Connective Tissue

Connective tissue joins cells and other tissues together. It provides a supporting frame for the body and the transport substances throughout the body (for example, blood). Connective tissue is characterized by large amounts of extracellular matrix that separates cells from each other. The extracellular matrix has three major components: protein fibers, ground substance consisting of nonfibrous protein and other molecules, and fluid. Connective tissue cells are named according to their functions. -Blast cells produce the matrix, -Cyte cells maintain it, and -Clast cells break it down for remodeling. For example osteoblasts form bone, osteocytes maintain bone, and osteoclasts break down bone.

Classification

The nature of the extracellular matrix determines the functional characteristics of the connective tissue and is used as a means of classifying the connective tissue.

Be sure to check out the electron micrographs by clicking on the thumbnails at the bottom of each section.

Matrix with protein fibers as the primary feature

The most common protein in the body is collagen which resembles microscopic ropes. It is flexible but resists stretching.

Dense connective tissue has an extracellular matrix consisting mostly of collagen fibers. The few cells found in dense connective tissue are fibroblasts, which are responsible for the production of the collagen fibers. Structures made up of dense connective tissue include tendons, which attach muscles to bone; ligaments, which attach bones to other bones; and the dermis of the skin, which is a layer of connective tissue under the stratified squamous epithelium.

In contrast to dense connective tissue, the protein fibers in loose or areolar connective tissue are widely separated from each other. Loose connective tissue is the "loose packing" material of the body, which fills the spaces between organs and holds them in place. It is found around glands, muscles, and nerves, and attaches the skin to underlying tissues.

Although adipose tissue has a matrix with protein fibers, it is not a typical connective tissue. There is very little matrix, and the adipose cells are large and closely packed together. Adipose cells are filled with lipids and function to store energy. Adipose tissue also pads and protects parts of the body and acts as a thermal insulator.

Examine these electron micrographs of the above connective tissues


Dense: Tendon	Loose: Reticular	Adipose

Adipose Droplet

Matrix with both protein fibers and ground substance

Cartilage is compose of cartilage cells, or chondrocytes, located in spaces called lacunae within an extensive matrix. Collagen in the matrix gives cartilage strength, and the ground substance of the matrix traps water, which enables the cartilage to spring back after being compressed. Cartilage is relatively rigid and provides support, but if it is bent or slightly compressed it will resume its original shape. Cartilage heals slowly after an injury because blood vessels do not penetrate the cartilage. Thus cells and nutrients necessary for tissue repair do not easily reach the damaged area.

Hyaline cartilage is the most abundant type of cartilage and has many functions. It covers the ends of the bones where bones come together to form joints, and provides a smooth, resilient surface that can withstand compression. Hyaline cartilage also forms the costal cartilage, which attach the ribs to the sternum (breast bone).

Fibro cartilage has more collagen than hyaline cartilage. In addition to withstanding compression, it is able to resist pulling or tearing forces. It is found in the disks between vertebrae (bones of the back) for example.

Elastic cartilage contains an elastic protein fiber in addition to collagen and ground substance and is able to recoil to its original shape when bent. The external ear contains elastic cartilage.

Examine these electron micrographs of Hyaline cartilage

Hyaline cartilage and Perichondrium

Solid Matrix

Bone is hard connective tissue that consists of living cells and a mineralized matrix. Bone cells, or osteocytes, are located within spaces in the matrix called lacunae. The strength and rigidity of the mineralized matrix enables bones to support and protect other tissues and organs of the body. There are two types of bone, compact and cancelous.

Examine this electron micrograph of bone tissue

Bone tissue

Fluid Matrix

Blood is unique because the matrix is liquid, enabling blood cells to move about freely. Some blood cells even leave the blood and wander into other tissues. The liquid matrix enables blood to flow rapidly through the body carrying food, oxygen, waste products, and other materials. The cellular component consists of red blood cells (for O2/CO2 transport), white blood cells (for disease fighting) and platelates (for blood clotting)

	Hyaline cartilage is the most abundant type of cartilage and has many functions. It covers the ends of the bones where bones come together to form joints, and provides a smooth, resilient surface that can withstand compression. Hyaline cartilage also forms the costal cartilage, which attach the ribs to the sternum (breast bone).
	Fibro cartilage has more collagen than hyaline cartilage. In addition to withstanding compression, it is able to resist pulling or tearing forces. It is found in the disks between vertebrae (bones of the back) for example.
	Elastic cartilage contains an elastic protein fiber in addition to collagen and ground substance and is able to recoil to its original shape when bent. The external ear contains elastic cartilage.