Abstract
The placenta is an active endocrine organ synthesizing a wide range of hormones, including growth hormone (GH) and insulin-like growth factors (IGFs). The GH-IGF system is of particular importance for the regulation of fetal growth (Bauer et al., 1998). The placenta may also be regarded as an organ in which the maternal and fetal circulations are brought into close juxtaposition for purposes of exchange. During the process of placentation, fetal tissues (e.g. chorionic villi in humans) erode the maternal vessels, seeking closer contact with the maternal blood stream. In early pregnancy, the trophoblasts dominate the placental phenotype and display both highly invasive and proliferative properties. These ‘pseudomalignant’ properties of the trophoblasts are reminiscent of some features of cancerous transformation where normal growth controls have been circumvented. The placenta serves as a selective barrier restricting the passage of protein hormones enabling mother and fetus a high degree of autonomy in regulating their respective endocrine environments. But it is also an active endocrine organ synthesizing a variety of hormones (placental lactogen or chorionic somatomammotropin, chorionic gonadotropin, variant growth hormone, steroid hormones such as oestrogens and progestins) as well as growth factors, such as insulin-like growth factors (Liu et al., 1985; Shen et al., 1986), fibroblast growth factor (Cattini et al., 1991) or transforming growth factor(Miettinen, 1993). Placental lactogen, progesterone and placental growth hormone are known to increase production of IGFs and subtly regulate the concentration of IGF-binding proteins. These factors are thus capable of regulating the metabolic and growth adaptation in a delicate manner to secure an adequate supply of substrates to the developing fetus (Owens, 1991). This review illustrates the achievements made since research on placental growth hormone and insulin-like
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