Proceedings of the 5th International Symposium on EQUINE ...

Equine Embryo TransferTHE INFLUENCES OF MATERNAL SIZE, AGE ANDPARITY ON PLACENTAL AND FETAL DEVELOPMENTIN THE HORSES. Wilsher and W. R. AllenEquine Fertility Unit, Mertoun Paddocks, Woodditton Road, Newmarket, Suffolk CB8 9BH, UKClear evidence that maternal size, and henceuterine size, pr<strong>of</strong>oundly effects <strong>the</strong> birthweight <strong>of</strong><strong>the</strong> foal was demonstrated by Walton andHammond (1938) in <strong>the</strong>ir classical experiment <strong>of</strong>between breed inseminations in Shetland poniesand Shire horses. Tischner and Klimszak (1989)also showed <strong>the</strong> effect <strong>of</strong> intrauterine environmenton <strong>the</strong> development <strong>of</strong> <strong>the</strong> fetus in horses bytransferring pony embryos to larger draft mares andcomparing <strong>the</strong> birth size and subsequentdevelopment <strong>of</strong> <strong>the</strong> foals with sex-matched siblingsborn from <strong>the</strong>ir natural pony mo<strong>the</strong>rs. In bo<strong>the</strong>xperiments <strong>the</strong> size differences between <strong>the</strong> foalsat birth persisted into adulthood. It is hardlysurprising that uterine size effects <strong>the</strong> size <strong>of</strong> <strong>the</strong>foal so dramatically since <strong>the</strong> diffuse, non-invasive,epi<strong>the</strong>liochorial equine placenta establishes a closemicrovillous interdigitation with <strong>the</strong> entireendometrial surface <strong>of</strong> <strong>the</strong> maternal uterus.Endometrosis may compromise <strong>the</strong> uterineenvironment during pregnancy and Bracher et al.(1996) showed a close relationship between <strong>the</strong>health <strong>of</strong> <strong>the</strong> endometrium and <strong>the</strong> normality anddensity <strong>of</strong> <strong>the</strong> microcotyledons per area <strong>of</strong> placentaduring <strong>the</strong> first two thirds <strong>of</strong> gestation. Thecondition appears to be associated more with agethan parity and Ricketts and Alonso (1991)showed that aged maiden mares developendometrosis, without <strong>the</strong> endometrium havingsuffered <strong>the</strong> challenges <strong>of</strong> semen, infection,pregnancy or parturition. Parity is also associatedwith foal birthweight and Hintz et al. (1979)observed that Thoroughbred mares < 7 years <strong>of</strong>age had lighter foals with smaller cannon bonesthan mares 7–11 years old. Fur<strong>the</strong>rmore, <strong>the</strong>sedifferences persisted until at least 510 days <strong>of</strong> age.Barron (1995) surveyed racetrack performance <strong>of</strong>Thoroughbreds and concluded that foals born fromparous mares aged 7–11 years were moresuccessful than those from maiden mares or olderanimals. Similarly, Finocchio (1996) reported thatthat third foals were most likely to become Stakeswinners, followed closely by foals produced inparities 5, 2 and 4, in that order.In <strong>the</strong> present experiment <strong>the</strong> effects <strong>of</strong>maternal size, age and parity on <strong>the</strong> gross andmicroscopic development <strong>of</strong> <strong>the</strong> placenta and onfoal birthweight were investigated. Normal termplacentae were recovered at spontaneous thirdstage labour from 2 cohorts <strong>of</strong> mares and, afterweighing <strong>the</strong> allantochorion and measuring itsgross area and volume, stereological techniqueswere applied to 10 biopsies selected randomlyfrom all parts <strong>of</strong> <strong>the</strong> allantochorion to measure <strong>the</strong>microscopic surface area <strong>of</strong> <strong>the</strong> microcotyledons.The microscopic fetomaternal contact per unitvolume <strong>of</strong> chorion (surface density <strong>of</strong>microcotyledons) and, by multiplication with <strong>the</strong>gross volume <strong>of</strong> chorion; <strong>the</strong> total area <strong>of</strong>fetomaternal contact across <strong>the</strong> entire placenta,were recorded.In <strong>the</strong> first cohort <strong>of</strong> mares between-breedembryo transfer was used to create 8Thoroughbred-in-Pony (Tb-in-P) pregnancies inwhich <strong>the</strong> genetically larger Thoroughbred fetusexperienced cramping and nutritional deprivationin utero, 7 P-in-Tb pregnancies in which <strong>the</strong>smaller Pony fetus was exposed to nutritionalexcess in utero, with 7 normal Tb-in-Tb and 7 P-in-P pregnancies as controls. Strong positivecorrelations were demonstrated between maternalweight and foal birthweight and betweenbirthweight and both <strong>the</strong> weight and gross area <strong>of</strong><strong>the</strong> placenta. Unexpectedly, surface density <strong>of</strong> <strong>the</strong>microcotyledons, which was also was positivelycorrelated with foal birthweight, was <strong>the</strong> highest in74