Conceptus-Maternal Communication In Horses

Early conceptus loss causes a significant financial burden to horse breeders. In the January 1998 issue of the Lloyd's Equine Disease Quarterly, maternal recognition of pregnancy and the relationship of progesterone, prostaglandin F2a(PGF) and oxytocin were discussed. This article will examine some aspects of the communication that occur between the conceptus and the maternal environment during the first few critical weeks of gestation.

One of the current researcher's approaches in studying conceptus-maternal interactions is to look at changes in protein production. They have examined proteins synthesized and released by equine conceptuses and from uterine endometrium and oviducts of cycling and pregnant mares. Major proteins synthesized and secreted by the uterine endometrium include retinol binding protein and oxytocin.

Retinol binding protein carries metabolites of vitamin A to the developing embryo and placenta. Studies in laboratory animals have shown that these metabolites are responsible for proper formation of the developing embryo. Either too much or too little of the vitamin A metabolites are lethal to the embryo. Uterine oxytocin may be involved in regulating PGF production and maternal recognition of pregnancy, as discussed previously.

Major protein products of the equine conceptus include transferrin and alpha-fetoprotein. Transferrin services to carry iron to the developing embryo and placenta. Iron is important in a variety of functions, including formation of hemoglobin, and is necessary during early development to establish the circulatory system for the embryo and the placenta. Transferrin also functions in cell growth, immunoregulation and organ differentiation.

Alpha-fetoprotein serves a variety of functions, including acting as a transport protein for estrogens and prostaglandins. Alpha-fetoprotein is the main protein of the fetal circulation; it serves many of the same functions that serum albumin serves in the adult animal.

The signals between the conceptus and the maternal reproductive system are controlled primarily by changes in, and regulation of, gene expression. Conceptus-maternal communication can be successful only if the appropriate genes are expressed in the appropriate tissues at the appropriate times during gestation. For example, “turning off” expression of certain genes will allow the conceptus or uterus to start or stop making particular products.

What are some of the genes which the horse conceptus expresses around the time of maternal recognition of pregnancy? In the past 2 years, we have identified 50 gene sequences which are expressed during this time (Simpson, Adams, Behrendt, Baker & McDowell; Identification of genes expressed in day 12 and 15 horse conceptuses by suppression subtractive hybridization. Biol Reprod 1997, Suppl 1:88). In addition to alpha-fetoprotein and transferrin, these latest studies identified conceptus production of phospholipase A2 (PLA2), calcyclin, and equine pregnancy associated glycoprotein (ePAG).

PLA2 is involved in PGF synthesis. We know that the conceptus produces a variety of prostaglandins, including PGF, but at this time we do not know how conceptus prostaglandins interact with uterine prostaglandins. Calcyclin is a protein that binds calcium. Studies in laboratory animals have indicated that it may help regulate secretion of other that the conceptuses produe. It may also be important for the differentiation of the nervous system in the developing embryo.

The function of ePAG is unclear at this time, but it may be involved in mediating maternal immune responses during pregnancy. In addition, studies in ruminants have shown that the PAG's of sheep and cattle conceptuses play a role in maternal recognition of pregnancy in those species.

Understanding more about gene regulation is essential in deciphering how the conceptus and mother communicate with each other. Appropriate conceptus-maternal communication is critical for a successful outcome to pregnancy of all species.

—Equine Disease Quarterly, Funded By Underwriters at Lloyd's , London, Brokers And Their Kentucky Agents

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