Bovine Freemartinism

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).

• 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.

FREEMARTIN SYNDROME (i.e. Chimeras)

• A freemartin is a masculinised female born co-twin with a normal male.
• 11/12 female calves born co-twin to a male will be freemartins.
• Mainly recognised in cattle, but occurs in sheep, goats and pigs.

• During foetal development placental anastomosis allows blood to be shared by the twins (anastamosis occurs in 90% of bovine twins, <2% in other species).
• Each twin acquires genetically distinct population of blood cells from the other twin - CHIMERISM.
• Blood tests can be used to demonstrate different chromosome complement in nucleated WBC and presence of two blood groups in each twin.

Aetiology: Testes develop earlier in embryonic life than ovaries and impair normal female development by the production of TDF, MIS and testosterone which enters the female twins circulation via the placental anastomosis.

Consequences:

1. Paramesonephric (Mullerian) ducts do not form or atrophy.
2. Mesonephric (Wolffian) ducts persist in the female twin and may form rudimentary male tubular genitalia.

Morphological features:

1. External genitalia may appear relatively normal - condition only detected at puberty or
2. Enlarged clitoris, long tufts of hair round vulva, skin fold from groin to umbilicus.
3. Vulva and vestibule patent but vestibule or vagina is blind-ended - can be checked using speculum or probe.
4. Vagina never communicates with uterus. Uterus variably under developed - fibrous cords or non-patent tubes.
5. Ovaries are usually small, cord-like thickenings in the ovarian ligament  seminiferous tubules.