Effects of endocrine disrupters on the oocytes and embryos of farm animals.

Currently, approximately 60 chemicals have been identified as endocrine disruptors (EDs): exogenous agents that interfere with the synthesis, secretion, transport, metabolism, binding, action, or elimination of natural blood-borne hormones. Farm animals ingest these substances with food and drinking water. Their stability and lipid solubility has led to increased concern that these substances may compromise the reproductive health of both humans and animals. Oocytes are a permanent cell population established before birth which is exposed to environmental stimuli for a period that, in farm animals, can be as long as several years. Oocyte competence is acquired within the ovary during the developmental stages that precede ovulation and its role is critical during the interval between fertilization and the so-called maternal to embryonic transition, when the transcriptional activity of the embryonic genome becomes fully functional. Any perturbation of these delicate process is likely to reduce oocyte developmental competence and, therefore, to cause an arrest of embryonic development at any given stage. A critical analysis of the doses and time of exposure is presented together with a description of the effects of different EDs on farm animal oocytes and early embryonic development. Finally some of the mechanisms mediating EDs effects on the oocytes will be described. In particular the role of arylhydrocarbon receptor, maternal mRNA stability and cytoplasmic remodelling during oocyte maturation will be discussed in some details.