Collagen fibers (typically type I collagen) are acidophilic when stained with H & E, as seen in the dermis of skin. They tend to have a wavy appearance and may be sectioned obliquely, transversely or longitudinally. Nuclei of fibroblasts (fibrocytes) are numerous among the collagen fibers.

Elastic fibers stain black with Verhoeff's Hematoxylin and are seen as branching black lines. In this slide, the elastic fibers are clearly visible around ducts and vessels (collagen fibers are green).

Both elastic and collagen fibers and various connective tissue cells are visible in this preparation. Verhoeff's Hematoxylin stains elastic fibers black and Van Gieson stains collagen acidophilic. In your slide the acidophilic collagen fibers may not be obvious. Mast cells are easily identified due to the metachromasia of granules with toluidine blue.

Reticular fibers (type III collagen) are thin collagen fibers not typically detected with routine H & E staining. However, these fibers stain black with silver stain and are often called argyrophilic fibers . The spleen demonstrates the supportive network of reticular fibers present in many organs. In this slide, the delicate reticular fibers are black and the thick collagen fibers are red/brown.

Fibroblasts are the most common cell type of connective tissue. They produce both fibers and amorphous ground substance. Typically only the oval nuclei are visible. These cells are found associated with the fibers listed above. In the tendon, fibroblasts are seen as elongate nuclei found sandwiched between collagen fibers .

Mast Cells are round/oval cells that contain granules that are metachromatic because of their glycosaminoglycan content; these cells are easily seen in the connective tissue spread. The toluidine blue component of the stain applied to this slide renders the mast cell granules blue-purple.

Examine the lamina propria underlying the epithelium of the pyloric stomach to find plasma cells . The cells are ovoid with basophilic cytoplasm, due to rER. The diagnostic feature of plasma cells is their eccentric round nuclei commonly described as "clock face" nuclei . This appearance is due to heterochromatin clumps. These cells are readily identified in the mucosa of the digestive tract.

In the lung, alveolar macrophages (dust cells) are found easily in the air spaces where these cells have either ingested carbon particles or erythrocytes. Some may appear as vacuolated cells. One can infer the identity of a macrophage by its indented nucleus . Macrophages are phagocytic cells that are difficult to find in normal tissues because there is not sufficient cause for them to increase in number.

Adipocytes , fat cells are large cells specialized in storage of neutral fats. Lipid is removed in routine tissue preparation. Consequently the cell appears as a thin rim of cytoplasm surrounding the vacuole of dissolved lipid. The nucleus is eccentric and flattened. Adipose tissue is a connective tissue with a predominance of adipocytes.

Lipid is preserved and stained black when the tissue is prepared using osmium tetroxide as a fixative.

Leukocytes are white blood cells that are readily found in connective tissue. Lymphocytes (similar in size to red blood cells) are the most common connective tissue leukocyte. Aggregates of lymphocytes are often found associated with the mucosal epithelium of the GI tract, such as this slide of the esophagus. They have a small amount of slightly basophilic cytoplasm and a large, darkly stained nucleus because of condensed chromatin. Use Slide 51 (pyloric stomach) to compare lymphocytes (no visible cytoplasm) to plasma cells which contain abundant cytoplasm.

Observe the eosinophils surrounding the large duct in the center of this slide. These cells are another type of leukocyte that are identified by their bilobed nucleus and refractile specific granules that are stained by eosin.

Loose connective tissue (areolar) is located under the thick eosinophilic basement membrane of the respiratory epithelium in the trachea. A major component of loose connective tissue is amorphous ground substance which does not stain with routine H & E. The most numerous cell types are fibroblasts . In addition, other fibers such as collagen, elastic, and reticular fibers are present. Also, look for this type of connective tissue surrounding blood vessels and underlying the epithelium of the digestive tract.

Loose connective tissue (areolar) is located directly beneath the epidermis of the skin. Dense irregular connective tissue forms most of the dermis below the loose connective tissue. Dense irregular connective tissue has similar components as loose connective tissue. However, the collagen fibers predominate and there are fewer cells and less amorphous ground substance. The collagen is arranged in bundles without any specific orientation.

Dense regular connective tissue has collagen fibers arranged in a definite pattern according to the direction of stress. The tendon clearly shows this arrangement.