Chapter 3 - Connective Tissue
Connective tissue provides support, binds together, and protects tissues and organs of the body.
Connective tissue consists of three principal components: cells, protein fibers, and an amorphous ground substance. Together the fibers and ground substance make up the extracellular matrix. Whereas the other tissue types (epithelium, muscle, and nervous tissue) are largely made up of cells, the extracellular matrix is the major component of most connective tissue.
This chapter will focus on the basic types of connective tissue, while subsequent chapters examine the specialized connective tissues (cartilage, bone, and blood).
CONNECTIVE TISSUE FIBERS
The three types of connective tissue fibers are:
- Collagen fibers - most abundant connective tissue fiber
- Tensile strength - resistance to stretching
- Elastic fibers - contain elastin
- Elasticity - can be stretched, yet still, return to its original length
- Reticular fibers - contain type III collagen
- Support - network of thin fibers
Collagen Fibers
Collagen fibers are the most abundant type of fiber in the extracellular matrix of connective tissue. At least 30 types have been identified in humans and comprise about 30% of the whole-body protein content.
Most common types are:
- Type I - abundant in skin, tendon, bone, and organs
- Type II - abundant in cartilage
- Type III - forms reticular fibers
- Type IV - forms the basal lamina
Type I collagen is the most common type of collagen fiber.
Type II collagen is found in cartilage.
Type III collagen forms reticular fibers. Reticular fibers form supporting networks for the cells of various tissues and organs. Such networks are widespread including the liver, bone marrow, lymphoid organs, capillary endothelium, and muscle fibers.
Type IV collagen cross-links to form a fine meshwork found in the basal lamina.
Elastic Fibers
Elastic fibers allow tissues to respond to stretching or expansion. Elastic fibers often form networks or occur as lamellar sheets, as in the wall of elastic arteries.
CONNECTIVE TISSUE
Connective tissue is classified based on the characteristics of its cellular and extracellular components. The main criteria are the type of cells, arrangement and type of fibers, and composition of the extracellular matrix.
Loose Connective Tissue
Loose (areolar) connective tissue is highly cellular with a sparse, random arrangement of fibers.
Dense Connective Tissue
Dense connective tissue contains mostly collagen fibers with little ground substance. The pattern of collagen fibers distinguishes between the two types.
- Dense Regular Connective Tissue - regular pattern of parallel fibers oriented in the same direction
- Dense Irregular Connective Tissue - irregular pattern of interwoven fibers in multiple directions.
CONNECTIVE TISSUE CELLS
Connective tissue cells are usually divided into two types:
- Fixed cells (or resident cells) - resident population of cells that develop and remain within connective tissue. Fibroblasts, adipocytes (fat cells), macrophages, and mast cells are regarded as resident cells.
- Transient cells (or wandering cells) - leukocytes (white blood cells) that migrate from the bloodstream into connective tissue in response to a signal (e.g., inflammation or tissue damage).
FIXED CELLS
Fixed cells are normal constituents of connective tissue.
Fibroblasts
Fibroblasts are the most common cell type in connective tissue. They produce and maintain the components of the extracellular matrix.
Adipocytes
Adipocytes (fat cells) are specialized for the storage of fats. They also function as a cushion for organs and insulate the body.
Macrophages
Macrophages are phagocytic cells that engulf and digest microbes, cellular debris, and foreign substances. Monocytes develop in bone marrow, circulate in the bloodstream, and migrate into connective tissue, where they differentiate into macrophages.
Mast Cells
Mast cells release molecules that dilate blood vessels and recruit more immune cells to a site of mast cell activation. Progenitor mast cells (agranular) develop in bone marrow, circulate in the bloodstream, and migrate into connective tissue, where they proliferate and differentiate into mature mast cells (granular).
Mast cells undergo rapid degranulation during an anaphylactic reaction.
TRANSIENT CELLS
Transient cells are leukocytes (white blood cells) that circulate in the bloodstream and migrate into connective tissue at sites of an immune response. These include neutrophils, eosinophils, basophils, lymphocytes, and monocytes.
Plasma Cells
Plasma cells are mature B lymphocytes that produce large quantities of antibodies. They are abundant wherever antigens may enter the body, such as the gastrointestinal tract and the respiratory system.
Eosinophils
Eosinophils are involved in many inflammatory processes, including parasitic infections, allergic diseases, and asthma.
Lymphocytes
Lymphocytes develop the ability to recognize and respond to antigens. Their number increases dramatically at sites of inflammation.
Mast Cell/Plasma Cells/Eosinophils
Example of multiple types of immune cells in connective tissue.
