The current paradigm of sexual differentiation states that in the presence of androgen masculine ExG develop and in the absence of androgen the default female pattern develops. Sparse data exist on internal morphological features of the adult penis, such as the position of the urethral meatus and especially the morphology of the distal penile tip. Morphology of the mouse clitoris is poorly described. Our study shows in detail the external and internal architecture of the wt adult murine male and female ExG, which are used as end points of sexually dimorphic development. Sexually dimorphic structures are remarkably uniform in wt male and female mice. MUMP in males, the amount of bone, and cartilage and erectile tissue appear to be influenced by androgen and estrogen action, based on our data on α
ERKO and XTfm
/Y mutant animals, and those reported previously.5
Initial development of the os penis is androgen independent based on its presence in androgen insensitive of XTfm
although subsequent bone growth is androgen dependent.14,15
The absolute length of the os clitoris was considerably greater in α
ERKO vs wt females (p <0.001). These differences may be explained by the effects of estrogen on osteogenesis.16,17
Also, studies of bone metabolism in aromatase KO mice implicate a role for androgen and estrogen in bone metabolism.17
Thus, estrogen insensitivity or androgen acting unopposed by estrogen may contribute to the increased amount of bone in α
ERKO female ExG.
A substantial cartilaginous element was consistently noted in the α
ERKO clitoris while cartilage is never observed in the wt clitoris. Neonatal administration of androgen augments bone growth in the mouse clitoris but does not induce cartilage formation.4
Our study suggests that signaling via ER may influence chondrogenesis. In this regard neonatally administered tamoxifen inhibits postnatal differentiation of skeletal elements in the mouse penis, resulting in a decrease in or absence of the os penis and distal cartilaginous segment.18
These changes may be inherent to tamoxifen since treatment with other antiestrogens did not result in cartilage loss. In mice tamoxifen can act as an estrogen agonist or antagonist.
Our findings of a potential role of estrogen in sexual dimorphic development are consistent with previous studies showing ERα
and aromatase in the developing rat penis,8,9
in the developing human male fetal ExG.19
Consistent with the ER in the developing ExG is the presence of T and estradiol in the serum of female rodents during the neonatal period, when ExG sex differentiation occurs.20
To our knowledge the biological significance of low T in newborn females is unclear and, thus, especially in α
ERKO females, must be considered since in this situation androgen can act unopposed by estrogen.
ExG sensitivity was previously established in studies by Goyal et al, in which exogenous estrogen at pharmacological levels was administered during the perinatal period.21
Such treatment elicited penile dysmorphogenesis, including malformations such as hypospadias, abnormal penile muscles and bone formation, and decreased penile length, diameter and weight.21
From these studies Pang et al suggested that ER and AR based mechanisms estrogen induce penile deformity.20
Unfortunately these findings represent nonphysiological conditions and, thus, are not relevant to normal development. Our findings document various developmental effects of the ERα
mutant state on normal clitoral development.
A potential explanation for the partial masculinization of the α
ERKO clitoris may be serum T, which is increased 40-fold in adult α
However, serum T fails to explain the phenotype of the α
ERKO clitoris for several reasons. Adult serum T does not necessarily have any relation to sex differentiation in the neonatal period. Serum T is not increased in female α
ERKO mice 21 days postnatally23
or at day 10, when sex differentiation occurs. Given that masculinization in the α
ERKO clitoris appears to be independent of systemic androgen, the masculinized phenotype of the α
ERKO clitoris may be due to absent ERα
signaling in developing ExG tissue.