Processing of fluorescently labelled insulin and insulin-like growth factor-I by the rat visceral yolk sac

Insulin is imprinted in the placenta of the marsupial, Macropus eugenii

Therian mammals (marsupials and eutherians) rely on a placenta for embryo survival. All mammals have a yolk sac, but while both chorio-allantoic and chorio-vitelline (yolk sac) placentation can occur, most marsupials only develop a yolk sac placenta. Insulin (INS) is unusual in that it is the only gene that is imprinted exclusively in the yolk sac placenta. Marsupials, therefore, provide a unique opportunity to examine the conservation of INS imprinting in mammalian yolk sac placentation. Marsupial INS was cloned and its imprint status in the yolk sac placenta of the tammar wallaby, Macropus eugenii, examined. In two informative individuals of the eight that showed imprinting, INS was paternally expressed. INS protein was restricted to the yolk sac endoderm, while insulin receptor, IR, protein was additionally expressed in the trophoblast. INS protein increased during late gestation up to 2 days before birth, but was low the day before and on the day of birth. The conservation of imprinted expression of insulin in the yolk sac placenta of divergent mammalian species suggests that it is of critical importance in the yolk sac placenta. The restriction of imprinting to the yolk sac suggests that imprinting of INS evolved in the chorio-vitelline placenta independently of other tissues in the therian ancestor of marsupials and eutherians.

Expression of insulin-like growth factor in the placenta of intrauterine growth-retarded human fetuses

Many cases of intrauterine growth retardation (IUGR) are the result of placental and fetal tissue insufficiency. Insulin-like growth factor-I (IGF-I) is known to play a role in placental and fetal growth. An immunocytochemical study was performed to localize IGF-I peptides in human placenta and umbilical cords of normal (n = 3) and IUGR (n = 3) fetuses. The peripartum fetal conditions were evaluated as well. Immunoreactive IGF-I was detected in the cytotrophoblast, syncytiotrophoblast, amnion, endothelial cells of fetal capillaries and in the decidua in both normal and IUGR placental tissue. A more robust immunostaining and increased numbers of positively stained cells were found in the decidua of IUGR placenta (p < 0.001). Intense immunostaining was also found in endothelial cells, smooth muscle cells and fibroblasts of the umbilical vein. IGF-I immunoreactivity was also present in stroma (Hofbauer cells and/or fibroblasts) of IUGR villi. Our results indicate that expression of IGF-I is high in specific sites in placenta and umbilical cords, which indicates a paracrine and/or endocrine function. The increased expression of IGF-I in placenta of IUGR fetuses indicates its involvement in restoring normal growth by means of a positive feed-back mechanism.

The growth promoting effects of bFGF, PD-ECGF and VEGF on cultured postimplantation rat embryos deprived of serum fractions

Serum components in which embryos are cultured in vitro are very important for normal embryonic development. In this study, rat serum was fractionated using Macrosep filters to study the effect of a single growth factor. The fractionated serum, both that containing only material greater than 30 kDa molecular weight (> 30 kDa) and that from which material between 30 kDa and 50 kDa had been removed (< 30 kDa+ > 50 kDa), caused significant embryonic growth retardation. Addition of different concentrations of basic fibroblast growth factor (bFGF, 18 kDa), vascular endothelial growth factor (VEGF, 45 kDa) and platelet-derived endothelial growth factor (PD-ECGF, 45 kDa), to fractionated serum (bFGF to > 30 kDa serum and VEGF or PD-ECGF to < 30 kDa+ > 50 kDa serum) partially restored embryonic growth and development according to a morphological scoring system and protein assay. This restoration was clear by all criteria, as well as in yolk sac vascularisation and heart development. The growth promoting effects of all 3 factors were significant but did not reach the level seen in embryos grown in whole rat serum. The effect of these growth factors was also investigated on anembryonic yolk sac development using a concentration for which maximum whole embryonic growth was seen (128 ng/ml bFGF, 1.6 ng/ml VEGF and 4 ng/ml PD-ECGF), and significant anembryonic yolk sac development was found. These findings suggest that the angiogenic factors may have a growth promoting effect on total embryonic development and vascularisation.

Growth-promoting effects of different fractions of extra-embryonic coelomic fluid on embryonic development

In the early stages of embryonic development, many growth-promoting molecules must be provided by the maternal system. These factors may be supplied locally to the embryo, by the decidua, the placenta, or the yolk sac. In this study the growth-promoting potential of extra-embryonic coelomic fluid (EECF) and its fractions was investigated. The embryonic requirement of growth-promoting molecules may be studied by reducing the growth-supporting capacity of serum. Thus, ultrafiltration of rat serum was carried out for 8 h using Millipore filters with a molecular weight exclusion of 30 kDa. Rat embryos at 9.5 days of age were cultured for 8 days for anembryonic yolk sacs, and then EECF was collected and divided into three different molecular weight fractions by ultrafiltration. Rat embryos were cultured for 48 h in whole rat serum and the serum retenate (which has low growth-supporting capacity) in the presence and absence of EECF, its fractions, or in EECF only. Embryos grown in retenate showed severe growth retardation, and the addition of EECF significantly improved embryonic growth. The fraction which contained the molecules with molecular weight between 10 and 30 kDa had significantly more effect on embryonic development than the other fractions. This fraction of EECF was analysed by gel electrophoresis. Three of the four protein bands observed in this fraction were identified by amino-terminal sequencing as alpha-fetoprotein precursor (22 kDa), apolipoprotein A1 precursor (24 kDa) and fetal haemoglobin Y2 chain (14 kDa), none of which are likely to be responsible for the growth-promoting activity. To further investigate growth-promoting proteins, EECF was Western-blotted to nitrocellulose membranes and probed with antisera against rat prolactin, epidermal growth factor, insulin-like growth factors I and II and human placental lactogen. No immunoreactive bands were detected in the EECF, suggesting that either these proteins are not present or are present at levels too low to be detected. Although the growth-promoting effect of the EECF was demonstrated in this study, the molecules responsible remain uncharacterized.

The role of exogenous growth-promoting factors and their receptors in embryogenesis

During organogenesis, the cells of the embryo may require growth factors that promote a cascade of intracellular events. An absolute requirement for exogenous insulin by presomite 9.5-d rat embryos grown in culture has been demonstrated. The uptake and processing of insulin and insulin-like growth factor-I showed different uptake and localization patterns. When epidermal growth factor (EGF) or "long EGF" is added to media depleted of low molecular weight material, a dose-dependent improvement in growth is observed. Furthermore, the specific EGF receptor signal transduction inhibitor Tyrphostin 47 can inhibit embryonic growth when it is administered in culture. When Tyrphostin 47 was microinjected into embryos on Day 11 and their growth and differentiation evaluated on Day 12 of gestation, a dose-dependent decrease in developmental score was observed. Thus, exogenous growth factors may be essential to normal rat development and these may be synthesized locally in the decidua or placental tissues. Perturbations to ligand-receptor interactions may be a mechanism for dysmorphogenesis.