Connective Tissue
Learning Objectives
- Identify and distinguish between the types of connective tissue: loose, dense, cartilage, bone, and blood
- Explain the functions of connective tissues
- Connective Tissue - Fibers and Ground Substance
- Functions of Connective Tissues
- Classification of Connective Tissues
- Loose Connective Tissue
- Dense Connective Tissue
- Cartilage
- Bone
- Blood
The most common cell found within connective tissue is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix.
Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the movement of the body.
Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column.
Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ).
All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of water, polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance.
Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column.
Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ).
All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of water, polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance.
Functions of Connective Tissues
Classification of Connective Tissues
- Loose Connective Tissue - large amounts of ground substance and fewer fibers
- Aerolar
- Adipose
- Reticular
- Dense Connective Tissue - large amounts of fibers and less ground substance
- Dense Regular
- Dense Irregular
- Elastic CT
- Cartilage - specialized cells called chondrocytes are within the matrix (cartilage cells)
- Hyaline Cartilage
- Elastic Cartilage
- Fibrocartilage
- Bone - strongest connective tissue with little ground substance, hard matrix of calcium and phosphorous and specialized bone cells called osteocytes
- Blood - fluid connective tissue, no fibers - only ground substance (plasma) and cells (red, white, and platelets)
Loose Connective Tissue
Adipose tissue consists mostly of fat storage cells called adipocytes that store lipids as droplets that fill most of the cytoplasm (figure 4.6). A large number of capillaries allow rapid storage and mobilization of lipid molecules. Fat contributes mostly to lipid storage, can serve as insulation from cold temperatures and mechanical injuries, and can be found protecting internal organs such as the kidneys and eye.
Figure 4.7. Areolar Tissue. This is a loose connective tissue widely spread throughout the body. It contains all three types of fibers (collagen, elastin, and reticular) with much ground substance and fibroblasts.
Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 4.8). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.”
Figure 4.8. Reticular TissueThis is a loose connective tissue made up of a network of reticular fibers that provides a supportive framework for soft organs. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Dense Connective Tissue
In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching (Figure 4.9).
Elastic connective tissue is a modified dense connective tissue that contains numerous elastic fibers in addition to collagen fibers, which allows the tissue to return to its original length after stretching Figure 4.10). The lungs and arteries have a layer of elastic connective tissue that allows the stretch and recoil of these organs.
Figure 4.10. Elastic Connective TissueElastic connective tissue consists of collagenous fibers with interwoven elastic fibers.
The distinctive appearance of cartilage is due to the presence of polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.Cartilage
The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 4.11). Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. The knee and jaw joints and the the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.
Bone
Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone is solid and has greater structural strength.
Figure 4.12. Bone Connective TissueThere are two types of bone tissue: compact and spongy. Picture shown is of compact bone tissue. This is most common and has the appearance of the matrix forming concentric rings around cavities for blood vessels. The osteocytes (bone cells) position themselves within the concentric rings.
Blood
Blood is a fluid connective tissues. Blood has two components: cells and fluid matrix (Figure 4.13). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix called plasma and transported through the body.
Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.
Figure 4.13. Blood: A Fluid Connective TissueBlood is a fluid connective tissue containing erythrocytes and various types of leukocytes that circulate in a liquid extracellular matrix. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.
Interactive Link
View the University of Michigan Webscope at http://virtualslides.med.umich.edu/Histology/Cardiovascular%20System/081-3_HISTO_40X.svs/view.apml to explore the tissue sample in greater detail.Interactive Link
Visit this link to test your connective tissue knowledge with this 10-question quiz. Can you name the 10 tissue types shown in the histology slides?Licenses and Attributions
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