Enigmatic Features of Penile Development and Functions

ENIGMATIC FEATURES OF PENILE DEVELOPMENT AND FUNCTIONS H. G. WILLIAMS-ASHMAN* The paramount utility of the penis in eutherian mammals is twofold: to eliminate urine and to inseminate females. These functions of the phallus depend on the distal segment of the male urethra traversing the penile shaft and glans and thus serving as the final common channel for delivery of urine or seminal fluid out of the organism. Micturition is the prerogative of the penis in its customary flaccid state, and its frequent occurrence throughout postnatal life is indispensable for survival. In contrast, penile intromissions into the vagina culminating in ejaculation of semen can be accomplished normally only by mature males capable of sustained erections, these events being essential only for perpetuation of the species but not for the continued existence of an individual male. Urination and semen delivery are normally mutually exclusive functions because (a) urine cahnot be discharged from the fully erect penis and (b) when spermatozoa together with seminal plasma are expelled into the proximal region of the urethra during seminal emission, the neck of the bladder normally shuts tightly, thereby preventing retrograde flow of semen into the bladder or outflow of urine therefrom. The morphology of the adult penis (and the adjacent scrotum when present) varies greatly among various orders of modern eutherian mammals and can differ extensively between even closely related species. Interspecific variations in the topology (but not, of course, the size) of the homologous vulva in sexually mature females are generally less evident . Speculations on the biological significance of the amazing diversity The author is deeply grateful to Drs. F. Fitch, M. K. McClintock, A. E. Pegg, A. H. Reddi, and N. Wagoner for valuable advice, and to Mrs. Frances Mills for expert assistance in the preparation of the manuscript. *Ben May Institute, and Departments of Biochemistry and Molecular Biology and Pharmacological and Physiological Sciences, University of Chicago, BH-N610, Box 241, 5841 South Maryland Avenue, Chicago, Illinois 60637.© 1990 by The University of Chicago. AU rights reserved. 0031-5982/3303-0696$01 .00 Perspectives in Biology and Medicine, 33, 3 ¦ Spring 1990 \ 335 of male external genitalia in contemporary eutherian species must take into account the following fundamental considerations. First, when they initially develop in the eutherian fetus before the onset of differentiation of extragonadal reproductive organs, the external genital anlagen appear to be morphologically identical in both sexes and can be distinguished only on the basis of the complement of sex chromosomes in the cells they contain. Yet a male mode of external genital morphogenesis in eutherians is essentially independent of the sex genotype, but rather can occur only if certain androgenic hormones (normally supplied by the fetal testis) are available in sufficient amounts to initially indifferent (ambisexual) precursor tissues at critical and restricted periods of embryonic development, and provided that the target cells contain adequate levels of functional androgen receptors and other biomolecules that are necessary for them to respond to androgens. The essential role of androgens in male external genital morphogenesis is illustrated by the formation of a vulva and lower vagina (though no upper vagina, uterus, or oviducts), and the absence of penis or scrotum, in XY male embryos in which either production of androgens by the fetal testis or androgen action mechanisms are defective for genetic reasons. Second, in various species the penis serves additional reproductive functions besides acting as a conduit for deposition of semen into the vagina. For example, excitation of tactile receptors in the penis generates sensory nerve impulses that influence central neurophysiological regulation of male mounting, erectile, intromittent, and ejaculatory processes . Intravaginal thrusts of the penis during copulation can stimulate sensory nerve endings in female vulvovaginal and cervical areas that bring about neuroendocrine events that affect fertility, such as facilitation ofsperm transport through the cervix and lumens ofthe uterus and oviducts; establishment of postcoital progestational states; and even the induction of ovulation in a restricted number of species. And in a few eutherian creatures, public display of the erect penis is a standard maneuver of male courtship behavior that fulfills an epigamie function in soliciting females in estrus to copulate with males. A third issue concerns the possible evolutionary adaptive significance of various...