The equine embryonic capsule practical implications of recent research.

Abstract: In most domestic animals, the zona pellucida is the outermost extracellular layer that covers the blastocyst before implantation. However, in the horse, an acellular membrane, the capsule, replaces the zona pellucida and envelops the developing conceptus during the 2nd and 3rd weeks of gestation. Although this structure was first described by Bonnet in 1889, it received little attention until the 1970s when its rediscovery by Marrable and Flood (1975) led to a series of reports (see review by Betteridge 1989). Nevertheless, until recently the structure, origin, and function of the capsule have been poorly understood, except for some noted ultrastructural and antigenic differences from the zona pellucida (Flood et al. 1982. Bousquet et al. 1987). The capsule is present during the period of gestation when considerable embryonic loss occurs, both in natural circumstances (Ball and Woods 1987. Woods et al. 1987. Chevalier-Cltment 1989) and after embryo transfer (Villahoz et al. 1985). Embryonic and early foetal loss have long been recognized as major causes of reproductive inefficiency in the mare (Ewart 1897), and these problems remain unresolved. Although the incidence and timing of equine embryonic loss have been defined, relatively few researchers have addressed the aetiology of these conditions (see Ball 1988). To do so, we need to understand the structure and function of the cells and cell products that are responsible for the maintenance of the equine conceptus in the uterus. Then we can address how failure of the complex interactions between them can lead to embryonic death. Because the equine capsule is at the maternal-foetal interface during the 2nd and 3rd weeks of gestation and is necessary for normal development (McKinnon et al. 1989), there is no doubt that it has a major involvement in embryonic survival and development during early pregnancy, a role that has usually been ignored (Heap 1991). In a detailed characterization of the capsule (Oriol 1993), the biochemical structure of the capsule, the changes occurring in the capsule during its development, and differences arising during equid evolution have been defined, using the horse (Equus caballus) and the donkey (Equus asinus) as models. As a result, we now have a better understanding of the structure, origin, and functions of the capsule during early pregnancy. The current work presents some of the practical implications of this study.