The freemartin condition in swine

XXII.—Placental Fusion in Mice

Since it is recognised that a female rodent will litter best if left alone, it is rare for breeders to see the placenta, which is usually devoured by the mother immediately after the birth of the young. Work upon the milk factor for mammary carcinoma in mice has necessitated frequent examination of mice about to litter in order to exchange litters as soon after birth as possible, and thus a number of placentas have been seen when the mouse was caught in the act of littering. Nevertheless these represent only a small proportion compared with the number of mice born during this work. It is of some interest that two cases of placental fusion have been noted among this small series, which suggests that this condition may be fairly common among mice. A rough estimate of the frequency in this series is about 1 per cent. No reports of similar cases have been found in the literature, though a note by Merton (1938) that two embryos had been “implanted in the same place” may refer to a similar case. Because of the implications of this finding in relation to sexual and genetical differentiation in the mouse, a description of this material seems justified.

Aetiology of intersexuality in female (XX) pigs, with novel molecular interpretations

This review considers the problem of ovotestis formation in animals of 38,XX chromosome complement. After a clinical description, attention focuses on the condition of the gonads and genital tract. A complete spectrum of gonadal types has been found, ranging from a single ovotestis almost invariably on the right-hand side to both gonads appearing as testicular-like structures, sometimes with a distinct tunica albuginea. The ovotestis or testis-like structure may have descended to an inguinal or scrotal location. Although interstitial cells of Leydig and seminiferous tubules were always abundant in testicular tissue, germ cells were never present. The lumen of the seminiferous tubules was packed with pale-staining, Sertoli-like cells. A bicornuate uterus was characteristic but suppression of the proximal portion of the Müllerian duct always adjoined an ovotestis; a corresponding development of the Wolffian duct featured as a convoluted epididymis. Inhibition of the Fallopian tube was attributed to a local influence of AMH from the Sertoli cells, as was the failure of small Graafian follicles within an ovotestis to respond to injected gonadotrophins. As to the aetiology of an ovotestis, defective colonisation of the genital ridges by primordial germ cells is considered, as is evidence for incorporation of adrenal cells into the embryonic gonad. Molecular probing has failed to reveal the classical sex-determining gene, Sry, and other Y-related DNA sequences such as Zfy and DYZI in almost all the intersex animals examined. Currently favoured as an explanation for ovotestis formation is a mutation in the inhibin gene within granulosa cells of Graafian follicles. Such a mutation would prompt secretion of the closely comparable glycoprotein molecule AMH in these genetic females, with a resultant progressive virilisation of gonadal tissue. The proposed mutation may be carried as an autosomal recessive gene by certain boars. Varying amounts of AMH secretion or differing timescales for the transition from inhibin to AMH could in part explain differing degrees of ovotestis formation. Despite this proposition, interactions between genes that prescribe functional testicular tissue, enhanced rates of gonadal development, and left-right asymmetries between the paired gonads now require systematic study.

Agonadal presumptive XX/XY leukochimeric pig

BACKGROUND

XX/XY chimeric pigs are uncommon and their reproductive anatomy is variable and unpredictable.

METHOD

A piglet was identified by its enlarged vulva as a possible intersex. Venous blood was collected at 1.5 and 9 months for karyotyping and determination of testosterone and estrone sulphate concentrations. At 1 year euthanasia was performed. The reproductive tract was carefully dissected and examined histologically.

RESULTS

As the animal matured the vulva did not develop relative to the size of the animal. Lymphocyte cultures indicated a 70% XX/30% XY chimera. The reproductive tract consisted of a strand of tissue enveloped by fascia. Histological study revealed presumptive Wolffian derivatives, coiled bilateral ducts along the tract, and a Müllerian derivative, a medially located duct in the caudal third of the tract. No gonads were found. Plasma levels of estrone sulfate and testosterone were negligible.

CONCLUSIONS

In utero exposure to exogenous androgens from a male co-twin or weak endogenous adrenal androgens may account for the enlarged vulva at birth and retention of the androgen dependent Wolffian duct primordia. An atesticular state is supported by retention of the Müllerian duct primordia and the negligible peripheral sex steroids.

Attempts surgically to induce chorionic fusion between embryos

Intercornual fistulas were made in the uteri of 19 ewes in an attempt to induce chorionic fusion between fetuses. Ewes were 28 to 40 days pregnant with at least one fetus per horn. Sections were removed from the dorsomedial wall of each uterine horn, and left and right horns were sutured together around the openings to produce the fistulas. In some cases a doughnut-shaped sheet of silastic was inserted between the horns, and in most cases the chorioallantoic membranes of the respective fetuses were sutured to one another prior to suturing the uterine walls. In the 16 animals in which intercornual fistulas were constructed in the area of the intercornual ligament, no chorionic anastomosis was induced. In one of the three animals in which fistulas were produced distal to that ligament, the fistula remained patent 25 days post-surgery, and membranes of two female fetuses were joined, through the opening, by a narrow strand of chorionic tissue. We concluded that this technique was not a promising one for induction of choriovascular anastomosis.

Cytogenetical studies of infertile pigs

Cytogenetical studies were made on 6 infertile pigs. Post-mortem examination of the reproductive organs of 5 of these pigs showed them to be intersexes. Regardless of the degree of gonadal deviation from the normal, chromosome karyotype of cultured peripheral blood lymphocytes, bone marrow, liver and kidney, revealed that 4 of the intersexes had a 38,XX constitution. One intersex exhibited sex chromosome mosaicism (38XX/38XY) in lymphocytes from the peripheral blood and bone marrow and a normal male karyotype (38XY) in cultured liver and kidney cells. The sixth pig, a phenotypically normal boar, also had a 38XX/38XY chromosome constitution.