This class of primary tissue supports and physically connects other tissues and cells to form functional organs. Its fluid provides a medium through which cells receive metabolic support, such as diffusion of both nutrients and waste products.

Generally, tissues can be said to consist of both cells and an extracellular matrix (ECM). ECM is composed of both protein fibers (collagen, elastic, reticular fibers) and ground substance (glycosaminoglycans [GAGs], proteoglycans, and multiadhesive glycoproteins). In connective tissue (CT), the ECM component predominates relative to the cellular component. In other words, cells tend to be widely separated among ECM fibers and ground substances.

Functions of CT include the following:

• Structure, support, and mechanical protection: capsules and fibrous septa of organs, support and space filling (padding) of organs and tissue elements, provides resistance to stress and shearing forces
• Nutrition: absorption in gastrointestinal tract and energy storage in adipose tissue
• Defense: immune cells in CT are a first-line of defense against microorganisms that breach an epithelium
• Repair: scar formation during wound healing
• Transport: continuous formation and flow of interstitial fluid

Various combinations of cells, fibers, and ECM components — both in numbers and types — create different types, or classifications, of CT; however, the lines separating these classifications are sometimes not distinct but graded. We can classify different types of the CT into three broad categories: embryonic, CT proper (loose and dense), and specialized CT. We can further classify CT in the following way:

Embryonic CT

• Mesenchyme
• Mucous CT (Wharton’s jelly)

Loose CT Proper

• Areolar (seen in the image on this page)
• Reticular

Dense CT Proper

• Dense irregular CT
• Dense regular CT

Specialized Connective Tissue

• Cartilage: hyaline, elastic, fibrocartilage
• Tendons and ligaments
• Bone: spongy and compact
• Blood
• Adipose tissue: white and brown (Of the specialized CTs, only adipose tissue will be described in this module; the others are discussed in other modules)

The learning objectives for this module are:

• Recognize the major types of connective tissue proper in light micrographs.
• Explain how the structure of each major type of connective tissue reflects its function.
• Distinguish the various cells found in connective tissue (fibroblasts, adipocytes, mast cells, plasma cells, macrophages, and undifferentiated mesenchymal cells), and describe their functions and key features.
• Recognize variations in distribution of ground substance and the three types of extracellular fibers in connective tissue proper.
• Recognize the unique features of adipose tissue, comparing and contrasting these in white and brown adipose tissue.
   

We will start with embryonic (primitive) connective tissue.