Cells of the endocrine islet (of Langerhans) (En) of the pancreas also have an epithelioid arrangement. The cells are in contact but lack a free surface, although they have developed from an epithelial surface by invagination. In contrast, the surrounding alveoli of the exocrine pancreas (Ex), which developed from the same epithelial surface, are made up of cells with a free  surface onto which the secretory product is discharged. Capillaries (C) are prominent in endocrine tissues. Similar examples of epithelioid tissue are seen in the adrenal and the parathyroid and pituitary glands, all of which are endocrine glands.
GENERAL STRUCTURE AND FUNCTION OF CONNECTIVE TISSUE / 158 EMBRYONIC CONNECTIVE TISSUE / 159 CONNECTIVE TISSUE PROPER / 160 CONNECTIVE TISSUE FIBERS / 161 Collagen Fibers and Fibrils / 161 Biosynthesis and Degradation of Collagen Fibers / 164 Reticular Fibers / 171 Elastic Fibers / 171 EXTRACELLULAR MATRIX / 173 CONNECTIVE TISSUE CELLS / 178 Fibroblasts and Myofibroblasts / 178 Macrophages / 181 Mast Cells / 182 Basophils / 187 Adipocytes / 187 Adult Stem Cells and Pericytes / 187 Lymphocytes, Plasma Cells, and Other Cells of the Immune System / 189 Folder 6.1 Clinical Correlation: Collagenopathies / 170 Folder 6.2 Clinical Correlation: Sun Exposure and Molecular Changes in Photoaged Skin / 173 Folder 6.3 Clinical Correlation: Role of Myofibroblasts in Wound Repair / 183 Folder 6.4 Functional Considerations: The Mononuclear Phagocytotic System / 185 Folder 6.5 Clinical Correlation: The Role of Mast Cells and Basophils in Allergic Reactions / 188
Connective tissue comprises a diverse group of cells within a tissue-specific extracellular matrix.
In general, connective tissue consists of cells and an extracellular matrix (ECM). ECM includes structural (fibers) and specialized proteins that constitute the ground substance. Connective tissue forms a vast and continuous compartment throughout the body, bounded by the basal laminae of the various epithelia and by the basal or external laminae of muscle cells and nerve-supporting cells.
Different types of connective tissue are responsible for a variety of functions.
The functions of the various connective tissues are reflected in the types of cells and fibers present within the tissue and the composition of the ground substance in the ECM. For example, in loose connective tissue, many cell types are present (Fig. 6.1). One type, the fibroblast, produces the extracellular fibers that serve a structural role in the tissue. Fibroblasts also produce and maintain the ground substance. Other cell types, such as lymphocytes, plasma cells, macrophages, and eosinophils, are associated with the body’s defense system; they function within the ECM of the tissue. In contrast, bone tissue, another form of connective tissue, contains only a single cell type, the osteocyte. This cell produces the fibers that make up the bulk of bone tissue. A unique feature of bone is that its fibers are organized in a specific pattern and become calcified to create the hardness associated with this tissue. Similarly, in tendons and ligaments, fibers are the prominent feature of the tissue. These fibers are arranged in parallel array and are densely packed to impart maximum strength.
Classification of connective tissue is based on the composition and organization of its cellular and extracellular components and on its functions.