The main point of this set of slides is to look at the different specialized epithelial secretory cells of the GI tract and consider the process of breakdown and absorption of foods. This means looking primarily at the details of the GI mucosa. The first slide shows the long, straight mucosal glands of the main body of the stomach: the so-called gastric or fundic glands. The simple columnar epithelium lining the main lumen of the stomach secretes a mucoid substance (often called mucigen). This same epithelium continues on down into short "pits", which might be considered to be the ducts of these glands. Notice that there are no goblet cells in this lining; this is crucial to deciding whether you have stomach or intestines. The narrowest part of the pit, just before the main secretory cells begin, is called the "neck" and is lined with rather small, ill-defined "mucous neck cells", which secrete mucus. This is also the place where mitotic activity for replenishment of gland cells and surface epithelium takes place. Two or three secretory tubules may branch off of a single pit. In the long glands you can see granular chief cells and pink parietal cells.
Detail of gastric (fundic) glands. The chief cells are paler, more granular looking, and tend to be basophilic at their base because of RER. Their zymogen granules are precursors of pepsin, the primary enzyme produced in the stomach. Seeming to hang off the sides of the glands are the rounded, pink parietal which have quite a clear acidophilic cytoplasm. Their secretions pass to the tubule lumen via channels between the chief cells; the lumens of the glands are too narrow to see here except perhaps in the gland at lower right center. Parietal cells secrete the precursors of HCI and the anti-pernicious anemia factor (also known as intrinsic factor). While viewing this slide, notice the loose areolar connective tissue of the lamina propria which packs around the individual glands.
EM of chief cell, which has the features of so many other serous, protein- secreting cells of the body. The nucleus is round and lies near the base. There is a lot of RER at the base. The secretory zymogen granules lie apically, are quite large, and will be expelled into the lumen of the gland by merocrine secretion.
EM of an endocrine (or argentaffin) cell in the wall of a gastric gland. Secretory peptide granules are small and dark and leave the cell at the base, going directly to capillaries in the lamina propria. The RER of a chief cell is visible just to the left of the endocrine cell; at the upper left-hand edge of the picture there is a large, round, pale gray zymogen granule half showing in the chief cell cytoplasm. Note its size in comparison with the small granules of the endocrine cell.
This jejunum will serve as an example of the mucosa of the small intestine. Fairly broad villi at the top are much wider than the spaces between the intestinal crypts of Lieberkuhn below. A cellular lamina propria constitutes the core of villi and the packing around crypts. Two (or perhaps three) narrow layers of muscularis mucosae lie below the glands; then the fibrous submucosa begins. Goblet cells are evident in the epithelial lining of both the villi and the crypts. Paneth cells, if present, would lie in the bases of the crypts.
Detail of base of a crypt, showing large zymogen granules in the apical region of several Paneth cells. Notice goblet cells among the simple columnar lining cells higher up in the tubule. Notice also that there is a capillary in the lamina propria below the gland.
Two more bases of crypts of Lieberkuhn. The one on the right has a couple of Paneth cells with large, acidophilic granules. The one on the left has an endocrine cell with bright orange-pink granules toward the base of the cell. Wandering eosinophils and lymphocytes are prominent among the connective tissue cells of the lamina propria.
Detail of Paneth cells with bright red secretory granules at their apices. These are the precursors of peptidase and various lysozymes. (NOTE: Paneth cells look very much like gastric chief cells in EM, with the large apical zymogen granules and a lot of basal RER.
Detail of another cross cut intestinal gland showing both: (1) Paneth cells with large, red, apical granules and (2) argentaffin (enterochromaffin) cells with small, orange-red, basal granules. The former are exocrine secreters; the latter, endocrine.
EM of enterochromaffin (endocrine) cells from a crypt. Granules lie basal to the nucleus in each case, ready to be released to the capillaries outside in the lamina propria. Notice the capillary in the lower right corner.
Scanning EM of the surface of a villus, showing cell outlines of the columnar epithelial cells. Their fuzzy looking surface is covered with microvilli. This, of course, increases greatly the surface area available for absorption.
Detail of lining epithelium of the intestines, especially as seen covering a villus. The goblet cell is pouring out mucus. The columnar cells have a clear striated (or brush) border of microvilli. If you'll recall, these are the cells we studied much earlier for the terminal web lying just below the microvilli and for the terminal bars (dark points between cells at their top borders). The bars represent junctional complexes.
Detail of a villus stained with Mallory. The central space is a lacteal lined with endothelium. Around it is the cellular lamina propria c.t.. In the columnar epithelium of the surface of the villus several features are particularly clear: (1) The blue goblet cells discharging mucus; (2) The "terminal web" forming a thin red line at the base of the microvilli (striated border); and (3) The bright blue basement membrane separating epithelium from lamina propria. Notice several lymphocytes (round nuclei) passing through the epithelium.
EM of striated border (microvilli). It is within the mucopolysaccharide surface coat (the glycocalyx) that lactase and some other digestive enzymes become activated. The cell membrane of the microvilli extends carbohydrate "arms" which interlace to form this tangled-looking glycocalyx.