Lab 4 Mucous Membranes
In this lab you will look at a number of mucous membranes. Mucous membranes line the luminal surfaces of a number of structures, such as those of the alimentary canal, the respiratory system and the urinary system. A mucous membrane is composed of (1) an epithelium, (2) the basement membrane on which the epithelium rests, and (3) a layer of loose connective tissue, called the lamina propria, underlying the basement membrane. The lamina propria contains cells, and a non-cellular matrix consisting of fibres and ground substance. The fibres and ground substance are secreted by cells called fibroblasts. Other cells, such as lymphocytes, plasma cells, macrophages and mast cells, are also present. The fibres of connective tissue are collagen fibres, elastin fibres and reticular fibres (the latter two types are only identifiable with special stains). The ground substance is amorphous and usually not identifiable in tissue sections (except in cartilage).
Mucous membrane of the duodenum (From Slide 9, Monkey Duodenum, PAS):
The jejunum is very similar in structure to the duodenum. However, there are no Brunners glands in the submucosa. Also, the goblet cells will appear pale with this stain.
In Figure 19, the luminal surface with its villi is on the left. The epithelium and lamina propria is similar to that seen in the duodenum. Intestinal glands open into the bases of the villi. A line of asterisks delineates the muscularis mucosae which is difficult to distinguish at this magnification. (The muscularis mucosae, a thin band of muscle running along the inner surface of the mucosa, is a distinguishing feature of the alimentary canal). The submucosa, with its irregular bands of collagen fibres, is much easier to see in the jejunum in the absence of Brunner`s glands. Although not shown in this picture, the submucosa can be thrown into folds called plicae circulares, which raise the overlying mucosa. The plicae serve to increase the absorptive area of the small intestine. They are most abundant in the jejunum. The muscularis externa, with its circular and longitudinal layers, and the serosa are also evident.
This image is a close-up of the top of a villus. The epitheium is simple columnar epithelium with microvilli. Many goblet cells are visible. In the loose connective tissue of the lamina propria, one sees many lymphocytes. Small blood vessels are also present, especially near the epithelium. Each villus also has a lymphaatic vessel called the central lacteal (not evident in this picture) which is involved in the uptake of lipids. Strands of smooth muscle in the villus allow it to contract independently. (At this resolution, it is difficult to distinguish smooth muscle from collagen fibres).
Figure 21 shows the very tall columnar epithelium from the gut of the worm Ascaris. The nuclei of some of the epithelial cells can be seen. The basement membrane is prominent and the microvilli are very long. Note that there is no lamina propria underlying the basement membrane. Terminal bars are evident near the apical surface of the cells. The terminal bars are the sites of specialized attachments, called junctional complexes in electron microscopy, between epithelial cells. These junctional complexes consist of tight junctions (aka zonula occludens), zonula adherens, and desmosomes (aka macula adherens). (See Ross et al., 3rd edition, pg. 67-8 for more details).
To the left of the lumen, the full length of epithelial cells can be seen, only the upper part of the epithelial cells can be seen to the right of the lumen.
The epithelium is on the left, underlying it on the right is the lamina propria containing many blood vessels. (The rabbit trachea has more blood vessels and fewer glands than the human trachea). The epithelium is pseudostratified columnar and ciliated. In pseudostratified epithelium, all cells rest on the basement membrane (so the epithelium is simple, not stratified), however, the cells are not all the same height and only some reach the lumen. The nuclei of the epithelial cells are found at different levels and the epithelium appears stratified. The most abundant cells are the ciliated columnar cells, followed by the mucous-secreting goblet cells.
In the trachea, the submucosa (not shown here) is similar in appearance to the lamina propria, from which it is separated by a band of elastic fibres, which are only seen with special stains.
This low magnification image of the ureter shows its three layers: the epithelium, the lamina propria and the muscularis. No distinct submucosa is present.
The lumen of the undistended ureter is convoluted, due to folds in the mucosa. The epithelium is transitional, which is a stratified epithelium made of several cell layers. Transitional epithelium has the ability to become thinner and flatter and allows the distension of urinary passageways. It is also essentially impermeable to salts and water.
The lamina propria is fairly dense and collagenous. There are usually two layers of smooth muscle in the muscularis underlying the lamina propria, an inner looser spiral described as longitudinal, and an outer tighter spiral described as circular. (Note that this is the reverse of the situation in the digestive tract). The smooth muscle is mixed with connective tissue and forms bundles rather than sheets.
The arrangement of cells in the transitional epithelium can be seen in this image. The surface cells are rounded and bulge into the lumen. They typically stain more deeply at their apical surface. The intermediate cells are pear-shaped or irregularly polyhedral. In the basal row, the cells are low columnar or cuboidal. A bit of the lamina propria, with its abundant collagen fibres is visible. A blood vessel containing blood cells is seen in the lamina propria.
The structure of the bladder is similar to that or the ureter, in that is has a transitional epithelium, a lamina propria and a muscularis. However, the wall of the bladder is much thicker than that of the ureter. The muscularis is arranged in more or less three layers of anastomosing muscle bundles with abundant connective tissue between them. (A smooth muscle bundle and some collagen of the muscularis are indicated by sm and c, respectively). The mucosa of the bladder is also folded in the undistended state.
This figure shows the appearance of the bladder epithelium with rounded surface cells. In the distended state, these cells can become broad squamous. Some epithelial cells can be binucleate.
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