Comparative aspects of relaxin and relaxin-like factor (Insulin 3) in the placental trophoblast

Abstract

Developing as a complex temporary system of membranes surrounding the fetus of viviparous vertebrates, the placenta consists of fetal membranes apposing or fusing with the maternal mucosa. The dominating placenta of most mammals is the chorio-allantoic placenta. Both members of the relaxin family, relaxin itself and the structurally closely related relaxin-like factor (RLF) also known as INSL3 [1, 2], are expressed at the fetomaternal interface of various species including the human [3–5]. Relaxin is regarded a marker of pregnancy in the horse [6], the dog [7], and the cat [8] and has been implicated to play a role in embryo implantation [9–11], placentation [12, 13] and parturition [14, 15]. Given the detection of specific binding sites for relaxin in the uterus/ placenta [14, 16–19] and RLF in the uterus [20, 21], both hormones [22] have both been implicated in paracrine functions within the placenta. Previous work on the placental expression of relaxin mainly in rodents and the human indicated that (a) there appears to be no clear pattern of pregnancy-related physiological factors that might predict the primary source of relaxin in species and (b) relaxin production is not restricted to the trophoblast but can also be expressed by maternal cells (camel [23]; pig [24]; rat [25]; guinea pig [26]; hamster [27]).