Main Difference – Compact Bone vs Spongy Bone
Bones provide structural support and aid in the movement of the body parts. Compact and spongy bone are two types of bones found in the bone tissue. The main difference between compact and spongy bone is that compact bone is the hard outer layers of the bone whereas spongy is the more porous, inner layers of the bone. Compact bones lack spaces between lamellae whereas spongy bones consist of spaces between lamellae. Compact bones are called cortical bones. Spongy bones are called cancellous bones. Compact bone consists of yellow bone marrow in the marrow cavity whereas spongy bones consist of red bone marrow in the spaces between lamellae. The yellow bone marrow stores fat while the red bone marrow produces red blood cells and granular white blood cells.
Key Areas Covered
1. What is a Compact Bone
– Definition, Features, Function
2. What is a Spongy Bone
– Definition, Features, Function
3. What are the Similarities Between Compact and Spongy Bone
– Outline of Common Features
4. What is the Difference Between Compact and Spongy Bone
– Comparison of Key Differences
Key Terms: Cancellous Bones, Compact Bone, Cortical Bone, Diaphyses, Haversian Canal, Lamella, Marrow cavity, Osseous Tissue, Osteons, Spongy Bone, Trabeculae
What is a Compact Bone
A compact bone is the hard outer shell of a bone. It is also called the cortical bone. The strongest and densest bones in the body are compact bones. However, compact bones consist of tiny passages for both blood vessels and nerves for the maintenance and repair of the bone. A compact bone is covered by a periosteum from the outside. The inner surface of the bone is covered by the endosteum, a thin, vascular connective tissue, which lines the marrow cavity of the long bones. The marrow cavity occurs at the shaft of the long bones called diaphyses. It consists of yellow bone marrow, which stores fat. The structure of a compact bone is shown in figure 1.
Figure 1: Compact Bone
The osseous tissue of a compact bone consists of osteocytes, surrounded by a solid extracellular matrix. The mineral that is found in the extracellular matrix of bones is hydroxyapatite. Hydroxyapatite is rich in calcium and phosphorous. The collagen fibres, which are interspersed in the hydroxyapatite, give some sort of flexibility to the bone. The structure of a compact bone is made up of osteons that surround tiny central canals. During fetal development, the osteogenic cells secrete the bone matrix as a ring called lamella. The osteogenic cells with lamellae are called osteocytes. The cavity that contains the osteocyte is called lacuna. The tiny canals, which provides nutrients and oxygen to osteocytes are called canaliculi. The canaliculi also connect lacunae, allowing the intercellular communication of osteocytes. This produces several concentric layers of lamellae around a central canal, forming an osteon. The central canal is called Haversian canal. The structure of an osteon is shown in figure 2.
Figure 2: Osteon
The compact bones provide structural support to the animal body and protect the internal organs of the body. They also provide a shape to the body. The thickness of compact bones is maintained by a layer of osteoblasts and osteoclasts. Compact bones also serve as a mineral store, aiding the mineral homeostasis of the body.
What is a Spongy Bone
Spongy bone is the osseous tissue found in the middle of the bones. Spongy bones are less dense and lighter than compact bones. They are also called cancellous bones. The interior portion of the long bones is composed of spongy bones. The matrix of the bone is composed of mineral bars called trabeculae, which forms a three-dimensional latticework. The spaces of the latticework are filled with red bone marrow and blood vessels. The spaces are connected to each other by canaliculi. Red bone marrow produces blood cells in a process called hematopoiesis. The structure of the spongy bone is shown in figure 3.
Figure 3: Spongy Bone
Spongy bone forms the enlarged ends of the long bones called epiphyses. They are the major components of the ribs as well as the shoulder blades and flat bones of the skull. Most of the short flat bones of the skeleton are also composed of spongy bones. Spongy bones show relatively high metabolic activity as well. The spongy bones are transformed to the compact bones by the action of osteoblasts. Osteoblasts deposit new bone matrix around the trabeculae. Therefore, the space between trabeculae become limited eventually.
Similarities Between Compact and Spongy Bone
- Compact bone and spongy bones are the two types of bones in animals.
- Both compact and spongy bones are made up of osseous tissue, which is a type of connective tissue that makes the bone.
- Both compact and spongy bones are involved in the movement of the body.
- Both compact bone and spongy bones contain calcium.
Difference Between Compact and Spongy Bone
Definition
Compact Bone: Compact bone is the non-cancellous portion of a bone, which largely consists of closely packed osteons and forms the hard exterior of the bone.
Spongy Bone: Spongy bone is the osseous tissue, which fills the interior cavity of bones, consisting of mineralized bars called trabeculae.
Alternative Names
Compact Bone: Compact bones are also called cortical bones.
Spongy Bone: Spongy bones are also called cancellous or trabecular bones.
Composition
Compact Bone: Compact bones are made up of osteons.
Spongy Bone: Spongy bones are made up of trabeculae.
Spaces between Lamellae
Compact Bone: Compact bones do not have spaces between lamellae.
Spongy Bone: Spongy bones consist of spaces between lamellae.
Marrow Cavity
Compact Bone: Compact bone has a marrow cavity.
Spongy Bone: Spongy bone does not have a marrow cavity.
Amount of Calcium
Compact Bone: There is a high amount of Calcium in compact bones.
Spongy Bone: There is a very low amount of Calcium in spongy bones.
Contribution to the Weight of the Skeleton
Compact Bone: Compact bones takes 80% of the weight of the skeleton.
Spongy Bone: Spongy bones takes 20% of the weight of the skeleton.
Shape of the Bone
Compact Bone: Compact bones are cylindrical.
Spongy Bone: Spongy bones are cuboidal.
Bone Marrow
Compact Bone: Compact bones contain yellow bone marrow.
Spongy Bone: Spongy bones contain red bone marrow.
Long/Short Bones
Compact Bone: Compact bones form major parts of the long bones such as arms and legs.
Spongy Bone: Spongy bones form major parts of the short bones such as wrists and ankles.
Structure
Compact Bone: Compact bones are hard.
Spongy Bone: Spongy bones are softer and more porous.
Location
Compact Bone: Compact bones occur in the inner surface of a bone.
Spongy Bone: Spongy bones occur in the outer layers of a bone.
Strength
Compact Bone: Compact bones can withstand the weight of up to 5000 pounds.
Spongy Bone: Spongy bones are unable to withstand high weights.
Function
Compact Bone: Compact bones provide structural support to the body.
Spongy Bone: Spongy bones act as a buffer for compact bones.
Conclusion
Compact bone and spongy bone are the two types of bones that occur in animals. Compact bones are made up of osteons. They are hard and contain high amounts of minerals. Spongy bones are made up of trabeculae. They are softer and contain a lot of spaces in the bone. Compact bones occur in the outer surface of the long bones and spongy bones occur in the middle of the long bones. The main difference between compact and spongy bone is their structure and function.
Reference:
1.“Cortical (Compact) Bone.” InnerBody, Available here. Accessed 9 Sept. 2017.
2.“Cancellous bone.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 16 Apr. 2015, www.britannica.com/science/cancellous-bone. Accessed 9 Sept. 2017.
Image Courtesy:
1. “605 Compact Bone” By OpenStax Anatomy and Physiology – (CC BY 4.0) via Commons Wikimedia
2. “Transverse section of bone en” By Source digital bitmap graphics: BDBRecreated in vector format: Nyq – Original analog graphics: Gray’s Anatomy of the Human Body from the classic 1918 publication (CC BY-SA 4.0) via Commons Wikimedia
3. “606 Spongy Bone” By OpenStax College – Anatomy & Physiology, Connexions Web site. Available here, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
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