Chapter 3. Stromal Connective Tissue

A. Introduction

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Connective tissue is a primary tissue that encompasses a wide variety of animal tissues, including blood, cartilage, bone, and adipose tissue. Structurally, all connective tissues share one fundamental characteristic: they all have a significant extracellular component. Another common fundamental characteristic that places tissues in this broad primary tissue category is their developmental origin from embryonic mesenchyme.

Connective tissues are divided into three major groups: connective tissue proper, which includes the connective tissue of the stroma, specialized connective tissue and embryonic connective tissue. During this laboratory session, we will microscopically examine the stromal connective tissues of connective tissue proper.

Connective Tissue Proper is a group of connective tissues designed generally to bind parenchymal tissues, blood vessels and nerves together to form organs and to passively transfer mechanical tension from point to point within the organ or within the animal body.   The stromal connective tissues belong to this group.  Stromal connective tissues are the common connective tissues found in the stroma, namely loose connective tissue and dense irregular connective tissue.

Stromal connective tissues are composed of a significant amount of extracellular matrix, in which are embedded connective tissue cells.  The extracellular matrix of stromal connective tissue consists of an amorphous ground substance and connective tissue fibers. The ground substance is a porous, well-hydrated, compressible gel made largely of proteoglycan aggregates. The large number of proteoglycans in each proteoglycan aggregate contains large numbers of negatively-charged sulfated glycosaminoglycans. The negative charge associated with the glycosaminoglycans attracts and holds high volumes of water.The ground substance typically does not stain well in routine H&E preparations.

Three types of fibers are embedded in the ground substance. The relatively thick, eosinophilic collagen type I fibers, the relatively thin, basophilic elastic fibers, and the extremely thin, usually unresolvable, collagen type III (reticular) fibers.Collagen type I fibers are the predominant type of fiber in stromal connective tissues. They are relatively thick, straight (sometimes wavy), unbranched, and acidophilic. Collagen type I is the principal type of collagen in the animal body, accounting for approximately 90% of the animal's total collagen content. Its fibrillar nature provides it with a high degree of tensile strength, enabling it to withstand stretching forces. In LCT, the collagen type I fibers provide the delicate tissue with the tensile strength needed to prevent tearing under normal use.

Elastic fibers are are thin, compared to the collagen fibers, and branched. They are primarily composed of the protein elastin.  Elastic fibers are very resilient and provide elasticity to the connective tissue.

Collagen type III (reticular) fibers are extremely thin fibers. They are PAS-positive but nearly impossible to visualize in H & E preparations. Reticular fibers are primarily composed of collagen type III fibrils, which do not combine to form conspicuous, thick fibers.

The Cells of Stromal Connective Tissues

The cells of stromal connective tissues are subdivided into two groups. Fixed cells are those cells that are stable, permanent residents of the tissue; they do not migrate into the tissue in response to an inflammatory stimulus. Wandering cells are cells that migrate into the tissue from the blood stream in response to an inflammatory stimulus.  Among all the stromal cells, the single most abundant type of cell present in connective tissue proper is the fibroblast. Fibroblasts produce and secrete the organic components of the amorphous ground substance and extracellular fibers of the extracellular matrix.

Classification of Stromal Connective Tissues

There are two specific types of stromal connective tissues:

  • Loose connective tissue (LCT): Loose connective tissue consists of an abundance of amorphous ground substance in which is embedded a loose, apparantly-random weave of extracellular fibers and a wide variety of fixed and wandering connective tissue cells. LCT is widely distributed about the animal body. It is primarily located beneath epithelial membranes and glandular epithelium, binding these epithelia to other tissues and contributing to the formation of organs. LCT physically supports the blood vessels and nerves that supply epithelia and serves as the principle site of the inflammatory response in the animal body.
     
  • Dense irregular connective tissue (DICT): Dense connective tissue consists of a dense arrangement of extracellular fibers embedded, with connective tissue cells, in a relatively sparse amorphous ground substance. It is designed to provide a binding of relatively high tensile strength between tissues and organs and passively translate tension from one organ to another.