Reproductive Physiology: The Mare

Introduction

Significant early embryonic death is a continuing source of economic loss in the equine industry.  An estimated 30% of all pregnancies are lost within the first two weeks.  These early pregnancy failures may result from a breakdown of critical communication or ‘signalling’ between the growing embryo and the lining of the mare’s uterus (Anderson, Walters and Roser, 2000).

Maternal recognition of pregnancy (MRP) is the process whereby luteolysis is prevented by the presence of the conceptus, thus prolonging progesterone secretion for the maintenance of pregnancy.  In both ruminants and non-ruminants the conceptus’s strategy for maintaining luteal function beyond the period of the oestrous cycle, and the hormone support for an appropriate uterine environment, is to produce a hormone signal before it attaches to the endometrium that reduces or blocks the secretion of prostaglandin F2α (PGF2α) from the uterus (Berglund et al., 1982, Sharp and McDowell, 1984, Zavy et al., 1984, cited in Sharp et al., 1989a).

In ruminants, the trophoblast of the developing conceptus blocks luteal regression by the production of interferons ensuring MRP (Jainudeen and Hafez, 2000).  In the mare, events that occur between approximately 11 and 15 days of gestation are of critical importance for the establishment and maintenance of successful pregnancy (Eberhardt, 1999). The exact mechanism by which the equine conceptus signals its presence to the mare to prevent the destruction of the primary corpus luteum (CL) is unknown (Pycock, 1999).

It is known that MRP in the horse corresponds temporally with a high incidence of early embryonic loss, suggesting that the process may occasionally fail (Sharp et al., 1989a).  The long-term goal of research into the ‘maternal recognition of pregnancy’ is therefore to identify causes of pregnancy loss in horses, and to develop methods of reducing the loss rates.  The direct benefits to the equine industry are to increase efficiency and decrease costs of horse breeding programs (Eberhardt, 1999).

The maternal recognition of pregnancy in the mare

The mare is a seasonal polyoestrous breeder; throughout the breeding season she has several oestrous cycles normally during late spring, summer and early autumn.  The oestrous cycle has two components; oestrus, 5-7 days during which the mare is receptive to the stallion, and dioestrus, 14-16 days during which the mare rejects the stallion.  The main endocrine changes that occur during the oestrous cycle are shown below.

Oestrous cycle of the mare

Approximately 14 days after ovulation, the mare must decide whether to maintain progesterone production or not.  In the absence of successful mating and fertilisation, the

uterus synthesises and releases luteolytic PGF2α to initiate regression of the CL and the cessation of progesterone production.  This process initiates a new cycle of sexual receptivity in preparation for mating and fertilisation in another attempt to establish pregnancy (McCracken, Schramm and Okulicz, 1984, cited in Allen, 2001; Geisert and Malayer, 2000).

If there is a viable conceptus in the uterus, it must signal its presence to the maternal system and block the regression of the CL in order to maintain luteal progesterone production.  This process is essential for the establishment of pregnancy in both ruminants and non-ruminants (Berglund et al., 1982, Sharp and McDowell, 1984, Zavy et al., 1984, cited in Sharp et al., 1989a; Sharp et al., 1989b, cited in Geisert and Malayer, 2000).