Extracellular matrix
The extracellular matrix of connective tissues consists of ground substance and fibers and plays a critical role as a scaffold within which tissues organize. The ground substance is the component of the matrix located between the cells and the fibers. It is typically a semifluid gel medium that serves for the passage of molecules through connective tissues and for the exchange of metabolites with the circulatory system (Young & Heath, 2000). It contains a mixture of long unbranched polysaccharide chains and water stabilized by glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. The glycoproteins include structural (fibrillin and fibronectin), nonfilamentous (laminin, entactin, and tenascin), and integrin proteins (cell adhesion molecules). The typical gel‐like property of the extracellular matrix is imparted by the large volume of these globular complexes and their hydrophilic nature (their highly charged side chains attract large volumes of water and ions), which, when combined, produces the characteristic turgor of the ground substance. The mechanical properties of a tissue’s particular ground substance depend on the reinforcing fibrous proteins (collagen subtypes or elastin) or minerals (bone) to which these aggregates and other components are bound.
A number of types of fibrous proteins can be found in the ground substance of connective tissue. Collagen type I is the primary collagen in the dermis of skin (Figure 3.2), tendons, ligaments, and bone. It is formed by tropocollagen triple chains into larger fibrils (Iannarone, Cruz, Hilliard, & Barbe, 2019) that are visible under light microscopy. In contrast, collagen type II is the main collagen of hyaline cartilage and consists of fine filaments that cannot be visualized under light microscopy. Collagen type III is the major component of the fine reticular network within the bone marrow. It is also the first type of collagen deposited into developing or repairing tendons and bones. Elastin is a fibrous protein typically present as discontinuous fibrils (Figure 3.2) or sheets and confers the properties of stretch and elastic recoil (Young & Heath, 2000).
General Subtypes: Structure and Function
In this section, we summarize the general features of connective tissues and their function (see Table 3.1).
Loose connective tissue
Loose connective tissues are the most abundant in the body. In this general type of connective tissue (often divided into adipose, areolar, and reticular), the fibers are loosely woven and there are many cells. It is located around nerves and blood vessels, among others, and is composed of thin and relatively few fibers (collagenous, elastic, and/or reticular) and cell types, all embedded in a semifluid ground substance (Figure 3.1a). There are large numbers of cells and cellular processes, including fibroblasts, often adipocytes, immune cells, blood vessel and lymph vasculature cells, and neuronal processes (nerves) (Figure 3.1b). Functionally, this tissue provides cushioning, support, elasticity, and immune functions.
Table 3.1 General Features of Connective Tissues
| Characteristic | Description |
|---|---|
| General types | Loose (adipose, areolar, reticular); dense (e.g., tendon, cartilage, and bone); fascia (e.g., epimysium) |
| Cells | Main cell types: Fibroblasts, adipocytes, resident macrophages, plasma cells |
| Extracellular matrix (ECM) | Main composition: Polysaccharides, water, glycosaminoglycans (GAGs), proteoglycans, glycoproteinsAdditional components: Collagen I/III, elastin, depending on subtype |
| Function | Envelops, separates tissues and cells, cushions, supports, immune function, and more |
Figure 3.1 Loose and adipose connective tissues. (a) Loose connective tissue stained with hematoxylin and eosin (H&E). Elastic fibers (EF) and fibroblasts (F) are indicated. (b) Arteries and nerve in loose connective tissue (CT); H&E stained. (c) Adipose tissue near muscle fibers surrounded by dense fibrotic tissue induced by repetitive strain injury; Masson’s Trichrome stained. (d) Adipocytes around skeletal muscle fibers; H&E stained.
Areolar tissue
This type of loose connective tissue is the most widely distributed and is present in the dermis, around blood vessels, and nerves. It contains at one time or another, nearly all of the cell types normally found in connective tissue, including fibroblasts, macrophages, plasma cells, mast cells, adipocytes, and a few white blood cells. Its loose, randomly arranged fibers include collagen, elastic, or reticular. The ground substance is semifluid or gelatinous and contains primarily hyaluronic acid, chondroitin sulfate, dermatan sulfate, or keratan sulfate.
Adipose tissue
Adipose tissue is primarily composed of adipocytes. It can be found in the subcutaneous layer of skin, the marrow of long bones, between muscles, and around nerves and joints (Figure 3.1c,d). It reduces heat loss through the skin and provides energy reserves, support, and protection.
Reticular tissue
This type of loose connective tissue contains a fine network of collagen III fibers, often termed reticular fibers. It is present around blood vessels and muscle, within bone marrow, and in basement membranes.
Dense collagenous connective tissues
Dense connective tissues contain either regularly or irregularly arranged collagen fibers and fewer intercellular substance and cells than found in loose connective tissues (Figure 3.2). Examples of dense irregular connective tissues include the dermis of skin, deep fascia, the periosteum of bone, the perichondrium of cartilage, and organ capsules. Examples of dense regular connective tissues include tendons, ligaments, aponeuroses (thin flat tendon bands that connect one muscle to another or to bone), cartilage, and bone. Each is discussed in further detail separately in subsequent sections. Elastic tissue has a preponderance of elastic fibers and constitutes the ligament flava of vertebrae and arterial walls, among others.
Figure 3.2 Dense irregular connective tissue in the dermis of the skin. (a) Masson’s Trichrome staining detects the dense amount of collagen fibers. (b) Verhoeff Gieson Elastin Staining is used to detect elastic fibers (EFs). The black‐stained EFs are surrounded by collagen
