ReviewMorphology of the external genitalia of the adult male and female mice as an endpoint of sex differentiation
Highlights
► In this review we describe the morphology of the adult mouse penis and clitoris. ► We describe 10 homologous features distinguishing the adult mouse penis and clitoris. ► A simple metric is presented for evaluating sex differentiation in normal and mutant mouse external genitalia. ► We briefly review development of external genitalia.
Introduction
Contemporary understanding of the morphogenetic and molecular mechanisms of sex differentiation of mammalian external genitalia (ExG) is still rudimentary and remains based mainly on principles enunciated by Alfred Jost over half a century ago (Jost, 1953). Jost demonstrated that masculinization of the urogenital system was dependent upon fetal testicular androgens. In the presence of androgens a male phenotype develops, whereas in the absence, the default female pathway occurs. Subsequently dihydrotestosterone (DHT) was shown to be the primary active metabolite responsible for masculinization of external genitalia in mammals (Wilson et al., 1995). This general account is supported by a wide array of clinical observations (Bin-Abbas et al., 1999, Conte et al., 1994, Ito et al., 1993). While Jost emphasized the role of androgens in masculinization of ExG, he had very little to say about the detailed anatomy of external genitalia other than two types develop, male and female. It is now apparent that male and female ExG each contain a constellation of component parts of considerable complexity and species variability (Yamada et al., 2003, Simmons and Jones, 2007).
The evolution of internal fertilization has resulted in the development of male and female organs required for this complex reproductive process. Accordingly, unique sets of male and female ExG have evolved, the penis and scrotum in males and the clitoris and vagina in females along with internal urogenital organs. Sexually dimorphic ExG have evolved in invertebrates and vertebrates (Yamada et al., 2006), and within these groups there is broad diversity of anatomical forms particularly in the penis (Simmons and Jones, 2007, Williams-Ashman, 1990). The primary penile functions are urination, copulation and sperm transfer (Simmons and Jones, 2007, Suzuki et al., 2003). Secondary penile functions include stimulation of the female and rival sperm displacement, which are achieved via morphologic specializations such as penile shape, length, papillae (spines) and other appendages (Simmons and Jones, 2007, Williams-Ashman, 1990).
Given the importance of reproduction for perpetuation of the species, sex differentiation of the ExG is an essential developmental event, which culminates in the formation of the adult penis and clitoris. Since the adult penis and clitoris are the endpoints of the process of sex differentiation, an understanding of the gross and microscopic anatomy of the normal adult penis and clitoris is essential for understanding the process of normal ExG development and its abnormalities, such as hypospadias, the most common malformation of the penis. Hypospadias occurs in approximately 1:200 to 1:300 live male births (Baskin, 2000). In the United States an unexplained doubling in the incidence of hypospadias has been reported over the last three decades (Paulozzi, 1999, Paulozzi et al., 1997). The only treatment for hypospadias is surgical repair, which can be performed successfully in the majority of patients. However, a number of patients have poor functional (may need to sit to void) and cosmetic outcomes despite multiple surgeries, in what has been described as “hypospadias cripples”. These unfortunate patients not only experience physical scarring but may also suffer from low self-esteem. Male patients with hypospadias may also have difficulty with sexuality and developing normal relationships (Baskin and Duckett, 2004). The psychological trauma to parents of newborn males with genital abnormalities must also be significant. The costs of hypospadias surgery, hospital and physician fees and time away from work to care for these young patients are a significant burden to the healthcare system.
To better understand normal development of the ExG and to prevent abnormalities such as hypospadias, it is imperative to understand sex differentiation of the ExG, its timing in appropriate animal models, the role of cell–cell interactions, the endocrine parameters of sex differentiation, morphogenesis of each component comprising the penis and clitoris, and the molecular biology of the developmental process. For many years we have had an abiding interest in development of the ExG, which has been pursued in studies of mice, moles and spotted hyenas (Yamada et al., 2003, Glickman et al., 2006, Rubenstein et al., 2003). As we have become focused specifically on the process of sex differentiation, our studies have utilized the mouse as an experimental model. First and foremost investigation of ExG development requires a detailed knowledge of the morphology of the adult mouse penis and clitoris, the endpoints of the developmental process. As we embarked upon our studies, it became apparent that the literature on the anatomy of the adult mouse penis and clitoris was both inadequate and in some cases inaccurate and certainly in need of modern morphological re-investigation. This review will focus on the updated anatomy of the adult mouse penis and the clitoris, which are the respective endpoints of masculine and feminine sex differentiation of the ExG.
Section snippets
Development of mouse genital tubercle
In mice, the penis and clitoris develop from the embryonic ambisexual genital tubercle (GT), an elevation in the fetal perineum that forms over a 4-day period (day E12–16) (Yamada et al., 2006, Perriton et al., 2002, Petiot et al., 2005). It is generally acknowledged that assembly of the ambisexual GT is complete at 16 days of gestation and that sex differentiation of the GT begins thereafter (Suzuki et al., 2002). Even though mice can be sexed visually at birth based upon anogenital distance
Morphology of the adult mouse penis
In a strict sense, the adult penis is a mesoderm-derived appendage covered on its surface by ectoderm-derived skin and containing an endoderm-derived urethra. In the mouse perhaps as much as 90% of the penis is derived from mesoderm, which differentiates into erectile bodies, connective tissue, smooth muscle, bone and cartilage arranged in precise patterns required for penile function. The adult mouse penis is comprised of a body proximally and a glans distally that connect at a right angle
Morphology of the adult mouse clitoris
Since ExG of the adult mice represent endpoints of the process of sex differentiation in which the ambisexual genital tubercle develops into either a penis or clitoris, the genesis of the morphological differences between these organs becomes paramount in understanding the process of sex differentiation of the ExG. Unfortunately, previous descriptions of the adult mouse clitoris lack sufficient detail to fully understand the anatomy and histology of this important organ of female sexual
Discussion
Since the penis and clitoris arise from a common precursor, the fetal GT, and are homologous organs, homologies exist between the individual components comprising these organs. These homologies are illustrated in Table 1 and Fig. 7 in which all male features are ascribed the value of 1, and all homologous female features are given a value of zero. Accordingly, objective comparative measures can be performed on ExG of adult wild-type male mice which score 10, and adult wild-type female mice
Conclusion
While Jost’s theory of ExG development focusing exclusively on the presence or absence of androgen action is undoubtedly correct, it neglects that possibility of other endocrine mechanisms such as estrogen action and relegates female development as a default pathway, which discouraged search for active processes involved in the development of female ExG (Drea et al., 1999). We emphasize that normal female sex differentiation of the ExG is a unique active process and not simply a default pathway
Acknowledgments
Supported by NSF Grant IOS-0920793, NIH grant RO1 DK0581050 and NH&MRC grant APP1002733.
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