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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.

Compact Bone - Spongy Bone - Other Bone Components

The hard mineral component of bones is additionally referred to as bone. Bone comes in two forms, both of which are present in every bone within the body: compact bone and spongy bone. These two forms are dependent upon two factors - the organization of bone minerals and the proportion of empty spaces in the extracellular matrix. Spongy bones consist of a web whereas compact bones appear solid. 

While compact bone appears initially to be solid and uninterrupted, closer inspections reveal that the bone only makes up 70-95% of the available volume. Even compact bones consist of spaces and pores. Except at its edge, the compact bone is arranged in cylindrical osteons. Each osteon may be a compact cylinder of concentric lamellae. The sole cells in an osteon are the osteocytes that are found on the sides of every lamella. The cell-shaped spaces called lacunae are present in Osteocytes. They prevent the crushing of the Osteocytes in the bone. At the center of every osteon may be a central canal through which blood vessels, lymph vessels, and nerves can visit service and signal the cells throughout the compact bone. At the bottom of individual osteons are perforating canals, which are empty spaces that allow blood vessels, lymph vessels, and nerves to travel across the bone, linking up with the vessels and nerves within the central canals. Compact bone is usually called cortical bone.

At the outer edges of compact bone, instead of being arranged in osteons, the bone is arranged in circumferential lamellae. These travel parallel to the fringes of the bone and are usually only a couple of lamellae deep before the osteons startup.

Only the outermost surfaces have compact bones and are not deep hence the bones of the body consist of compact bones. The majority of bone tissue is formed of spongy bone. In spongy bone, there's much more uncompacted space. The bone only makes up somewhere between 10-70% of the available volume, counting on how spongy it's. The remainder of the quantity is formed from mostly bone marrow, although there also are blood vessels, lymph vessels, nerves traveling through the spaces. The bone is organized into trabeculae and is interconnected bones creating a spongy grid of spongy bone. Within one trabecular, there are concentric lamellae, with osteocytes in lacunae connected to at least one another via canaliculi, almost like the tissue arrangement within the osteons of compact bone. However, unlike osteons, trabeculae don't have central canals or perforating canals containing blood vessels, lymph vessels, and nerves. Spongy bone is usually called cancellous bone or trabecular bone.

The outsides of all the bones of the body are covered with a layer of irregular dense animal tissue called the periosteum. There's a special layer of animal tissue proper that lines all the interior cavities of bones – the central canals of osteons in compact bone, and therefore the exterior spongy bone is called endosteum. 

The long bones of the body found within the arms, legs, hands, and feet of the body, have a further feature unique to their long shape. within the diaphysis or shaft, of every os longum, the maybe a central hollow cavity, called the medullary cavity. Having no heavy bone within the center of the long bones makes them lighter. The non-long bones just believe in having spongy bones in their interior to scale back their overall mass. The medullary cavity, which lives in spaces in spongy bone, is crammed with bone marrow.

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 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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FAQs on Difference Between Compact and Spongy Bones

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

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 compact bones. Instead, they have a yellow bone marrow majorly consisting of fat.

2. What are the functions of bones other than providing structural support to the body?

As explained earlier, bones have many functions other 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 marrow, 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.

3. Are spongy bone and bone marrow the same?

In the human body, there are two different types of marrow that are present. Red marrow contains blood stem cells and these blood cell stems then become platelets, white blood cells(WBC), and red blood cells (RBC). The red marrow is found at the ends of bones and is called the spongy bone. The yellow marrow is found at the center of most of the bones and it has a lot of blood vessels too. Hence, Spongy bone and bone marrow are not the same. Bone marrow is found in the spongy bone located at a specific location.

4. What is the function of Osteocytes?

An osteocyte is a cell found within the material of fully formed bone. It lives in a small chamber known as a lacuna that is confined within the calcified matrix of bone. Osteocytes are derived from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded by the released products. The osteocyte's cytoplasmic processes stretch away from the cell in small channels called canaliculi toward neighboring osteocytes. Nutrients and waste products are exchanged through these canaliculi to keep the osteocyte alive. In mature bone tissue, osteocyte cells are the most numerous types of cells.

5. What is the structure of the Cancellous bone?

Cancellous bone is also known as a spongy bone because it has many open spaces joined by flat planes of bone known as trabeculae, much like a sponge or honeycomb. There are three types of bone cells inside the trabeculae: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are the cells responsible for the formation of new bones. They develop layers of hard tissue mostly composed of calcium and phosphate until they are completely surrounded, at which point they are classified as osteocytes. Finally, osteoclasts are bigger cells that break down and destroy old or damaged bone in order for osteoblasts to repair and replace it. This cycle of repair and rebuilding is continuing in order to keep bones strong and healthy.

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