Reverse, hormone-dependent sex difference in molar tooth mass in pubertal mice

Increased tooth crown size in females with twin brothers: Evidence for hormonal diffusion between human twins in utero

In rodents, the position of a fetus in utero is associated with the expression of sexually dimorphic traits. This phenomenon has been explained by prenatal diffusion of sex hormones among litter mates. To test for such effects in humans, female-male twin pairs provide a natural experiment. The size of dental crowns is a sexually dimorphic trait which can be measured with a high degree of reliability. Thus, two crown diameters of 28 permanent teeth were recorded for 56 opposite-sexed (OS) and 242 same-sexed (SS) twin pairs, and 150 singletons. Comparisons of OS twins with SS twins and singletons within each sex reveal that OS females have consistently larger teeth (on average) than other females, while there is no consistent difference between OS and SS twin males. It is proposed that diffusion of sex hormones from male to female co-twins in utero may account for the increased tooth size in OS females. This study is one of the first to report such an effect on a morphological variable in humans. The finding that the maxillary canine, one of the most sexually dimorphic teeth, exhibits the least effect in OS female twins, suggests that prenatal sex hormone levels may have less impact on sexual dimorphism in the maxillary canines than in other permanent teeth. Am. J. Hum. Biol. 11:577-586, 1999. Copyright 1999 Wiley-Liss, Inc.

Histological studies on eyelid opening in normal male mice and hemizygotes for the mutant gene Tabby (Ta) with and without epidermal growth factor treatment

Previous work has shown that mice hemizygous or homozygous for the mutant gene Tabby have delayed eyelid opening, as compared to unaffected, wildtype littermate controls; exogenous treatment with epidermal growth factor reverses this delay. We performed histological studies to explore the mechanisms of action of the Tabby gene and of epidermal growth factor in these processes. These show that eyelid opening is associated with keratinization of the fusion junction and conjunctival sac formation. Both these processes occur earlier in normal male mice (days 4 and 7 respectively) than in Tabby hemizygotes (days 7 and 10, respectively). After epidermal growth factor injection, keratinization and conjunctival sac formation are both observed on postnatal day 1 in all control and mutant pups. Thus epidermal growth factor appears to accelerate eyelid opening by stimulating these morphological processes and the Tabby gene appears to delay eyelid opening by impairing them. It is possible that deficiency of epidermal growth factor at the tissue level may be involved in the development of some of the traits seen in Tabby mutants. In addition to analysing the effects of the Tabby gene and of epidermal growth factor on eyelid opening in the mouse, this study appears to be the first detailed histological description of normal eyelid opening. The findings have potential clinical significance; firstly, because the Tabby gene shows genetic homology to the human gene for hypohidrotic ectodermal dysplasia, and disturbed eyelid opening is a trait of some forms of human ectodermal dysplasia, and secondly, because the gene for epidermal growth factor receptor is an oncogene.

Induction of sweat glands by epidermal growth factor in murine X-linked anhidrotic ectodermal dysplasia

Tabby (Ta), a murine X-linked mutant gene, produces a syndrome of ectodermal dysplasia including anhidrosis (absence of sweat glands). Development of sweat glands is related to that of dermal ridges (dermatoglyphics) and abnormal ridges may be associated with absence of sweat glands in the human syndrome of hypohidrotic ectodermal dysplasia (HED). We have found that dermal ridges occur in normal mice but are lacking in Ta mutants. Previously we showed that epidermal growth factor (EGF) reverses delayed eyelid opening and incisor eruption in Ta mice. We now report that EGF induces development of dermal ridges and functional sweat glands in Ta/Y hemizygotes, indicating a role in mammalian morphogenesis. Ta seems to be genetically homologous to human X-linked HED, as Ta maps close to loci homologous to linkage markers of HED and the two syndromes share many traits, including absence of all or most sweat glands. Absence of these glands causes hyperpyrexia, a clinical emergency in infants with HED; reversal of the trait in the mouse homologue of the disease indicates that an important genetically determined congenital defect in humans may become treatable.

Beta-glucuronidase activity is present in the microscopic epididymis of the Tfm/Y mouse

The sex-linked recessive gene Tfm in the mouse produces a condition of testicular feminization (androgen insensitivity syndrome, AIS) in hemizygotes, comparable to the condition of the same name in humans. The murine mutant was originally believed to have no derivatives of the mesonephric duct system (MDS), and this absence was ascribed to dependence of these derivatives on androgens for survival. However, microscopical epididymides, retia testes, and vasa deferentia were identified in these animals in our laboratory. These micro-organs may play a role in meiosis induction in Tfm/Y animals. The present study was designed to determine whether survival of these organs is due to retention of an ability to respond to androgens, or whether they are unique amongst MDS derivatives in being independent of androgens. Previous studies in our laboratory demonstrated that the enzyme beta-glucuronidase (beta G) is androgen sensitive in the epididymis of the normal mouse. In the present investigation we used this enzyme as a marker to study androgen sensitivity in the microscopical epididymides of Tfm/Y hemizygotes and in the epididymides of control +/Y litter-mate brothers. Both mutant and control animals were studied with and without exogenous androgen stimulation. Tfm/Y hemizygotes demonstrated low levels of diffuse, cytoplasmic beta G activity that appears to be unresponsive to exogenous androgen stimulation. In light of our previous studies, this distribution of beta G reaction products suggests some degree of androgen sensitivity. The survival of these micro-organs and their partial androgen sensitivity may be related to the role of the MDS in inducing meiosis.

Anhidrosis and absence of sweat glands in mice hemizygous for the Tabby gene: supportive evidence for the hypothesis of homology between Tabby and human anhidrotic (hypohidrotic) ectodermal dysplasia (Christ-Siemens-Touraine syndrome)

I have suggested that the X-linked gene Tabby (Ta) and its autosomal mimics in the mouse may be homologous with the genes for sex-linked anhidrotic (hypohidrotic) ectodermal dysplasia (Christ-Siemens-Touraine syndrome, CST) and its apparent autosomal mimics in the human. In the present study, I examined whether anhidrosis, a cardinal feature of CST, is present in the putative mouse sex-linked model, Tabby. The results demonstrate that whereas normal mice perspire on the volar and plantar surfaces of their paws, hemizygous Ta/Y male mice show anhidrosis and absence of sweat glands, as do human hemizygous male sufferers of CST. This result is strongly supportive of the hypothesis that Ta is homologous to the gene for CST.

Pleiotropic effect of Tabby gene on epidermal growth factor-containing cells of mouse submandibular gland

The influence of the Tabby gene on the submandibular gland of the mouse was assessed by comparing the mass of the gland, and its content of granular convoluted tubules, in prepubertal, pubertal, and adult Tabby mice and their wild type brother controls. The Tabby mice showed reduction in mass of the adult submandibular gland, delayed development of the granular convoluted tubules, and reduction in relative content of granular convoluted tubules. Epidermal growth factor (EGF) is released from the granular convoluted tubules of the mouse submandibular gland, and it is known to experimentally influence the development of at least some of the structures that are affected by the Tabby gene. Accordingly, the question of a relationship between the Tabby gene and EGF is raised.