DEVELOPMENT OF THE SHEEP OVARY DURING FETAL AND EARLY NEONATAL LIFE AND THE EFFECT OF FECUNDITY GENES

In female sheep fetuses, the mesonephros and genital ridge can be iden tified at days 20 and 23 of gestation (term = 145 days), respectively. Moreover oogonia can be observed at the genital ridge from as early a s day 23. Around day 55 of gestation, some germ cells enter meiosis co incident with the arrival of mesonephric-derived somatic cells (i.e. t he rete ovarii). From days 75, 100, 120 and 135 of gestation, primordi al (one layer of flattened granulosa cells), primary (one complete lay er of cuboidal granulosa cells; early preantral), secondary (preantral ) and tertiary (antral) follicles, respectively, develop within the in nermost regions of the ovarian cortex. During the early neonatal perio d highly variable numbers of antral follicles may be present. After ex amination of Booroola fetuses from day 28 of gestation, it seems that the FecB(B) gene is associated with retarded development of the heart (day 28) mesonephros (days 30-40) and from day 30 to early neonatal li fe, the ovary. With respect to the ovary, fewer oogonia (days 30-40), primordial follicles (day 75-90) and growing follicles (day 120 to 6 w eeks after birth) have been observed in females carrying the FecB(B) g ene. By contrast, the FecB(B) gene is not associated with differences in plasma gonadotrophin or immunoreactive inhibin until early neonatal life. In Inverdale (I) fetuses heterozygous for the FecX(I) gene (I+) , retarded germ cell development was observed at days 40 and 90 of ges tation. In putative homozygous carriers (II) of the Inverdale gene, ge rm cell development appeared normal until day 100, but thereafter from day 120 normal secondary follicles were not observed, although many a bnormal follicular-like structures were present. In both I+ and II fet uses no obvious differences in gonadotrophin concentrations have been noted. Collectively, the evidence suggests that the fecundity genes Fe cB(B) and FecX(I), which affect ovulation rate in sexually mature fema les, are regulating organ differentiation or germ cell maturation or b oth processes during fetal life.