Connective tissue connects, supports, binds, and separates organs and tissues, forming a framework to support body tissues and organs. It plays a role in immune function, transporting nutrients and wastes, storing fat, and repairing tissue damage.
Connective tissue is made from cells, fibers, and a gelatinous material called ground substance. Bone, cartilage, blood, fat, and lymphatic tissue are types of connective tissues.
The Role of Connective Tissue
Connective tissue is found throughout the body in a range of locations including tendons, ligaments, skin, membranes, elastic tissue, and others. It helps support and protect the body's organs and tissues. It also plays a role in a number of functions within the body, including:
- Repairing damaged tissue
- Fighting infection
- Storing fat
- Transporting nutrients
- Removing waste
Structure
Connective tissue is made up of cells and extracellular matrix. Extracellular matrix is the term used to describe the spaces outside of the cells. The extracellular matrix is composed of:
- Glycosaminoglycans (GAGs), long chains of sugar molecules that help support collagen
- Proteoglycans, GAGs attached to a core protein
Variations in the composition of the extracellular matrix determine properties of the connective tissue.
The cells in connective tissue are few and dispersed, so they are not in close contact with each other as they are in other types of tissue. Most connective tissues—with the exception of cartilage—are vascularized, which means they contain blood vessels.
The components of connective tissue include:
- Ground substance, a gelatinous material composed of water, proteins, GAGs, and proteoglycans
- Fibers including collagen, reticular fibers, and elastic fibers
- Cells such as bone, fat, muscle, and cartilage cells
Connective Tissue Types
Connective tissues fall into one of two primary categories.
Connective Tissue Proper
Connective tissue proper is classified as either loose irregular connective tissue or dense irregular connective tissue.
- Loose irregular connective tissue contains numerous cells and a loose fiber arrangement in a moderately viscous fluid matrix.
- Dense irregular connective tissue has a dense woven network of collagen and elastic fibers in a viscous matrix. Dense connective tissue is found in joint capsules, muscle fascia, and the dermis layer of skin.
Specialized Connective Tissue
Specialized connective tissue examples include:
- Dense regular connective tissue (found in tendons and ligaments)
- Cartilage (a type of supporting connective tissue that consists of chondrocyte cells, collagen fibers, and elastic fibers; semi-solid or flexible matrix; includes hyaline cartilage, fibrocartilage, and elastic cartilage)
- Adipose tissue (a type of supporting connective tissue that cushions, stores excess fat and energy; contains reticular cells and reticular fibers)
- Hemopoietic or lymphatic tissue (a fluid connective tissue involved in blood cell production; contains leukocytes and fibers of soluble liquid proteins formed during clotting; extracellular portion is plasma)
- Blood (contains erythrocytes, leukocytes, and thrombocytes; fibers are soluble proteins; extracellular substance is plasma)
- Bone (a type of supporting connective tissue that contains osteoblasts or osteocytes; consists of collagen fibers and is rigid or calcified)
Conditions
Under normal circumstances, the fibers, proteoglycan, and GAGs are regulated and controlled by a balance between synthesis and degradation. The balance is maintained by:
- Cytokines, proteins that help boost the immune response
- Growth factors, proteins that regulate cell growth
- Matrix metalloproteinases (MMPs), enzymes that help break down collagen
If there is an imbalance, connective tissue diseases can develop. Some conditions happen because of net degradation of connective tissues, while others happen because of a net increase in synthesis.
There are more than 200 diseases and conditions that affect connective tissue. Some connective tissue diseases are consequences of infection, autoimmune diseases, injury, or genetic abnormalities. The cause of some connective tissue diseases remains unknown.
Autoimmune Conditions
An autoimmune condition is a condition that causes the body's immune system to mistakenly attack healthy tissue. Autoimmune conditions that affect connective tissue include:
- Systemic lupus erythematosus
- Rheumatoid arthritis
- Scleroderma
- Myositis
- Sjögren's disease
Inherited Conditions
Some connective tissue diseases are caused by defective genes that impact the formation or strength of connective tissue. These include:
- Ehlers-Danlos syndrome
- Marfan syndrome
- Osteogenesis imperfecta
Other Connective Tissue Diseases
Connective tissue diseases may develop due to inflammation of the joints, membranes, blood vessels, or other parts of the body that contain connective tissue. Some of these conditions include:
- Osteoarthritis
- Osteoporosis
- Ankylosing spondylitis
- Soft tissue sarcoma
Summary
Connective tissue is the tissue that supports the organs and tissues in your body. Connective tissue also helps with body functions such as immune processes, transport of nutrients, fat storage, and tissue repair.
Most connective tissue contains fibers, cells, and ground substance. Cartilage, adipose tissue, lymphatic tissue, blood, and bone are examples of connective tissue.