ABSTRACT
In different species a wide variety of structures are formed: from pig with simple apposition of uterine epithelium and trophoblast; to deep penetration into, together with loss of, the surface layers and endothelium of the endometrium with syncytial transformation of the trophoblast in guinea pig and human. The process is under multifactorial control with growth factors, cytokines and hormones all directly involved (Ben Rafael and Orvieto, 1992; Strickland and Richards, 1992; Wathes, 1992; Wooding and Flint, 1994).
