Now, let's get a closer look. The large intestine consists of the appendix, cecum, colon, rectum, and anal canal. The colon is mainly responsible for reabsorbing water and electrolytes from the feces within its lumen. The colon is continuous with the rectum, where the feces is stored before defecation. Similar to the rest of the large intestine, the colon has 4 main layers: the mucosa, submucosa, muscularis propria, and a surrounding serosal layer of connective tissue that isn’t seen in this image.
Even at low magnification, we can see that the colon’s mucosa at the top of this image doesn’t have the distinct long villi or finger-like projections that would normally be seen in the small intestine. Taking a closer look at the colon’s mucosa, the lumen of the colon is seen at the top of the image and the first layer of cells lining the mucosa is the epithelium of the mucosa. The epithelium consists of two types of cells, enterocytes and goblet cells. The enterocytes or absorptive cells are the simple columnar cells with microvilli. They’re also called the absorptive cells because of their main function of absorbing water from the colon lumen. And the goblet cells are responsible for secreting mucus. Although the cells aren’t clearly seen in this image, the mucus they produce is easily seen as the globular structures that are stained dark purple from the hematoxylin and eosin stain. The surface epithelium is continuous with straight, unbranched, tubular glands called the crypts of lieberkühn. Unlike the crypts in the small intestine, these crypts extend through the majority of the mucosa, from their openings at the intestinal surface all the way to the muscularis mucosa along the deepest portion of the overall mucosa. It may not always appear to be continuous on histological slides because the path of the crypts may not always travel along the same plane as the section of tissue taken from the colon. The superficial portions of the crypts will typically have a higher concentration of enterocytes and the deeper portions will have a high concentration of goblet cells. The tissue found between the crypts and the epithelium is the lamina propria, which consists of many types of immune cells, including plasma cells, lymphocytes, eosinophils, and macrophages. And finally, the deepest layer of the mucosa is a thin layer of smooth muscle called the muscularis mucosa.
Beneath the mucosa is the next major layer of the colon, the submucosa. This layer consists mostly of dense irregular connective tissue, but also contains blood vessels, lymphatic vessels, and the submucosal or meissner’s plexus. Meissner’s plexus is a network of nerves that innervate the goblet cells in the mucosa as well as the smooth muscle of the muscularis mucosa. In this image, there’s a relatively large cross-section of a bundle of nerve cells that are part of meissner’s plexus. We can also see a number of large blood vessels and a lymphatic vessel in the upper left of this image.
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