Early Canine Pregnancy – A Battle for Successful Growth and Angiogenesis

Contents Currently, no early pregnancy marker has been identified in the bitch. However, significantly decreased concentrations of heat‐shock protein ( HSP ) 70 and increased activity of matrix metalloproteinases ( MMP ) 2,9 were detected in serum from bitches during the pre‐implantation period between days 5 and 13 after mating, that is, 2–3 days after ovulation as determined by the measurement of progesterone and vaginal cytology. Especially during the implantation period and thereafter (days 15–55), high serum concentrations of antibodies against desmin are present, which is believed to indicate or regulate decidualization. Pre‐implantation embryos express m RNA for enzymes and cytokines, known to promote and regulate trophoblast growth, and some intrauterine changings like the increased activity of MMP 2,9 in maternal endometrium are dependant on the presence of embryos. Some mechanisms that protect canine embryos from attack by the maternal immune system can also be identified. The embryos express CD 4, a receptor known to interact with immune cells. They, furthermore, do not express MHC I and II, which might prevent them from being recognized as foreign antigen. Pre‐implantation embryos express F as L , which probably renders them able to destroy F as‐bearing cytotoxic T cells. Furthermore, the uterus during pre‐implantation and implantation expresses cytokines that modulate the intrauterine milieu towards a predominance of T h2 cells. During pre‐implantation and implantation, an increased uterine expression of platelet activating factor ( PAF ) and PAFR , vascular endothelial growth factor ( VEGF ) and EGFR 2 as well as epithelial growth factor ( EGF ) is characteristic. Towards placentation, the upregulation of leukaemia inhibiting factor ( LIF ) and at placentation the expression of insulin‐like growth factor( IGF )2 and granulocyte–macrophage colony‐stimulating factor ( GM ‐ CSF ) are striking. Progesterone receptor ( PR ) appears to be downregulated inside the uterus except at placentation sites, presumably where it is essential for maintenance of pregnancy. In addition, receptor‐bound P 4 regulates the activity of MMP 2,9. Apoptosis seems to be a further regulatory mechanism. Expression of F as and F as L m RNA in uterine tissue is maximum until implantation, both factors then decreased significantly. These changings might indicate increased endometrial apoptosis and defence against maternal cytotoxic T cells, probably promoting trophoblast invasion. In human decidual stromal cells, G n RH is involved in the regulation of apoptosis, which is proposed to be similar in pregnant bitches, as G n RH ‐ R is expressed at canine implantation sites. Our work investigating immunological changes in pregnant bitches has elucidated aspects of the complex physiology of implantation but raises important questions about the mechanisms involved.