(C9-13) Slide 51: Ileum - fast green, hematoxylin & PAS

Begin by looking at the lumen of the ileum. Its surface is covered with finger-shaped structures, called villi (C9). The epithelium covering the villi has many different appearances, but the thinnest area is simple columnar epithelium. Next, look for any modifications that should be added to its name. Examining the epithelium at high power reveals two stripes at the apical surface, one pink stripe and one green stripe (C10). The pink stripe is the glycocalyx, a layer of carbohydrates linked to the plasmalemma that stains pink with the sugar-specific PAS stain. The green stripe is the microvillous border, and at the base of the microvilli there is a darker green line, the terminal web. The terminal web is interrupted at every intercellular boundary by a slight thickening called the terminal bar, where a junctional complex exists between the cells. Scattered throughout the epithelium there are magenta ovals, which represent the apical cytoplasm of goblet cells, unicellular exocrine glands that secrete mucus onto the apical surface. The complete name of this epithelium is simple columnar epithelium with microvilli and goblet cells. This type of epithelium is found throughout the small and large intestines.

Follow the epithelium as far as possible away from the tips of villi, deep into the wall of the ileum, to get a glimpse of cell turnover (C11). The mitotic cells in this part of the epithelium provide a constant supply of new cells, while other cells are being sloughed at the tip of each villus (C12). This epithelium is replaced completely every 5-7 days.

Although the basement membrane is not shown well, you may see it in some places as a very thin line, stained faintly pink, at the base of the epithelium (C13). In the lamina propria, there is a wide variety of cells giving the connective tissue a granular appearance at low power. We will study these cells in the connective tissue labs. For now, look at the blood vessels that are lined by endothelium.