Puma! of Reproduction and Fertility Supplement 48, 271-278 Earlyembryonic development in prolific Meishan pigs S. P. Ford and C. R. Youngs Iowa State University, Ames, IA 50011 -3150, USA Prenatal mortality in European pigs is estimated at 30-40%, the majority of which occurs between days 12 and 18 after mating. Chinese Meishan pigs are prolific, averaging three to five more pigs per litter than do European breeds. Early reports into the fecundity of Meishan females suggested that their prolificacy resulted from lower embryonic mortality when compared with European females exhibiting the same ovulation rate. The preponderance of evidence suggests that there are no differences between Meishan and European breeds in either morphological embryo diversity within a litter or embryo mortality before day 12 after mating. Recent studies from our laboratory suggest that preimplantation embryos from Meishan females exhibit markedly reduced growth rates and oestrogen secretory activities through day 12 when compared with embryos from Yorkshire females. The signifi- cantly reduced conceptus sizes of Meishan versus European breeds on day 30 of gestation provide additional evidence of the reduced growth rate of Meishan embryos. Furthermore, because embryonic oestrogen production is known to alter uterine secretion of histotroph, the lower oestrogen production by Meishan embryos in the Meishan uterus may result in more gradual alterations in the uterine environment that are beneficial for conceptus survival and subsequent litter size. Recent studies using cross-transfer of Meishan and Yorkshire embryos on day 2 after mating have led to the suggestion that there is a major effect of recipient genotype on embryonic growth rate and oestrogen secretion. In these studies, both Meishan and Yorkshire embryos transferred to Meishan uteri exhibited marked decreases in morphological development and oestrogen content on day 12 when compared with embryos transferred to Yorkshire recipients. These data suggest the presence of factor(s) in endometrial secretions from Meishan females that reduce the growth rate and oestrogen secretory potential of preimplantation conceptuses. Introduction It is during the first 30 days of gestation in pigs that about 75% of the total prenatal embryonic loss occurs (Hammond, 1914; Comer, 1923; Squires et al., 1952; Perry, 1954; King and Young, 1957; Baker et al., 1958; Spies et aL, 1959; Hanly, 1961; Perry and Rowlands, 1962; Pope and First, 1985; Dziuk, 1987). This embryo loss appears to be independent of fertilization rate (Anderson, 1978; Polge, 1982) or limi- tations of uterine capacity (Dziuk, 1968). A large body of evidence exists suggesting that few embryos are lost before day 12 (Polge, 1982; Pusateri et aL, 1990; Anderson et aL, 1993) or from day 18 to day 30 (Spies et al., 1959; Perry and Rowlands, 1962). Thus, the period most critical to embryo inrvival occurs between day 12 and day 18 of gestation. • A wide range of embryonic developmental stages has been observed both within and between litters from day 9 to day 13 of gestation (Anderson, 1978; Pusateri et al., 1990; Anderson et al., 1993). It is during this period that critical events occur, including equidistant spacing of embryos throughout the 0 1993 Journals of Reproduction and Fertility Ltd