4.3 Connective Tissue Supports and Protects

Learning Objectives

Describe the structural characteristics of the various connective tissues and how these characteristics enable their functions.

By the end of this section, you will be able to:

  • Identify and distinguish between the different type of connective tissue: proper, supportive, and fluid – and associate each with their function and location
  • Describe the common structural elements of connective tissue
  • Describe how the structural properties of connective tissue relate to the unique functions of the tissue

Functions of Connective Tissues

Connective tissues perform many functions in the body, most importantly, they support and connect other tissues: from the connective tissue sheath that surrounds a muscle, 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. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of gases, 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.

Embryonic Connective Tissue

All connective tissues derive from the mesodermal layer of the embryo (see Figure 4.2.2). The first connective tissue to develop in the embryo is mesenchyme, the stem cell line from which all connective tissues are later derived. Clusters of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after a connective tissue injury. A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton’s jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.

Structural Elements of Connective Tissue

Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. Unlike epithelial tissue, which is composed of cells closely packed together, cells of connective tissue are more widely dispersed within an extracellular matrix (ECM). The matrix plays a major role in the functioning of this tissue. The major component of the matrix is ground substance. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones.  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.

Cell Types

Each class of connective tissue is formed by fundamental cell types. The cells can be found in both an active form (suffix –blast), where they are dividing and secreting the components of ground substance, and an in-active form (suffix –cyte).  The most abundant cell in connective tissue proper 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 and cells, forms the extra-cellular matrix. Chondroblasts and osteoblasts are the primary specialized cell type located in cartilage and bone, respectively.

Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population.

The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue.

The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. The macrophage cells are an essential component of the immune system, which is the body’s defense against potential pathogens and degraded host cells. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. In contrast, fixed macrophages are permanent residents of their tissues.

The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness (in people with light skin), recognized as an allergic response. Mast cells are derived from hematopoietic stem cells and are part of the immune system.

Connective Tissue Fibers and Ground Substance

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, straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience.

An 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, the walls of large blood vessels, and in a few ligaments which support the spine.

A reticular fiber is formed from the same protein subunits as collagen fibers, however, these fibers remain narrow and are arranged in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as the 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 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 ground substance.

Classification of Connective Tissues

The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix (Table 4.1). Connective tissue proper includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue—bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins.

Table 4.1
Connective tissue proper Supportive connective tissue Fluid connective tissue
Loose connective tissue:

  • Areolar
  • Adipose
  • Reticular
Cartilage:

  • Hyaline
  • Fibrocartilage
  • Elastic
Blood
Dense connective tissue:

  • Regular
  • Irregular
  • Elastic
Bone:

  • Compact bone
  • Spongy bone
Lymph

Connective Tissue Proper

Fibroblasts are present in all connective tissue proper (Figure 4.3.1). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body.

The left image shows a diagram of connective tissue. As a whole, the connective tissue appears somewhat disorganized, with fibers and cells mixed together heterogeneously. There are many open spaces between the embedded elements, suggesting that the connective tissue is somewhat loosely packed. The thickest fibers are collagen fibers; the thinner fibers are elastic fibers. Both the collagen fibers and the elastic fibers crisscross randomly throughout the tissue. In addition, a net of reticular fibers appear in the upper part of the diagram. Two yellow and oval shaped adipocytes are embedded below the reticular fiber net, with a small dark nucleus squeezed into one corner of the cell. A mesenchymal cell is next to one of the adipocytes. The cell is rectangular and has four projections stemming from each corner of the cell. The projections appear to attach to the nearby collagen fibers. A fibroblast is located at the center of the diagram. The fibroblast appears similar to the mesenchymal cell, except that it is larger and has more projections. Finally, a white macrophage is in the lower right of the diagram. The macrophage is a white, oval shaped disc with a prominent nucleus. The right diagram is a micrograph of connective tissue. The tissue is mostly stained pink, however, the thick collagen fibers crisscrossing the tissue are white. Five adipocytes also appear white, except for their cell membrane and nucleus, which stained dark. A mesenchymal cell occupies the space between two adipocytes. It stains a very deep purple, but its shape is unclear in the micrograph. A fibrocyte is also visible as an oval shaped cell with a deep purple nucleus.
Figure 4.3.1 – Connective Tissue Proper: Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells (LM × 400). (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

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, with little extracellular matrix (Figure 4.3.2). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys, cushioning the back of the eye, within the abdomen, and in the hypodermis. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.

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 4.3.2 – 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 relatively little specialization and is the most widely distributed connective tissue in the body. It contains all the cell types and fibers previously described and is structured in an apparently 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.

Areolar tissue
Figure 4.3.2a – Areolar tissue

Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 4.3.3). The reticular fibers 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 4.3.3 – 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 and a higher tensile strength. 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 mostly formed from dense regular connective tissue.

In dense irregular connective tissue, the arrangement of proteins fibers is irregular and lacks the uniformity seen in dense regular . This arrangement gives the tissue greater strength in all directions and less strength in any 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 elastic tissue contains elastin fibers in addition to collagen fibers, which allows the tissue to return to its original length after stretching. Dense elastic tissues give arterial walls the strength and the ability to regain original shape after stretching (dense CT figure).

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 4.3.4 – 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)

 

Figure 4.3.4a – Dense Elastic Connective Tissue: Dense elastic connective tissue consists of high proportion of elastic fiber.

Disorders of the Connective Tissue: Tendinitis

Your opponent stands ready as you prepare to hit the serve, but you are confident that you will smash the ball past your opponent. As you toss the ball high in the air, a burning pain shoots across your wrist and you drop the tennis racket. That dull ache in the wrist that you ignored through the summer is now an unbearable pain. The game is over for now.

After examining your swollen wrist, the doctor in the emergency room announces that you have developed wrist tendinitis. She recommends icing the tender area, taking non-steroidal anti-inflammatory medication to ease the pain and to reduce swelling, and complete rest for a few weeks. She interrupts your protests that you cannot stop playing. She issues a stern warning about the risk of aggravating the condition and the possibility of surgery. She consoles you by mentioning that well known tennis players such as Venus and Serena Williams and Rafael Nadal have also suffered from tendinitis related injuries.

What is tendinitis and how did it happen? Tendinitis is the inflammation of a tendon, the thick band of fibrous connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a joint. Most often, the condition results from repetitive motions over time that strain the tendons needed to perform the tasks.

Persons whose jobs and hobbies involve performing the same movements over and over again are often at the greatest risk of tendinitis. You hear of tennis and golfer’s elbow, jumper’s knee, and swimmer’s shoulder. In all cases, overuse of the joint causes a microtrauma that initiates the inflammatory response. Tendinitis is routinely diagnosed through a clinical examination. In case of severe pain, X-rays can be examined to rule out the possibility of a bone injury. Severe cases of tendinitis can even tear loose a tendon. Surgical repair of a tendon is painful. Connective tissue in the tendon does not have abundant blood supply and heals slowly.

While older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people of all ages can develop tendinitis. Young athletes, dancers, and computer operators; anyone who performs the same movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and may lead to tendinitis, precautions can be taken that can lessen the probability of developing tendinitis. For active individuals, stretches before exercising and cross training or changing exercises are recommended. For the passionate athlete, it may be time to take some lessons to improve technique. All of the preventive measures aim to increase the strength of the tendon and decrease the stress put on it. With proper rest and managed care, you will be back on the court to hit that slice-spin serve over the net.

External Website

tendonitis

Watch this animation to learn more about tendonitis, a painful condition caused by swollen or injured tendons.

Supportive Connective Tissues

Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs.

Cartilage

The distinctive appearance of cartilage is due to 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 4.3.5 – Types of Cartilage). 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 forms the 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 intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue provides 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 4.3.5 – 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. Osteoblasts are the active bone forming cells, producing the organic part of the extracellular matrix. The mature bone cells, osteocytes, are located within lacunae.  Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time.

The histology of a cross sectional view of compact bone shows a typical arrangement of osteocytes in concentric circles around a central canal. This structural unit of compact bone is called the osteon. There is no such structural unit in cancellous bone, or spongy bone, which looks like a sponge under the microscope and contains empty spaces between trabeculae. It is lighter than compact bone and found in the interior of bones and at the end of long bones. Compact bone is solid and has greater structural strength.

Fluid Connective Tissue

Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow (Figure 4.3.6 – Blood: A Fluid Connective Tissue). Erythrocytes, red blood cells, transport oxygen and 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 and transported through the body.

Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are highly permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph vessels return molecules and fluid to the venous 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 4.3.6 – 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)

External Website

cardiovascular_new

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.

External Website

10quiz

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?

Chapter Review

Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfills in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injuries. Connective tissues also provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis.

Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen fibers (which are thick, strong, flexible, and resist stretch), reticular fibers (which are thin and form a supportive mesh, and elastin (fibers that are thin and elastic).

The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may be regular, with fibers oriented in parallel as in ligaments and tendons,  irregular, with fibers oriented in several directions, or elastic, with a large amount of the protein elastin embedded within the fibers. Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense, irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibers and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibers.  Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid matrix and no supporting fibers.

Interactive Link Questions

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?

Click at the bottom of the quiz for the answers.

Review Questions

 

 

 

 

 

Critical Thinking Questions

One of the main functions of connective tissue is to integrate organs and organ systems in the body. Discuss how blood fulfills this role.

Blood is a fluid connective tissue, a variety of specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins in a liquid extracellular matrix. Blood contains formed elements derived from bone marrow. Erythrocytes, or red blood cells, transport the gases, oxygen and carbon dioxide. Leukocytes, or white blood cells, are responsible for the defense of the organism against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some cells have the ability to cross the endothelial layer that lines vessels and enter adjacent tissues. Nutrients, salts, and waste are dissolved in the liquid matrix and transported through the body.

Why does an injury to cartilage, especially hyaline cartilage, heal much more slowly than a bone fracture?

A layer of dense irregular connective tissue covers cartilage. No blood vessels supply cartilage tissue. Injuries to cartilage heal very slowly because cells and nutrients needed for repair diffuse slowly to the injury site.


This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax, licensed under CC BY. This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax, are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.

License

Icon for the Creative Commons Attribution-ShareAlike 4.0 International License

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.