The anatomy and physiology of the appendix


vesalius appendix

Vesalius' 1540 dissection of the human colon and appendix

Anatomy and physiology are the studies of how a body structure is built, and then how that structure works. I've always found anatomy and physiology to be two sides of the same coin - I learned anatomy by figuring out how something works, and I learned physiology by studying how something was built.

Stomach colon rectum diagram.svg

The gut is wonderful in this regard as it not only has to somehow propel food down a twenty foot tube, but it also has to break that food up into molecules, break the molecules into smaller molecules, and then absorb the smaller molecules into the blood stream, all while filtering out toxins, maintaining a balance of good and bad bacteria, and even holding onto waste products until elimination is convenient (and safe). It is just remarkable.

Gut wall.svg

To that end, the entire GI tract is a long tube with several layers (layers of bowel wall), wrapped by muscles (bowel muscles) and lined with a variety of cells that seal the body from the contents (bowel mucosa), have an absorptive surface, make antibodies (GI antibodies), and which generate hormones that respond to the contents and adjust the entire system (the neuroendocrine system).

The appendix is a small eddy off of the GI tract. It still contains all of the important elements of the GI anatomy and physiology, but is adapted to be off the beaten path. In order to better appreciate the anatomy and physiology of the appendix, I find it convenient to break it down a bit; to start large and then work our way into smaller bits and see how they work together.


Anatomy

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The appendix is a smal tubular extension of the right side of the colon, right near where the small intestine also inserts into the colon. Its length is quite variable, from an inch or so to up to 8 or nine inches in length. Most of the time is looks like a stubby #2 pencil.

The colon has three outer longitudinal muscle bands, called the tenia, that run the lenvth of the colon as strips, equidistant around the circumference of the colon. The appendix arises from the blind pouch of the cecum where the three tenia merge. In fact, the easiest way of finding the appendix in surgery is usually to pull up the colon, find a tenia, then run it backwards until the appendix is found.

The appendix has its own blood supply in leaves of fat arising off the mesentery of the cecum. There is a small appendiceal artery that runs as an arcade along the lower edge of the organ.


Microanatomy

appendix microscopy

A cross section of the appendix shows its tubular nature. The appendix has the usual outerserosal 1 cell thick peritoneal covering, then the muscle layers, which stain light pin, and then a layer of connective tissue, called the submucosa (this is,interestingly, the strongest layer of bowel, from which "catgut" and old tennis racquet strings and cello strings were made). Beneath the submucosa is a thin muscle layer and then, finally, the lining layer, called the mucosa.

The mucosa of the appendix is very similar to that of the colon, in that the mucosa is studded with pits, called the Crypts of Lieberkuhn.

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Crypts of Lieberkuhnare very interesting. They exist throughout the intestine and serve several functions. First, the intestinal contents, food, stool, etc. are always rubbing off mucosal cells as they brush past. The intestinal mucosal cells that replace them are always growing out of the crypts to resurface the bowel lining. The dark purple cells at the base of the crypts are intestinal stem cells.

Secretions and Hormones

The Crypts also have large clearer cells, called goblet cells, which produce mucous, which flows up and out of the crypt, washing the crypts, and then lubricating and protecting the mucosa of the bowel from its contents.

The appendix is different from the colon in that the appendix has a very large number of lymphoid aggregates in the submucosa. These collections of white blood cells are a lot like lymph nodes elsewhere. These aggregates are thought to produce secretory antibodies (IgA's) which assist in handling toxins within the gut.

Peristalsis

The muscles of the gut are designed to propel material forward. To accomplish that, the muscles of the gut are programed to squeeze in a rhythmic fashion creating a wave, called peristalsis, which moves the contents within the lumen of that section forward.

Tying it all together

The appendix is an extension of bowel that creates an eddy , or sump, off the first portion of the colon. Stool is mixed in the distensable well of the cecum before being pushed on. Some of the stool may pass into the appendix, but appendiceal peristalsis pushes it back into the colon. The appendix also makes mucous and antibodies which are also pushed into the cecum by peristalsis.