Specialized Structure or Vestigial Organ?
The appendix is a long, narrow, worm-shaped tube connected to the cecum, the blind pouch that forms the first portion of the large intestine. Average length is three inches with variations from less than one inch to more than eight inches. The end farthest from the cecum, the distal end, is closed.
A mucous membrane, known as the mucosa, lines the appendix. This lining is similar to the moist tissue layer that lines the hollow organs and cavities of the body; such as the large intestine. Sometimes the orifice, opening, is guarded by a semilunar, or crescent-shaped, mucosal fold which forms a valve.
Currently there is debate concerning whether or not the appendix is a vestigial organ or has a more specialized function. According to Taber's Cyclopedic Medical Dictionary the word vestigial means: "a small degenerate or incompletely developed structure that has been more fully developed in the embryo or in a previous stage of the species." For further clarification, Meriam-Webster's Medical Dictionary defines degenerate as: "having declined (as in nature, character, structure or function) from an ancestral or former state; especially having deteriorated progressively (as in the process of evolution) especially through loss of structure and function. Taber's Cyclopedic Medical Dictionary, Magills's Medical Guide and the Mayo Clinic Family Healthbook all subscribe to the opinion that the appendix is a vestigial organ.
The Thirty-Seventh Edition of Gray's Anatomy holds a differing view. Upon thorough microscopic examination their researchers found the appendix to contain a muscular layer, Lymphoid follicles, columnar epithelium and glands. The muscular outer layer provides shape and support for the appendix.
Lymphoid follicles are in irregular aggregation, clustering, of lymphocytes. Lymphocytes are the individual cells that make up the lymphatic fluid. The cells are also present in the blood. There are a number different types of lymphocytes. The different lymphocytes perform different functions. Stem cells, from which all blood cells arise, are where lymphocytes begin. They are the main means of providing the body with immune capability.
These lymphoid follicles also produce B-Cells and T-Cells. B-Cells start as lymphocytes and, upon contact with foreign antigens, become mature functioning cells. Mature B-Cells are able to identify foreign antigens and transform them into antibody producing plasma cells and memory cells. Plasma cells are the only source of antibodies, also called immunoglobulins. Memory cells quickly produce antibodies when our body is invaded by the same organism at a later date. T-Cells begin the same way as B-Cells but, at maturity, they possess highly specific cell-surface antigen (antibody) receptors. B-Cells produce the antibodies in response to foreign invaders and T-Cells intercept these antibodies and help distribute them throughout the body.
Columnar epithelium is a layer of tall, narrow epithelial cells that form a lining for the appendix and provide a "gateway" for the B-Cells and T-Cells to travel from the Lymphoid follicles into the open cavity of the appendix. Study of the appendix also revealed glands. These glands are a group of cells which selectively remove materials from the blood and either concentrates or alters the material. The transformed material is then secreted for further use in, or elimination from, the body.
At the end of their report Gray's Anatomy states the following: "In view of its rich vascularity and histological differentiation, the appendix is probably a specialized rather than a degenerate or vestigial structure." (Williams, Warwick, Roger et al. 1989).
Thomas, M.D., M.P.H., Clayton, L. Edition 18 Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia. 1997.
Hawthorne, Jr. Robert M., ed. Magill's Medical Guide: Health and Illness Vol. 1. Salem Press, Inc. 1995
Larson, M.D., David; William, E. Mayo Clinic Family Healthbook. Morrow and Company, Inc. 1990.
Williams, Peter L.; Warrick, Roger; Dyson, Mary and Bannister, Lawrence. Thirty-Seventh Edition: Gray's Anatomy. Longman Group, UK Limited. 1989.