Connective tissue encompasses a variety of tissues with differing functional properties but with certain common characteristics that allow them to be grouped together. For
FIGURE 6.1 • Loose connective tissue. a. Photomicrograph of a mesentery spread stained with Verhoeff’s hematoxylin to show nuclei and elastic fibers; it has been 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 discernible 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 (e.g., 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. 150. 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.
TABLE Classification of Connective Tissue6.1 Embryonic connective tissue Mesenchyme Mucous connective tissue Connective tissue proper Loose connective tissue Dense connective tissue Regular Irregular Specialized connective tissuea Cartilage (Chapter 7) Blood (Chapter 10) Bone (Chapter 8) Hemopoietic tissue (Chapter 10) Adipose tissue (Chapter 9) Lymphatic tissue (Chapter 14) aIn 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).
tissues, including subtypes.
, the middle embryonic germ layer, gives rise to alall of the connective tissues of the body. An exception is head region, where specific progenitor cells are derived a primitive connective tissue referred to as mes(in the head region, it is sometimes called ectomes) is established in the early embryo. Maturation and tissue is classified into two subtypes: Mesenchyme is primarily found in the embryo. It contains small, spindle-shaped cells of relatively uniform appearance (Fig. 6.2a). 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.
FIGURE 6.2 • Embryonic connective tissue. a. Photomicrograph of mesenchymal tissue from a developing fetus stained with H&E. Although morphologically the mesenchymal cells appear as a homogeneous population, they give rise to cells that will differentiate into various cell types. Their cytoplasmic processes often give the cell a tapering or spindle appearance. The extracellular component of the tissue contains a sparse arrangement of reticular fibers and abundant ground substance.
480. b. Photomicrograph of Wharton’s jelly from the umbilical cord stained with H&E. Wharton’s jelly consists of a specialized, almost gelatinlike ground substance that occupies large intercellular spaces located between the spindle-shaped mesenchymal cells. 480.