a. Fibroblasts - This most common and characteristic C. T. cell type secretes the fibers and ground substance of connective tissue. The fibroblast has a large, flattened, euchromatic, ovoid nucleus, a fusiform shape, and long, tapering cytoplasmic processes often indistinct in outline. When not actively engaged in synthesis, these cells may be called fibrocytes.

b. Adipose cells (fat cells) - Specialized for storage of lipid. The solubilization of the large central lipid droplet during paraffin embedding leaves only the nucleus and a thin rim of cytoplasm, giving the cells a "signet ring" appearance.

c. Mast cells - Commonly occur along blood vessels, but do not stain distinctly with H&E. The cells are large, ovoid and densely packed with metachromatic granules containing heparin, serotonin, and histamine. The granules usually obscure the nucleus unless, as readily happens, degranulation occurs.

d. Macrophages (histiocytes) - These are phagocytic cells derived from blood monocytes. They may be fixed in position or may migrate into the C.T. after tissue injury or infection. Their nucleus is smaller and darker staining that that of the fibroblast, and is ovoid or somewhat indented. The cell is irregular in shape, but has a distinct outline. Ingested particles may be visible in the cytoplasm.

e. Plasma cells - These cells synthesize antibody molecules which circulate in the blood stream. Plasma cells are commonly found in the C. T. of the respiratory, gastrointestinal and genitourinary systems. These cells are oval and have an abundant, more-or-less basophilic cytoplasm. The nucleus is eccentrically placed and is small and dense, with clumps of heterochromatin dispersed in a cart-wheel pattern around the periphery ("clock face" nucleus). In H&E preparations, the cells typically have an clear perinuclear halo marking the position of the pale-staining Golgi complex.

f. Lymphocytes - These blood cells are normally found in C.T. in small numbers but produce dense tissue infiltrations during chronic inflammatory states. The cell is usually small (7-8 mm in diameter) with a relatively large nucleus and scanty cytoplasm. In H&E preparations they appear as dark purple staining spheres scattered in the C.T.

g. Neutrophils and Eosinophils - Blood cells which infiltrate C.T. in large numbers only during tissue inflammation. The neutrophil is characterized by a multilobed nucleus. The eosinophil contains reddish-orange granules and has a horseshoe-shaped (bilobed) nucleus.

a. Collagen Fibers - The most common C.T. fiber; these are long, non-branching and straight (or slightly wavy in the absence of tension). They do not impregnate with silver (non-argyrophilic) or stain strongly with PAS. The fibers consist of bundles of fine collagen fibrils, composed of repeating tropocollagen molecules. The partially overlapping, 280 nm long, tropocollagen units impart to the fibrils a 64nm periodicity visible at the EM level.

b. Reticular Fibers - Fine collagen fibrils bound to glycoprotein components. They form delicate supporting networks in adipose and lymphoid tissue and along the basement membranes where C.T. lies in contact with epithelial cells or muscle fibers. Reticular fibers are not detectable with H&E, but stain strongly with PAS* and are intensely argyrophilic (have an affinity for silver staining). The fibrils have the typical 64 nm periodicity and may develop into larger diameter collagen fibers during maturation.

c. Elastic Fibers - Do not stain with H&E and are best demonstrated by either orcein or Weigert's resourcin-fuchsin stain. They are long, thin, branching threads, exhibiting no periodicity at the EM level. The fibers are composed of tiny peripheral microfibrils and a central core of elastin, a protein which contains two unusual amino acids, desmosine and isodesmosine.