Functions Of The Human Appendix2,4,7,8,20,21
The appendix completes most of its functions at the early end of the spectrum of life. The vital aspects of these are probably complete at least by early infancy. While it is freely admitted that the precise functions of the human vermiform appendix are still unclear, so much more is now known that clarification is at hand. It is my intention to discuss this further under the following headings:
Embryological
Physiological
Microbiological (Bacteriological)
Biochemical
Immunological
1. Embryological2,22
During the fifth foetal week it is the appendix which develops from a bud at the junction of the small and large bowel and undergoes rapid growth into a pouch. In the sixth week there is a transient nubbin surmounting the pouch indicative of being involved in the rapid development of the pouch which is very strategically placed near the apex of the highly significant mid-gut loop. It is only after the fifth foetal month that the proximal end of this pouch, which has appeared to be a very insignificant structure up until this stage, starts growing differentially to give rise to the true caecum which continues to develop into infancy.
The embryonic appendix has finger-like projections (villi) on its inside surface and it is only around birth that the long ribbons (taeniae) causing the sacculation of the large bowel start to develop. These ribbons, of course, converge on the base of the appendix.
2. Physiological2,23,24,25
The goblet cells lining the appendix and adjacent caecum and colon secrete a special type of mucus which can be regarded as an antibacterial paint controlling the organisms which develop in the bowel in the region. The paint contains a high concentration of IgA type immunoglobulins, secretory antibodies produced for mucosal or surface immunity and part of the bowel-blood barrier .
3. Bacteriological2,23,24,25
Through the cells within and overlying the lymphoid follicles and their production of secretory and humoral antibodies the appendix would be involved in the control of which essential bacteria come to reside in the caecum and colon in neonatal life. As well it would be involved in the development of systemic tolerance to certain antigenic agents within the alimentary tract whether they are derived from bacteria, foodstuffs or even the body's own proteolytic enzymes.
4. Biochemical2
One in three hundred or so appendectomy specimens contains a carcinoid tumour composed of a highly specialised type of cell rich in vaso-active peptides such as serotonin. The exact function of such agents in the entire bowel is still being elucidated, but the fact that the majority4 of such tumours occur within the appendix is indicative that the appendix could well be involved in some way with such substances.
5. Immunological2,4,9, 13,14,20,2.1,23,25,26,27,28,29
This is the area where the appendix would seem to have its predominant functions due to its content of lymphoid follicles, which are highly specialised structures. Although it was thought the appendix itself could be the site for B-lymphocyte induction (a Bursa of Fabricius equivalent)26 the latest opinions favour this programming being more centralised in the bone marrow. The appendix may still have a role in this highly significant function, but not alone, and its lymphoid tissue is known for certain to be involved in antibody production (the function of B- type lymphocytes). These antibodies are of two types:
IgA type immunoglobulins for secretory or mucosal surface immunity, and
IgM and IgG immunoglobulins for humoral or bloodstream immunity.
The above type functions have proven the appendix to be part of the G.A.L.T.2,4,23,25 (Gut Associated Lymphoid Tissue), but it has also been shown that the appendix after the neonatal period is dispensable, meaning that normal G.A.L.T. functions remain after appendectomy. This result is not unexpected as similar lymphoid tissue is distributed up and down the alimentary tract, there being a considerable reserve potential. Experiments in rabbits have shown that the appendix alone can provide normal humoral antibody levels if necessary27 and also replenish depleted lymphocyte populations secondary to neonatal thymectomy.28
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