To the Editor: Is the body perfect?
The human body is a wonderful thing. Some parts of it, however, may simply be poorly designed. The paranasal sinuses are prone to infection. Drs. Altemeier and Ward Pediatric Annals, December 1998) offer an interesting theory on why this may not be a completely bad thing. Could they, I wonder, rationalize next why the male urethra is surrounded by the hypertrophy-prone prostate gland?
Howard Fischer, MD
Division of Ambulatory Pediatrics
Children's Hospital of Michigan
Dr. Altemeier's response:
Dr. Fischer's challenge is in reference to the editorial, "Why Do We Have Paranasal Sinuses?" This editorial indicates that many functions have been offered, but none seems to clearly explain why we have these cavities that are prone to infections and that occasionally lead to severe complications. The editorial goes on to postulate that some important positive function must be present or evolution would have selected against a design so susceptible to infection. Our hypothesis was that perhaps one possible function of the paranasal sinuses, especially the ethmoid and the maxillary sinuses, is to serve as reservoirs for production and absorption of infectious antigens to enhance immunity. Dr. Fischer uses the example of the prostate gland to question the assumption that the body is designed with sufficient perfection to warrant raising a question when a specific organ does not seem to be designed correctly.
This is a great challenge because evolution would be expected to exert strong selective force on organs involved in reproduction. A search was made to see if there is a benefit that could explain why the urethra runs through the prostate gland. Because of the negative impact of benign prostatic hypertrophy, if there is no adequate explanation for this anatomic design, a flaw may be presumed.
A brief review of the anatomy and biology of the prostate gland1,2 revealed that this organ is approximately 2.5 to 3 cm long and 3 to 4 cm in diameter, weighing 18 to 25 g in young adult men. It contains 30 to 50 compound tubuloalveolar glands that share 16 to 32 excretory ducts that open directly into the part of the urethra that is surrounded by the prostate. The prostate also surrounds the paired ducts from the seminal vesicles and the deferent ducts that carry sperm. These merge within the prostate to form the ejaculatory ducts that project dorsally through the prostate and open out into a compartment that projects into the lumen of the prostatic urethra. The glands of the prostate are embedded in a connective tissue stroma consisting mainly of fibroblasts and smooth muscle cells.
The prostate and the seminal vesicles produce the seminal fluid. The prostate produces 30% by volume and the most important constituents are citric acid, zinc, and spermine, which all seem to protect sperm cells. Acid phosphatase, of unclear function, and prostate specific antigen, a proteolytic enzyme important in normal function of the prostatic fluid, are also produced. The major constituents of seminal vesicle secretion are fructose, which provides energy for sperm, and other protective agents. The synthesis and secretion of prostatic fluid are under neurologic and hormonal (predominantly androgen) control. The fibromuscular stroma of the prostate acts as a sphincter of the urethra that, with pelvic musculature, controls micturition, and plays a major role in the process of ejaculation of semen, under neurologic control.
So some explanations can be offered for why the prostate gland surrounds the urethra. The design facilitates the direct delivery of prostatic secretion into the urethra. Malfunction or obstruction of one or even several glands or ducts would leave backup function by this design. And the muscular contraction during ejaculation propels prostatic fluid with seminal vesical fluid and sperm into the urethra. Thus, passing the urethra through the prostate gland provides a simple and direct design for this necessary reproductive function. One could call the design "foolproof," but somehow this term does not fit here in a global context.
Also of note is that the power of selection would be expected to wane as aging made reproductive function less important. So selection would have much less of a reason to design the prostate gland and urethra to avoid benign prostatic hypertrophy in middle age than it would to ensure reproductive function in young adults.
Of course, all of this is based on theory, but it is fun to speculate about such matters. Thank you, Dr. Fischer.
1. Dulbecco R. Encyclopedia of Human Biology, vol. 7. 2nd ed. New York: Academic Plot; 1997:146-148.
2. WaIs PC, Retik AB. Vaughan ED, Wein AJ. Anatomy of die lower urinary tract and male genitalia. In: Campbell's Urology, 7di ed. Philadelphia: W. B. Saunders; 1997:110-114-