This slide is not a histological section, but was made by fixing and staining the mesentery of a rat in toto, then spreading a small piece onto a slide. It illustrates the three main cell types in considerable number, and the collagen and elastic fibers. Once again, the ground substance is not preserved, and therefore not visualized.

The simple squamous epithelium lining both sides of a mesentery is called a mesothelium. Expect to see these cells on the top and bottom of the tissue but be cautioned that they are very thin and only their pale staining nuclei are discernible (mesothelial cells C1-3 m).

The predominant cell in the CT is the fibroblast (C1-3 F). This cell has a round to oval nucleus, a prominent nucleolus and densely staining cytoplasm.

The second major connective tissue cell type is the mast cell (C1). Mast cells are small, densely staining, and frequently exhibit a round or oval morphology. The very round nuclei are often obscured by large numbers of cytoplasmic granules, which may stain either red or green on this slide. These granules contain several important compounds including heparin and histamine or serotonin (depending on the animal species--histamine in humans). The heparin (being a sulfated GAG) is the main constituent that is stained.

Macrophages (C2, 3 M) are readily identified in this spread, since they have phagocytosed particulate carmine dye, with which the rats were injected the day before the spreads were prepared. Macrophages are generally elongate or bipolar cells, whose nuclei appear smaller, more elongate, and denser than those of fibroblasts. (Note that we ran out of different M's, so while we used M for macrophage on C2 and 3, M indicates mast cells on C1.)

Collagen fibers (C3 c) appear as diffuse, wavy yellow bundles in the matrix. Which collagen type predominates in this loose irregular connective tissue?

Elastic fibers (arrows) appear as sharp, straight lines which exhibit occasional branching. Note the relative num-bers of collagen and elastic fibers.

In addition to the resident CT cells, one can also find numerous white blood cells (polymorphonuclear leukocytes and lymphocytes) These cells play a role in surveillance and defense.