Key words:
Equine twins, abortion, blastocyst capsule, diapause.
Introduction
Early pregnancy in the mare is a fascinating period that comprises numerous profound developmental changes and events, many of which are characteristic to the horse. (Stout,2009). At ovulation, the oocyte is released into the relatively small oviductal fimbria and dropped into the ampullary region where, if capacitated sperm are present, fertilization will take place (Boyle et al., 1987 and Hunter 2005). Once fertilized, the newly formed “zygote” embarks on a series of regular cell divisions, such that it reaches the two-cell stage within 24 hours, four to six cells within 48 hours, and by 72 hours contains eight to ten cells (Bezard et al., 1989). Between days 4 and 5, a “morula” is formed, and the as yet undifferentiated cells “compact” into a homogenous ball until late on day 5 or early on day 6 when the embryo starts to develop into a “blastocyst” (i.e., to form a central cavity while the constituent cells differentiate visibly into either inner cell mass or trophectoderm). At around the same time, day 6–7 after ovulation, the embryo finally enters the uterus. (Freeman et al., 1991; Battut et al., 1997).
A blastocyst capsule is one of the defining characteristics of pre-fixation horse conceptus development. Although the exact functions of the capsule are not known, it’s tough, elastic nature almost certainly allows it to provide mechanical protection (Stout and Allen, 2001).
The most well-known historical and important cause of abortion in thoroughbreds was twin formation (Acland, 1987, 1993). Twins are usually originating from diestrus ovulation. The outcome for the majority of twin pregnancies is early fetal resorption or loss, late term abortions, or the birth of small growth retarded foals. Mares aborting twins in late gestation frequently have dystocia, damaging effect on their reproductive tracts, and are difficult to rebreed. If foals are born alive they are frequently small, showing the effects of intrauterine growth retardation, and have a very poor survival rate, with many essential expensive complicated critical care (McKinnon, 2011).
There is a phenomenon called an embryonic diapause in mammals which is a period of developmental arrest, in which the blastocyst is maintained in a dormant state for an extended period of time. Over 100 species of mammals undergo embryonic diapause (Renfree, Calaby 1981 and Renfree, Shaw 2000), including the mouse and around 30 species of marsupials. In diapausing mammals, the entry into diapause is controlled by signals including the suckling stimulus, the change in daylength or the availability of nutrition (Flint et al., 1981).