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Variation in Penile and Clitoral Morphology in Four Species of Moles


Most eutherian mammals possess sexually dimorphic external genitalia. Males have a penis that is traversed to near the tip by a urethra, a scrotum that encloses the testes, and a long anogenital distance. In females anogenital distance is short, and the typical clitoris is usually markedly smaller than the penis, and is frequently "internally" situated with the urethra exiting independent of the clitoris. In addition, the clitoris is associated with an externally visible vaginal opening (at least during the breeding season). This sexual dimorphism is usually associated with the presence (males) or absence (females) of androgens during development of the external genitalia. Females with naturally "masculinized" external genitalia challenge the typical mammalian androgen-dependent masculinization theory and are the focus of this dissertation. Research as to how they "do," or "do-not" fit the current widely accepted theory of sexual differentiation may reveal novel mechanisms of sexual differentiation.

Since adult external genitalia are the endpoints of sexual differentiation, developmental processes can be inferred from examination of adult morphology, providing that anatomy of external genitalia has been (is) accurately described and interpreted. The first revelation incurred in my study was that all previous reports on mole external genitalia were in error in regard to the following terms: penis, clitoris, penile clitoris, phallus, prepuce and urethra. Accordingly, by way of correcting errors of previous literature, my first task was a detailed anatomic and morphometric analysis of external genitalia in four species of moles. For an even broader perspective I also reviewed morphology of external genitalia of mouse, human and spotted hyena. Accurate morphological descriptions of external genitalia are the essential pre-requisite to a full understanding of the comparative anatomy of mole external genitalia and the potential role of hormones in development of the external genitalia in these species.

This dissertation is focused on four species of moles. All are members of the family Talpidae, in the order Insectivora. Three of these species defy the conventionally obvious visual distinctions between males and females in so far as the perineal appendage described previously as "penis" and "penile clitoris" is of similar size in males and females. Unfortunately, the visible perineal appendage in these male and female moles is prepuce. A major question is why the prepuce is similar in size in males and females. One theory is that the ovarian interstitial gland is capable of producing androgen. Of the three mole species with "masculinized" female external genitalia, broad-footed moles (Scapanus latimanus) do not possess an ovarian interstitial gland, star-nosed moles (Condylura cristata) have an ovarian interstitial gland, and in hairy-tailed moles (Parascalops breweri) ovarian structure has never been investigated. For these three mole species, it is difficult for the casual observer to distinguish between males and females during the non-breeding season, when the vaginal opening of female moles is "closed." The fourth species examined in the present investigation is the Japanese shrew mole (Urotrichus talpoides), which does not possess an ovarian interstitial gland. In Japanese shrew moles, the distinction between males and females is obvious, since the male prepuce is much larger than the female prepuce, and the latter does not have a urethra exiting its tip. Thus, sexual dimorphism of the external genitalia in Japanese shrew moles appears to follow the typical mammalian pattern.

Intriguing work has been done on mole species concerning the ovarian interstitial gland, its resemblance to testicular tissue, and its ability to produce androgens in relation to the corresponding presence of a "penile clitoris" in some mole species. However, other mole species that do not possess this ovarian interstitial gland also display a "penile clitoris" that is similar in size and shape to male external genitalia. Of course, this erroneous discussion of "penile clitoris" from the literature actually deals with the female prepuce. Discussion of the role of the ovarian interstitial gland in masculinization of mole external genitalia, first and foremost requires a detailed accurate anatomic analysis of the structure of the so-called "penile clitoris". Unfortunately no comparative work has adequately addressed the role of androgens derived from the ovarian interstitial gland in development of external genitalia in the different mole species. For the first time, my accurate detailed description of mole genital morphology provides the opportunity to address this question.

The penis and clitoris of typical mammals are strikingly different, anatomically complex organs composed of epithelial tissue, connective tissue, vascular tissue, nerves, cartilage, and bone that are organized into specific and precise morphological patterns. The common developmental history, architecture, and composition of the penis and clitoris across most mammalian species allow for multiple features to be used to assess sexual dimorphism of the various components that constitute male and female external genitalia. The size and location of several key anatomic features were noted with the aid of three-dimensional reconstructions for a more detailed comparison between the different mole species. In addition to these internal anatomical measurements, anogenital distance, a trait that is modulated by androgen action in utero, was used as another measure of "genital masculinization". Measures of prepuce length, termed "phallus" length in previous publications, were used to investigate the degree of sexual dimorphism in the external genitalia in my study. Ovarian tissue was examined histologically for the presence or absence of an interstitial gland (a potential source of androgen). I discovered in the breeding season, Japanese shrew moles display a large glandular tissue structure attached to the ovary that has never before been reported. The Japanese shrew mole penis is much larger than the clitoris, is vastly anatomically different from the clitoris, and males have a longer anogenital distance than females presenting a typical mammalian pattern of sexual dimorphism, presumably based upon the presence versus absence of adequate androgen levels. Broad-footed, star-nosed, and hairy-tailed moles have notable morphological variation in the penis and clitoris between these species as well as between the sexes. However, similar to star-nosed moles (that possess an ovarian interstitial gland), female broad-footed moles displayed several masculine morphological characteristics and an anogenital distance equal to the males' despite lacking an ovarian interstitial gland. This suggests that either development of the external genitalia is partially androgen-independent in these species or in females androgen production may be coming from another source. Lastly, I compared different patterns of external genitalia in human, mouse, 4 species of mole, and spotted hyena in relation to known endocrine profiles, and mechanisms of morphogenesis/differentiation noting gaps in the data.

In summary, my research provides the first accurate descriptions of the gross and histologic anatomy of male and female mole external genitalia. Also, comparative mouse-mole studies reported here have validated the remarkable similarity in external genitalia anatomy between these two species and have led to the conclusion that all previous literature on mole external genitalia suffers from consistent anatomical error. Being able to set the record straight allows for the first time an accurate definition of mole external genitalia anatomy and its relation to endocrine parameters.

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