Chapter 3 - Connective Tissue
Connective tissue provides support, binds together, and protects tissues and organs of the body.
Connective tissue consists of three main 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 are type I collagen (most abundant protein in the body)
- Tensile strength - resistance to stretching
- Elastic fibers - contain elastin and fibrillin
- Elasticity - can be stretched, yet still, return to its original length
- Reticular fibers - contain type III collagen
- Support - network of thin fibers
Different stains can be used to visualize each type of fiber.
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 has a sparse, irregular network of collagen and elastic fibers suspended within a relatively large amount of ground substance.
Dense Regular Connective Tissue
Dense regular connective tissue is composed of type I collagen fibers oriented in the same direction. It provides tensile strength in one direction.
Dense Irregular Connective Tissue
Dense irregular connective tissue contains type I collagen fibers woven in multiple directions. It provides tensile strength in multiple directions.
Embryonic Connective Tissue
Embryonic connective tissue is formed during the development of the embryo. Mesenchyme develops into the various connective tissues of the body. Mucoid connective tissue is a gelatinous substance found in the umbilical cord.
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 inflammation or tissue damage.
FIXED CELLS
Fixed cells are normal components of connective tissue.
Fibroblasts
Fibroblasts produce and maintain the extracellular matrix. They are the most common cell type in connective tissue.
Adipocytes
There are two types of adipose tissue:
- White fat - long-term storage of energy
- Brown fat - generation of heat (thermogenesis)
White adipocytes are specialized for the synthesis and storage of triglycerides. White fat also functions as a cushion for organs and to insulate the body.
Brown adipocytes are specialized to generate heat. Newborns have a higher proportion of brown fat (5% of body weight) than adults, which gradually decreases with age.
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).
Special stains are required to identify mast cells in connective tissue.
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. These cells are discussed in more detail in the chapter on Peripheral Blood.
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.
