Reproduction performances and conditions of group-housed non ...

- Appendix 1 -
Soon after ovulation the granulosa cells lining the follicle wall begins to multiply to form a
corpus luteum (CL) (Anderson, 2000). If fertilisation does not occur, prostaglandin pro-
duced by the uterus is transferred to the ovary were it induces regression of CL. Corpus
Luteum is essential for maintenance of pregnancy because CL is the only source of proges-
terone in the sow and progesterone is essential during the whole period of pregnancy (van
der Lende, 1989). The main function of CL is the secretion of progesterone. Progesterone
stimulates uterine secretions (among other things IGF-1) and progesterone is the primary
director of uterine development and secretion (Geisert & Yelich, 1997). Furthermore pro-
gesterone is necessary to prevent contractions of the uterus, which would be caused by ele-
vated levels of oestrogen (Hughes et al., 1996). Both oestrogen (Stroband & Lende, 1990)
and progesterone are involved in preparing the uterine wall for the attachment of the em-
bryos (Hughes & Varley, 1980). There are indications that LH is essential for maintenance
of the CL beyond the first 12 days of pregnancy but that this requirement for LH decreases
after day 29 of pregnancy (Peltoniemi et al., 1995; Hughes et al., 1996). To avoid regres-
sion of CL at day 16 in oestrus cycle the embryo around day 11-12 secretes oestrogen
(Bazer et al., 1982; Geisert & Yelich, 1997). This is also called ‘the embryonic signal’
(Peltoniemi et al., 2000) or ‘the maternal recognition signal’ (Geisert et al., 1987). Accord-
ing to Bazer et al. (1982;1984) oestrogen produced by the embryo directs the prostaglandin
synthesised in the uterus into the uterine lumen which prevent their release to the uterine
vascular bed and therefore prevents the regression of CL. Oxytocin (Geisert & Yelich,
1997), which secretion from the endometrium is stimulated by oestrogen, and prolactin
(Gross et al., 1990 cf Geisert et al., 1994) may also play a role in this redirection of pros-
taglandin. Bazer et al. (1984) suggested that prostaglandin is only luteolytic on day 12 or
later after onset of oestrus. However, Estill et al. (1993) found that repeated treatment with
prostaglandin from day five to day ten also resulted in luteolysis. Geisert et al. (1994) hy-
pothesise that, before the first embryonic signal, progesterone controls the release of pros-
taglandin. It seems that a second prolonged embryonic oestrogen signal is required after
day 14 to maintain pregnancy beyond day 30 (Geisert et al., 1987).
At the time of parturition foetal cortisol stimulate the prostaglandin production in the
uterus, which causes regression of the CL followed by a drop in progesterone level (Hughes
et al., 1996; Anderson, 2000).
Discussion
The above-mentioned emphasize that a successful reproduction depends upon a series of
precisely timed physiological and endocrine events. These events may be disturbed by
various factors. There are thus indications that the endocrine regulation may be disturbed
by e.g. stress, as indicated by increased cortisol. In several studies, administration of corti-
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