Connective Tissue

Characteristics of Connective Tissue

Connective tissue is incredibly diverse and contributes to energy storage, the protection of organs, and the body’s structural integrity.

Learning Objectives

Describe the main characteristics and functions of connective tissue

Key Takeaways

Key Points

  • Connective tissue is the most abundant and widely distributed of the primary tissues.
  • Connective tissue has three main components: cells, fibers, and ground substance. Together the ground substance and fibers make up the extracellular matrix.
  • Connective tissue is classified into two subtypes: soft and specialized connective tissue.
  • Major functions of connective tissue include: 1) binding and supporting, 2) protecting, 3) insulating, 4) storing reserve fuel, and 5) transporting substances within the body.
  • Connective tissues can have various levels of vascularity. Cartilage is avascular, while dense connective tissue is poorly vascularized. Others, such as bone, are richly supplied with blood vessels.

Key Terms

  • extracellular matrix: Cells of the connective tissue are suspended in a non-cellular matrix that provides structural and biochemical support to the surrounding cells.
  • fibroblast: A type of cell found in connective tissue that synthesizes the extracellular matrix and collagen.
  • connective tissue: A type of tissue found in animals whose main function is to bind, support, and anchor the body.

Connective tissue (CT) is a one of the four main classes of tissues. Although it is the most abundant and widely distributed of the primary tissues, the amount of connective tissue in a particular organ varies. Like to the timber framing of a house, the connective tissue provides structure and support throughout the body.

Structure of Connective Tissue

Connective tissue has three main components:

  1. Ground substance
  2. Fibers
  3. Cells

Together the ground substance and fibers make up the extracellular matrix. The composition of these three elements vary tremendously from one organ to the other. This offers great diversity in the types of connective tissue.

This is a drawing of the structural elements of connective tissue. It depicts cells suspended in a ground substance or matrix, with elastic and collagen fibers running throughout the matrix. CT is classified into two subtypes: soft CT and specialized CT. The soft CT subtype—loose, dense, and elastic tissues—are found in parts of our skin, tendons and blood vessels. The special CT subtype includes cartilage, bone, adipose tissue, blood, and lymphatic tissue, and provides a structural framework for the body and connects body tissues.

Structural elements of connective tissue: Connective tissues consist of three parts: cells suspended in a ground substance or matrix; and most have fibers running through it.

Ground substance is a clear, colorless, viscous fluid that fills the space between the cells and fibers. It is composed of proteoglycans and cell adhesion proteins that allow the connective tissue to act as glue for the cells to attach to the matrix. The ground substance functions as a molecular sieve for substances to travel between blood capillaries and cells.

Connective tissue fibers provide support. Three types of fibers are found in connective tissue:

  1. Collagen
  2. Elastic fibers
  3. Reticular fibers

Collagen Fibers

This is a black and white x-ray of collagen fibers, the strongest and most abundant of all the connective tissue fibers.

Collagen: Collagen fibers are the strongest and most abundant of all the connective tissue fibers.

Collagen fibers are fibrous proteins and are secreted into the extracellular space and they provide high tensile strength to the matrix.

Elastic Fibers

Elastic fibers are long, thin fibers that form branching network in the extracellular matrix. They help the connective tissue to stretch and recoil.

Reticular Fibers

Reticular fibers are short, fine collagenous fibers that can branch extensively to form a delicate network.

Function of Connective Tissue

The major functions of connective tissue include:

  1. Binding and supporting.
  2. Protecting.
  3. Insulating.
  4. Storing reserve fuel.
  5. Transporting substances within the body.

Types of Connective Tissue

Connective tissues encompass a diverse array of tissue types that are involved in binding and supporting body structure and tissues.

Learning Objectives

Describe the diverse types of connective tissue

Key Takeaways

Key Points

  • The lymphatic system is a part of the circulatory system, comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally towards the heart.
  • Blood is considered a specialized form of connective tissue. In vertebrates, it is composed of blood cells suspended in a liquid called blood plasma.
  • The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite.
  • Adipose tissue or body fat is loose connective tissue composed of adipocytes.
  • Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes, and the intervertebral discs.
  • In humans, adipose tissue is located beneath the skin (subcutaneous fat), around internal organs (visceral fat), in bone marrow (yellow bone marrow), and in breast tissue.

Key Terms

  • cartilage: A type of dense, non-vascular connective tissue, usually found at the end of joints, the rib cage, the ear, the nose, in the throat, and between intervertebral disks.
  • adipose tissue: Connective tissue that stores fat and cushions and insulates the body.
  • blood: A vital liquid flowing in the bodies of many types of animals that usually conveys nutrients and oxygen. In vertebrates, it is colored red by hemoglobin, is conveyed by arteries and veins, is pumped by the heart, and is usually generated in bone marrow.

Connective tissue is divided into four main categories:

  1. Connective proper
  2. Cartilage
  3. Bone
  4. Blood

Connective tissue proper has two subclasses: loose and dense. Loose connective tissue is divided into 1) areolar, 2) adipose, 3)
reticular. Dense connective tissue is divided into 1) dense regular, 2) dense irregular, 3) elastic.

Areolar Connective Tissue

These tissues are widely distributed and serve as a universal packing material between other tissues. The functions of areolar connective tissue include the support and binding of other tissues.

It also helps in defending against infection. When a body region is inflamed, the areolar tissue in the area soaks up the excess fluid as a sponge and the affected area swells and becomes puffy, a condition called edema.

Adipose Tissue or Body Fat

image

Adipose tissue: Yellow adipose tissue in paraffin section with lipids washed out.

This is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.

The two types of adipose tissue are white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue is found in specific locations, referred to as adipose depots.

Reticular Connective Tissue

This tissue resembles areolar connective tissue, but the only fibers in its matrix are the reticular fibers, which form a delicate network. The reticular tissue is limited to certain sites in the body, such as internal frameworks that can support lymph nodes, spleen, and bone marrow.

Dense Regular Connective Tissue

This consists of closely packed bundles of collagen fibers running in the same direction. These collagen fibers are slightly wavy and can stretch a little bit.

With the tensile strength of collagen, this tissue forms tendons, aponeurosis and ligaments. This tissue forms the fascia, which is a fibrous membrane that wraps around the muscles, blood vessels, and nerves.

Dense Irregular Tissue

This has the same structural elements as dense regular tissue, but the bundles of collagen fibers are much thicker and arranged irregularly. This tissue is found in areas where tension is exerted from many different directions. It is part of the skin dermis area and in the joint capsules of the limbs.

Elastic Connective Tissue

The main fibers that form this tissue are elastic in nature. These fibers allow the tissues to recoil after stretching. This is especially seen in the arterial blood vessels and walls of the bronchial tubes.

Cartilage

This is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes, and the intervertebral discs.

Cartilage is composed of specialized cells called chondroblasts and, unlike other connective tissues, cartilage does not contain blood vessels. Cartilage is classified in three types: 1) elastic cartilage, 2) hyaline cartilage, and 3) fibrocartilage, which differ in the relative amounts of these three main components.

Elastic Cartilage

This is similar to hyaline cartilage but is more elastic in nature. Its function is to maintain the shape of the structure while allowing flexibility. It is found in the external ear (known as an auricle) and in the epiglottis.

Hyaline Cartilage

This is is the most abundant of all cartilage in the body. Its matrix appears transparent or glassy when viewed under a microscope. It provides strong support while providing pads for shock absorption. It is a major part of the embryonic skeleton, the costal cartilages of the ribs, and the cartilage of the nose, trachea, and larynx.

Fibrocartilage

This is a blend of hyaline cartilage and dense regular connective tissue. Because it is compressible and resists tension well, fibrocartilage is found where strong support and the ability to withstand heavy pressure are required. It is found in the intervertebral discs of the bony vertebrae and knee meniscus.

Bone tissue is also called the osseous tissue. The osseous tissue is relatively hard and lightweight in nature. It is mostly formed of calcium phosphate in the chemical arrangement termed calcium hydroxyapatite, which gives bones their rigidity. It has relatively high compressive strength, but poor tensile strength, and very low shear stress strength.

The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. Its porosity is 5–30%. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton.

Filling the interior of the bone is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone), which is composed of a network of rod and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow.

Blood

This is considered a specialized form of connective tissue. Blood is a bodily fluid in animals that delivers necessary substances, such as nutrients and oxygen, to the cells and transports metabolic waste products away from those same cells.

It is an atypical connective tissue since it does not bind, connect, or network with any body cells. It is made up of blood cells and is surrounded by a nonliving fluid called plasma.