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

As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. Two major components of the matrix are ground substance and protein fibers. This ground substance is usually a fluid (water), but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells; for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris.

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.

Functions of Connective Tissues

Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body.

Classification of Connective Tissues

Categories of connective tissue include the following:

  • 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

Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues.

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.

Image A shows a collection of yellow adipocytes that do not have a consistent shape or size, however, most have the general appearance of a kernel of corn with a wide end that tapers to a point. Each adipocyte has a nucleus occupying a small area on one side of the cell. Nothing else is visible within the cells. Image B shows a micrograph of adipose tissue. Here, the adipocytes are stained purple. However, only their edges and their nuclei stain, giving the adipose tissue a honeycomb appearance. The adipocytes in the micrograph are large and round, but still show a diversity of shapes and sizes. The nucleus appears as a dark staining area very close to the cell membrane.
Figure 4.6. Adipose Tissue
This is a loose connective tissue that consists of fat cells with little extracellular matrix. It stores fat for energy and provides insulation. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.

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.”

This figure shows reticular tissue alongside a micrograph. The diagram shows a series of small, oval cells embedded in a yellowish matrix. Thin reticular fibers spread and crisscross throughout the matrix. In the micrograph, the reticular fibers are thin, dark, and seem to travel between the many deeply stained cells.
Figure 4.8. Reticular Tissue
This 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

Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are three major categories of dense connective tissue: regular, irregular, and elastic. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular 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).

Part A shows a diagram of regular dense connective tissue alongside a micrograph. The tissue is composed of parallel, thread-like collagen fibers running vertically through the diagram. Between the vertical fibers, several dark, oval shaped fibroblast nuclei are visible. In the micrograph, the whitish collagen strands run horizontally. Several dark purple fibroblast nuclei are embedded in the lightly stained matrix. Part B shows a diagram of irregular dense connective tissue on the left and a micrograph on the right. In the diagram, the collagen fibers are arranged in bundles that curve and loop throughout the tissue. The fibers within a bundle run parallel to each other, but separate bundles crisscross throughout the tissue. Because of this, the irregular dense connective tissue appears less organized than the regular dense connective tissue. This is also evident in the micrograph, where the white collagen bundles radiate throughout the micrograph in all directions. The fibroblasts are visible as red stained cells with dark purple nuclei.
Figure 4.9. Dense Connective Tissue
(a) Dense regular connective tissue consists of collagenous fibers packed into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like network. From top, LM × 1000, LM × 200. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)
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 Tissue
Elastic connective tissue consists of collagenous fibers with interwoven elastic fibers.

Cartilage

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.

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.

Part A of this diagram is a drawing and a micrograph of hyaline cartilage. The cartilage contains chondrocytes encapsulated in lacunae. Several of the lacunae are joined into groups or small stacks and embedded in the surrounding matrix. The micrograph shows the lacunae as white rings surrounding the purple staining chondrocytes. Some occur as joined pairs while others are embedded singly within the pink staining matrix. Image B shows a diagram and a micrograph of fibrocartilage that contains many fine collagen fibers embedded in the matrix. The collagen fibers are roughly parallel to each but run through the matrix in a wavy fashion. There are also four round chondrocyte cells embedded within the matrix. In the micrograph, the matrix is shaded red and the collagen fibers are visible in white. The lacunae are clearly visible as a faint purple ring containing several dark purple chondrocytes. Part C shows a diagram and micrograph of elastic cartilage. In the diagram, fine elastic fibers are seen crisscrossing the matrix. Many of the elastic fibers branch off from each other, unlike the collagen fibers depicted in parts A and B. The lacunae are clearly visible as white rings containing stained chondrocytes. The fibers stain deeply in this micrograph and can been seen crisscrossing through the tissue.
Figure 4.11. Types of Cartilage
Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. The example is from dog tissue. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but elastic support. From top, LM × 300, LM × 1200, LM × 1016. (Micrographs provided by the Regents of University of Michigan Medical School © 2012)

Bone

Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal (Figure 4.12). Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time.

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 Tissue
There 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.

This micrograph of a blood smear shows a group of red blood cells and a single white blood cell. The red cells are small discs which have a slight depression at their centers with no nuclei present. The white blood cell is larger and more darkly stained and has a large, prominent nucleus that is also darkly stained.
Figure 4.13. Blood: A Fluid Connective Tissue
Blood 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)
 

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?