**5. Embryo signals and pregnancy recognition**

Blockage of luteolysis during the recognition of gestation can be possible by inhibition of es‐ tradiol production because existence of estradiol is obligatory for luteolysis. Estradiol indu‐ ces PGF2α secretion. When compared with cyclic animals, follicular development and concentration of plasma estradiol are less in pregnant animals. How does estradiol affects PGF2α secretion in cellular and molecular levels is not known. However, estradiol has got a central role in luteolysis. For this reason; while antilteolytic strategies are developed, for the retardation or inhibition of luteolysis decrease of estradiol level is aimed [16].

Progesterone amount circulating in cows provides maternal recognition. This situation shows the importance of high level progesterone for the recognition of pregnancy in critical period. Another factor for the pregnancy recognition is bovine interferon-tau which is re‐ leased by the embryo. Bovine interferon-tau is also known as bovine trophoblast protein-1 (bTP-1). Bovine interferon-tau which is secreted to lumen of uterus inhibits the release of PGF2α from the endometrium in critical period. Stimulating of progesterone to bovine inter‐ feron-tau is another possible mechanism for the maternal recognition. In the cows, which have higher levels of progesterone in the critical period, more bovine interferon-tau is pro‐ duced by the embryo [16, 32].

Interferon-tau shows its affect by hindering estradiol receptors. Subsequently, oxytocin recep‐ tors diminish and cyclooxygenase inhibitors get activated. Interferon-tau insures the produc‐ tion of some endometrial proteins crucial for the life of embryo. The first of these proteins is bovine granulocyte protein-2. Second one is ubiquitin cross-reactive protein (UCRP). UCRP conjugates with cytosolic endometrial proteins in response to pregnancy and interferon-tau. Proteins conjugated with UCRP become a target for processing by proteasome. This affect of interferon-tau is mediated by the induction of signal transducer and activation of transcription 1 (STAT-1), STAT-2, and interferon regulatory factor 1. UCRP, alpha chemokines and induc‐ tion of these transcription factors procure pregnancy recognition by mother [33].
