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Aetiology
- Results from sexual modification of a female twin by the in-utero exchange of blood from a male foetus.
- Freemartin is XX genetic female, but becomes an XX/XY chimera.
- 90% of female calves born co-twin to a male will be freemartins.
- There are two theories to explain this:
- Hormonal Theory
- Hormones from the male twin reach the female through vascular anastamoses between the fused placentae to cause masculinisation of the female gonad.
- Cellular theory
- Exhange of blood-forming cells and germ cells between foetuses.
- Reciprocal exchange results in identical erythrocyte antigen types and sex chromosome chimerism (60 XX/XY).
- Hormonal Theory
- Paramesonephric (Mullerian) ducts do not form or atrophy.
- Mesonephric (Wolffian) ducts persist in the female twin and may form rudimentary male tubular genitalia.
Diagnosis
- Clinical genital abnormalities
- Presence of sex chromatin bodies in circulating leukocytes of the male co-twin
- Chimerism (XX/XY) of haematopoeitic cells. Blood tests can be used to demonstrate different chromosome complement in nucleated leukocytes and presence of two blood groups in each twin.
- Rectal palpation and vaginoscopy reveal a shortened vagina
- In calves 1-4 weeks old, the vagina is normally 13-15cm in length compared to 5-6cm in a freemartin.
- Vulva and vestibule patent but vestibule or vagina is blind-ended - can be checked using speculum or probe.
Morphology
- Gonads range from modified Ovaries to structures resembling Testes which are intra-abdominal and do not descend through the inguinal canal (Cryptorchid).
- No spermatogenesis, sterile.
- Produce Testosterone as the major hormone.
- Rudimentary Uterus and Small Vagina.
- Enlarged clitoris, long tufts of hair round vulva, skin fold from groin to umbilicus.
- Vagina never communicates with uterus
- Fibrous cords or non-patent uterine tubes.
- Ovaries are usually small, cord-like thickenings in the ovarian ligament are semniferous tubules.