Time and nature of the signal for maternal recognition of pregnancy in the pig

A vital link in a complex of physiological processes occurring during early pregnancy is the so-called maternal recognition of pregnancy: the prolongation of ovarian luteal function for continuation of progesterone secretion by an anti-luteolytic action of the developing embryos. Progesterone is crucial for the secretion of histotrophe, which in turn is essential for conceptus development and inhibition of uterine contraction activity. In the pig the corpora lutea (CL) remain the major source of progesterone throughout pregnancy.

To establish the time of maternal recognition of pregnancy in the pig (a) progesterone profiles of pregnant and cyclic gilts were compared (chapter III) and (b) blastocysts were flushed out of the uterine horns of gilts on Days 10-13 of pregnancy (chapter IV).

From Day 14, the progesterone concentration in pregnant gilts was higher than in cyclic gilts. This indicates that rescue of the CL during pregnancy occurs before Day 14.

Flushing of blastocysts out of both uterine horns on Day 10 of pregnancy resulted in a cycle with a normal progesterone profile and a normal length. Flushing of blastocysts on Days 11, 12 or 13 resulted in the maintenance of the CL for 3-13 days in those gilts in which at least some of the blastocysts were ≥8 mm or filamentous. Since CL function is only extended for on average 7 days, a second signal seems necessary to maintain the CL throughout pregnancy.

At the time that a first signal for maternal recognition of pregnancy is generated, the blastocysts start to secrete appreciable amounts of proteins, prostaglandins and oestrogens. So far, no anti- luteolytic role of proteins has been demonstrated in the pig, and data concerning an anti-luteolytic action of prostaglandin E2 are contradictory. Oestrogens exert an anti-luteolytic effect as indicated by CL maintenance in cyclic gilts after i.m. injection of large doses of oestrogens.

The ability of blastocysts to synthesize oestrogens has been demonstrated by conversion of labelled precursors into both oestrone and oestradiol-17βduring in vitro culture. The conversion of androgens to oestrogens by aromatase activity has first been detected in litters with spherical Day-11 blastocysts.

To gain a clear understanding of the participation of each blastocyst in the initiation of oestrogen production, aromatase activity was determined in individual Day-11 blastocysts (chapter V). The oestrogen-synthesis capacity of blastocysts during the period that a first signal for maternal recognition of pregnancy is generated, was determined by measurement of oestrogen release by spherical and filamentous blastocysts during in vitro culture without supplementation of precursors (chapter VI).

Blastocysts flushed out of both uterine horns of 10 gilts on Day 11 of pregnancy varied in diameter and aromatase activity. The variation in aromatase activity reflected a difference between and within gilts. Of the total variation between all blastocysts, 33% was due to differences within gilts. A positive exponential relationship between blastocyst diameter and aromatase activity was determined, but this relationship differed between gilts.

Day-11 spherical blastocysts and clewed Day-13 filamentous blastocysts released both free and conjugated oestrone and oestradiol-17βduring 2 successive periods of 24 h in vitro culture. Individually collected and cultured Day-13 filamentous blastocysts developed trophospheres, and differences existed in the release of oestrone and oestradiol-17βbetween and within gilts. Individual Day-13 filamentous blastocysts released on average 2.6 ng oestrone and 9.8 ng oestradiol-17βper 24 h. In all cultures of spherical and filamentous blastocysts, the oestradiol-17βrelease was significantly larger than the oestrone release. The data indicate that Day-11 and Day-13 blastocysts are able to synthesize oestrogens from endogenous precursors.

Assuming that CL function in cyclic gilts can be maintained by i.m. injection of large doses Of oestrogens, several mechanisms have been proposed to explain the anti-luteolytic action of oestrogens in the pig. Oestrogens released by blastocysts, however, are supposed to exert their anti-luteolytic effect at the level of the uterine endometrial epithelium. Large doses of intramuscularly injected oestrogens, however, may act anti- luteolytica1ly via a direct or indirect (pituitary) effect on the CL.

To investigate the influence of oestrogens at the level of the uterine endometrial epithelium, the concentration of endometrial oestradiol receptors at the expected time of maternal recognition of pregnancy in cyclic and pregnant gilts was determined (chapter VII).

Cytosolic oestradiol receptor concentrations did not change between Days 10-13 in cyclic gilts, but it decreased in pregnant gilts. The cytosolic oestradiol receptor concentration on Day 12 of pregnancy was higher in gilts with spherical blastocysts than in gilts with filamentous blastocysts.

To investigate the effect of a locally administered physiological dose of oestrogens, oestradiol-17βwas administered intrauterine in non-pregnant gilts and gilts pregnant up to Day 10 (chapter VIII).

Injection of 380 ng oestradiol-17βevery 6 h from Day 11 to Day 15 into both uterine horns resulted in normal cycles in non-pregnant gilts and gilts Pregnant up to Day 10. Implantation of three oestradiol-17βreleasing micropellets into each uterine horn of non-pregnant gilts on Day 10 of the cycle resulted also in normal oestrous cycles. The results indicate that administration of oestradiol-17βin physiological doses into the uterus lumen, the presumed site of action in the maternal recognition of pregnancy, does not result in CL maintenance in non-pregnant gilts.

From this study it may be concluded that a first signal for CL maintenance is generated in gilts in which at least some of the blastocysts were ≥8 mm or filamentous, but a second signal is necessary to maintain CL function throughout pregnancy. Already before the time of generating a first signal for CL maintenance, the blastocysts start to synthesize both oestradiol-17βand oestrone. The oestrogens exert an effect on the uterine endometrium, which is indicated by a change in endometrial oestradiol receptor concentration. However, since intrauterine oestradiol-17βadministration does not mimic the effect of the blastocysts, the first signal for maternal recognition of pregnancy is not a simple, merely oestrogenmediated mechanism.