood vessels, nerves, and bone marrow within the lamella of both types of bones.

  • They form joining sites to attach muscles and also contain cartilages and membranes such as the endosteum.

Fun Facts about Spongy Bones

Research on chimpanzees and skeletal structures of prehistoric humans show a higher percentage of spongy bones than the modern human skeletal system. It is found that trabecular bone density has decreased in the current human skeletal system. This is attributed to the sedentary lifestyle prevalent now, which has reduced the need for constant physical movement.

The following compact bone diagrams explain its structure along with the structure of spongy bones. It also shows how blood vessels are present within both types of bones. 

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FAQ (Frequently Asked Questions)

Q1. Why don't Compact Bones have bone marrow cavities like the Spongy Bones?

Answer: The bone marrow is made from millions of blood vessels crowded together to form myeloid tissue. In spongy bones, there are adequate sponge-like cavities called lamella. They form a trabecular matrix where the blood vessels can condense to form the bone marrow. Therefore they are porous enough to deliver adequate glucose, lipids, amino acids, and other trace minerals necessary to create new red blood cells. This process is called erythropoiesis. Further, when red blood cells are old or damaged, they return to sites like the spleen, liver or reach back to the bone marrow. The old cells are phagocytized by macrophages and removed from the bloodstream.

In compact cells, the osteons are densely packed, leaving no space for nutrients and minerals to reach blood vessels. Therefore erythropoiesis cannot take place in the compact bones. Instead, they have a yellow bone marrow majorly consisting of fat. 

Q2. What are the functions of Bones other than providing structural support to the body?

Answer: As explained earlier, bones have many functions than just providing support and shape to an organism's body. Below are some functions of bones.

  • The storing of bone marrow within the bone matrix happens in spongy bones. 

  • Erythropoiesis or formations of blood cells happen within the bone marrows, and hence it offers a sire for cell production. 

  • Spongy bones act as a shock absorber for all movements, especially at the joints while walking, jumping, etc.

  • The spongy bones in the body are responsible for storing 99% calcium in the body and 85% phosphorus. They also regulate the mineral content in the body by releasing the necessary amount and keeping the rest. The compact bones are chiefly responsible for releasing calcium into the body. This is an important function since varying mineral levels in the body adversely affect the muscles and nervous system's functioning.