Histology Guide

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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

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 collagen is the most common type of collagen fiber.

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Fibrils and Band Pattern
SEM
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Fibrils and Band Pattern
TEM
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Band Pattern
TEM

Type II collagen is found in cartilage.

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Type II Collagen
SEM

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.

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Type III Collagen
SEM

Type IV collagen cross-links to form a fine meshwork found in the basal lamina.

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Type IV Collagen
SEM

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.

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Elastic Fibers
SEM
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Elastic Fibers
TEM

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.

CONNECTIVE TISSUE CELLS

Connective tissue cells are usually divided into two types:

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.

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Phagocytosis Bacteria
SEM
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Phagocytosis Blood Cells
SEM

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.

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Degranulation
TEM

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.