Figure 51

Loose connective tissue, a. Photomicrograph of a mesentery spread stained with Verhoeff s hematoxylin to show nuclei and elastic fibers, counterstained with safranin for identification of mast cell granules and with orange G for identification of other proteins (mainly collagen fibers). The elastic fibers appear as blue-black, thin, long, and branching threads without discernable beginnings or endings. Collagen fibers appear as orange-stained, long, straight profiles and are considerably thicker than the elastic fibers. Most of the visible nuclei are presumed to be those of fibroblasts. Nuclei of other cell types, i.e., lymphocytes, plasma cells, and macrophages, are also present but are not identifiable. Mast cells are identified by the bright reddish granules within their cytoplasm. Note the presence of the small blood vessel filled with red blood cells. X150. b. Schematic diagram illustrating the components of loose connective tissue. Note the association of different cell types with the surrounding extracellular matrix, which contains blood vessels and different types of fibers.

phocytes, plasma cells, macrophages, and eosinophils, are associated with the body's defense system; they function within the ground substance 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

The term connective tissue includes a variety of tissues with differing functional properties but with certain common characteristics that allow them to be grouped together. For convenience they are classified in a manner that reflects these features. Table 5.1 provides a classification including subtypes of the principal connective tissues.

table 5/1. Classification of Connective Tissue

Embryonic connective tissue

Mesenchyme Mucous connective tissue

Connective tissue proper

Loose connective tissue Dense connective tissue Irregular Regular

Specialized connective tissue"

Adipose tissue (Chapter 6) Blood (Chapter 9) Bone (Chapter 8) Cartilage (Chapter 7) Hemopoietic tissue (Chapter 9) Lymphatic tissue (Chapter 13)

"In the past, the designations elastic tissue and reticular tissue have been listed as separate categories of specialized connective tissue. The tissues usually cited as examples of elastic tissue are certain ligaments associated with the spinal column and the tunica media of elastic arteries. The identifying feature of reticular tissue is the presence of reticular fibers and reticular cells together forming a three-dimensional stroma. Reticular tissue serves as the stroma for hemopoietic tissue (specifically the red bone marrow) and lymphatic tissue organs (lymph nodes and spleen, but not the thymus).

elastic fiber eosinophil b adipose cell lymphocytes plasma cells endothelial s7 embryonic connective tissue

Embryonic mesenchyme gives rise to the various connective tissues of the body

The mesoderm, the middle embryonic germ layer, gives rise to almost all of the connective tissues of the body. An exception is the head region, where certain progenitor cells are derived from ectoderm by way of the neural crest cells. Through proliferation and migration of the mesodermal and specific neural crest cells, a primitive connective tissue referred to as mesenchyme (in the head region, it is sometimes called ectomesenchyme) is established in the early embryo. Maturation and proliferation of the mesenchyme give rise not only to the various connective tissues of the adult but also to muscle, the vascular and urogenital systems, and the serous membranes of the body cavities. The manner in which the mesenchymal cells proliferate and organize sets the stage for the kind of mature connective tissue that will form at any specific site.

Embryonic connective tissue is present in the embryo and within the umbilical cord

Embryonic connective tissue is classified into two subtypes:

• Mesenchyme is primarily found in the embryo. It contains small, spindle-shaped cells of relatively uniform appearance (Fig. 5.2a). Processes extend from these cells and contact similar processes of neighboring cells, forming a three-dimensional cellular network. Gap junctions are present where the processes make contact. The extracellular space is occupied by a viscous ground substance. Collagen (reticular) fibers are present; they are very fine and relatively sparse. The paucity of collagen fibers is consistent with the limited physical stress on the growing fetus.

• Mucous connective tissue is present in the umbilical cord. It consists of a specialized, almost gelatin-like extracellular matrix whose ground substance is frequently referred to as Wharton's jelly. It occupies large intercellular spaces located between thin, wispy collagen fibers (Fig. 5.2b). The spindle-shaped cells contained in the matrix are widely separated and appear much like fibroblasts in the near-term umbilical cord, e.g., the cytoplasmic processes are thin and difficult to visualize in routine hematoxylin and eosin (H&E) preparations.

v connective tissue proper

Connective tissues that belong to this category are divided into two general subtypes:

• Loose connective tissue, also sometimes called areolar tissue

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