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Pregnancy and its diagnosis
3
Pregnancy and its diagnosis
MATERNAL RECOGNITION OF PREGNANCY In most domestic species, the establishment and maintenance of pregnancy require that the luteal phase of the oestrous cycle is prolonged by the persistence of a single corpus luteum (CL) or a number of corpora lutea (CLs). As a result of the persistence of the luteal tissue, progesterone concentrations remain elevated. This results in a negative feedback on the hypothalamus and anterior pituitary with a resultant inhibition of follicular development and ovulation and, in polyoestrous species, a prevention of return to oestrus. In many species, the placenta subsequently replaces or supplements the luteal source of progesterone. In Chapter 1 the importance of the CL in regulating the periodicity of the oestrous cycle was discussed, and the role of prostaglandin F2α (PGF2α), produced by the endometrium, in causing regression of the CL and the consequent return to oestrus was described. The presence of a viable, developing embryo(s), however, prevents the CL from regressing and thus, in polyoestrous species, inhibits the return to oestrus. This phenomenon was described by Short (1969) as the ‘maternal recognition of pregnancy’. It is particularly interesting because a maternal endocrine response is detectable before the blastocyst is attached to the endometrium by microvilli, which either directly or indirectly prevents regression of the CL. In five of the domestic species the time of maternal recognition of pregnancy has been determined (Table 3.1).
The sheep In sheep, early work by Moor (1968) and Martal et al. (1979) demonstrated that the conceptus produces a protein. In recent years it has been
Table 3.1 Time of maternal recognition of pregnancy (after Findlay, 1981) Species
Day of maternal recognition of pregnancy
Day of definite attachment
Sow Ewe Cow Mare Goat
12 12–13 16–17 14–16 17
18 16 18–22 36–38
characterised as existing in three or four isoforms of molecular weight about 18 000; originally it was named ovine trophoblast protein or oTP-1. This substance has been shown to be a type 1 interferon, which together with the same substance produced by the bovine conceptus, is classified as a tau interferon (IFN-τ). It is produced by the trophectoderm from about day 10, when the blastocyst starts to elongate. Details of the mechanism of luteolysis have been described in Chapter 1. The effect of IFN-τ in the maternal recognition of pregnancy is to alter the dynamics of PGF2α secretion at this early stage of pregnancy, compared with the same stage of the oestrous cycle. The number of luteolytic pulses is reduced between days 15 and 17, yet surprisingly the basal secretion of PGF2α is increased at this time. It is likely that IFN-τ prevents the rise in endometrial oestrogen receptors which precedes the rise in endometrial oxytocin receptors, which is necessary for the secretion of PGF2α.The consequence of this is that there is a reduction in the synthesis of PGF2α from arachidonic acid. Tamby et al. (1993) found that endometrial tissue from pregnant ewes (days 12, 14 and 16) had lower phospholipase A2 (PLA2) activity than tissue from non-pregnant ewes on the same days 69
Pregnancy and its diagnosis
MATERNAL RECOGNITION OF PREGNANCY In most domestic species, the establishment and maintenance of pregnancy require that the luteal phase of the oestrous cycle is prolonged by the persistence of a single corpus luteum (CL) or a number of corpora lutea (CLs). As a result of the persistence of the luteal tissue, progesterone concentrations remain elevated. This results in a negative feedback on the hypothalamus and anterior pituitary with a resultant inhibition of follicular development and ovulation and, in polyoestrous species, a prevention of return to oestrus. In many species, the placenta subsequently replaces or supplements the luteal source of progesterone. In Chapter 1 the importance of the CL in regulating the periodicity of the oestrous cycle was discussed, and the role of prostaglandin F2α (PGF2α), produced by the endometrium, in causing regression of the CL and the consequent return to oestrus was described. The presence of a viable, developing embryo(s), however, prevents the CL from regressing and thus, in polyoestrous species, inhibits the return to oestrus. This phenomenon was described by Short (1969) as the ‘maternal recognition of pregnancy’. It is particularly interesting because a maternal endocrine response is detectable before the blastocyst is attached to the endometrium by microvilli, which either directly or indirectly prevents regression of the CL. In five of the domestic species the time of maternal recognition of pregnancy has been determined (Table 3.1).
The sheep In sheep, early work by Moor (1968) and Martal et al. (1979) demonstrated that the conceptus produces a protein. In recent years it has been
Table 3.1 Time of maternal recognition of pregnancy (after Findlay, 1981) Species
Day of maternal recognition of pregnancy
Day of definite attachment
Sow Ewe Cow Mare Goat
12 12–13 16–17 14–16 17
18 16 18–22 36–38
characterised as existing in three or four isoforms of molecular weight about 18 000; originally it was named ovine trophoblast protein or oTP-1. This substance has been shown to be a type 1 interferon, which together with the same substance produced by the bovine conceptus, is classified as a tau interferon (IFN-τ). It is produced by the trophectoderm from about day 10, when the blastocyst starts to elongate. Details of the mechanism of luteolysis have been described in Chapter 1. The effect of IFN-τ in the maternal recognition of pregnancy is to alter the dynamics of PGF2α secretion at this early stage of pregnancy, compared with the same stage of the oestrous cycle. The number of luteolytic pulses is reduced between days 15 and 17, yet surprisingly the basal secretion of PGF2α is increased at this time. It is likely that IFN-τ prevents the rise in endometrial oestrogen receptors which precedes the rise in endometrial oxytocin receptors, which is necessary for the secretion of PGF2α.The consequence of this is that there is a reduction in the synthesis of PGF2α from arachidonic acid. Tamby et al. (1993) found that endometrial tissue from pregnant ewes (days 12, 14 and 16) had lower phospholipase A2 (PLA2) activity than tissue from non-pregnant ewes on the same days 69