Reproduction, nutrition, developpement最新文献

This paper considers how and why natural selection might promote or block the photoperiodic regulation of a mammal's reproduction. The factors most important in making this decision would seem to be the following: life expectancy, length of the female's cycle, feeding strategy, the presence or absence of survival mechanisms like hibernation, and the nature of the seasonal challenges offered by the mammal's habitat. A speculative scheme is offered for the potential utility of this type of regulation dependent upon life expectancy and latitude of residence.

Abnormal embryo development represents the major cause of implantation failures and accounts for the low rate of human fertility in vivo or in vitro. Chromosome abnormalities are widely involved in this process as 26% of oocytes, 8% of fertilizing spermatozoa and 29% of preimplantation embryos carry a chromosome aberration induced by meiotic (aneuploidy) or mitotic (mosaic) non disjunctions. Fertilization anomalies (possibly increased by in vitro procedures) were recorded: 1.6% of embryos resulted from parthenogenesis and 6.4% were polyploid (mainly polyspermic). A morphological, histological and ultrastructural study of embryos recovered after in vivo or in vitro fertilization showed some anomalies: multinucleated blastomeres, cytoplasmic fragments in the perivitelline space, vacuoles, associated or not with developmental impairement. Finally, a few embryos appeared to be free of abnormalities. The analysis of in vitro developmental capacities of normal or abnormal embryos showed great differences: parthenones exceptionally reached the blastocyst stage and therefore probably did not implant. The diploid embryos used in this study were (for ethical reasons) more or less fragmented and gave evidence of low developmental capacities, limited to the 3rd cleavage. Triploid embryos were able to further develop as some of them reached the early blastocyst stage; they represented the major cause of chromosomal 1st trimester abortions. It is interesting to note that 47% of tripronucleated ova divided directly into 3 and 6 cells (probably via a tripolar spindle) instead of 2 and 4 cells as classically described. Finally, tetraploid embryos expressed a precocious lethality as none developed beyond the 3rd cleavage. To conclude, many embryos carry genetic and/or cytological abnormalities which may be enhanced by superovulation treatments. The selection proceeds through all pre- and postimplantation steps, and as a matter of fact nor more than 0.6% newborns are abnormal.

The first part of this paper presents data concerning our knowledge of uterine proteins during early pregnancy in domestic mammals; the second part gives results of in vitro biochemical studies on embryo-uterine interactions in the ewe. We have developed an in vitro technique of the co-culture of ovine uterine epithelial cells with the blastocyst or its secretory proteins. The effects of a specific trophoblastic protein (oTPB), involved in the maintenance of the corpus luteum, have been particularly studied by this system. The modifications of endometrial protein synthesis have been measured by incorporation of radiolabelled amino acids and analysed by electrophoresis (SDS-PAGE or bidimensional). The results show that the presence of the blastocyst, or of its total secretory proteins or oTPB alone, decreased overall protein synthesis by the endometrial cells. The nature of the secreted proteins was apparently not affected by the blastocyst, but the addition of oTPB alone increased the production of 3 polypeptides (MW = 150.10(3); 74.10(3); 50.10(3) and pI = 7-8.2; 5.4-5.2; 6.4, respectively) and decreased the synthesis of 2 others (MW = 57.10(3); 35.10(3) and pI = 7-6.7; 5.3, respectively). We also studied the effects of co-culture with uterine cells on blastocyst DNA and protein synthesis. In no cases we obtained stimulation of blastocyst development during the co-culture period, and DNA or protein synthesis decreased in the presence of uterine cells. In conclusion, the presence of specific uterine proteins has been established in some domestic mammals (pig, rabbit) but not in all of them. Although local modifications of uterine protein synthesis are induced by the embryo or its secretory products, the nature and the role of the proteins which are affected need to be determined by further studies.

Many studies conducted on human or animal placenta (chorion) suggest that the trophoblast is not only a passive filter between maternal and foetal blood flow, but is also endowed with complex functions. Factors of trophoblast origin involved in the mechanism of pregnancy recognition or maintenance of the progesterone environment required for the embryo survival, are reviewed. The main proteins involved in pregnancy are reported in table 1. Emphasis is laid on early signals of pregnancy which are of practical interest in human clinical medicine and animal husbandry. Among them, human chorionic gonadotropin (hCG), protein SP1 ("Schwangerschaftsprotein" 1) and some pregnancy-associated plasma proteins such as the PAPP A are very useful in the diagnosis of pregnancy, abortion, foetal abnormality or tumor in the human species. The presence of trophoblastin (presently studied in our laboratory) of a pregnancy-specific protein B and of early pregnancy factor (EPF) attest the establishment of pregnancy in domestic animals and in other mammals. The biological properties of some hormones such as placental lactogens (PL) or chorionic somatomammotropins (CS), human placental growth hormone (hPGH) contribute to a better understanding of the gestation function. Many other factors participate in the foetal development, for example, proliferin. Some proteins can display immunosuppressive properties or be responsible for the immune tolerance between the mother and the foetus. Although many placental proteins have already been defined, their biological functions have not yet been elucidated.

In the past few years, one realizes that number of genes whose products are involved in regulating normal cell growth and development are also capable of inducing malignancy. These genes are called oncogenes and include 1 degree the cellular oncogenes (c-onc) or proto-oncogenes and 2 degrees the viral oncogenes (v-onc) which are most likely derived from c-onc and which are the transforming genes of several strains of animal retrovirus. The c-onc are present in the genome of all vertebrate cells and show a remarkable degree of evolutionary conservation, suggesting that they serve essential cellular functions. The analysis of biological properties of both types of oncogenes is of importance to understand the role of proto-oncogenes in normal cell proliferation and/or differentiation and development and to determine how alterations in their structures play a role in malignant transformation. In order to give an overview of the diversity of oncogene activities, several oncogene products involved in growth control, signal transduction, gene expression and development will be described.

The ruminal ecosystem illustrates the main concepts of system analysis. Due to its structure and functional complexity and to its determinant role in ruminant nutrition and feeding, the reticulo-rumen has been the target of modelling works. The proposed models have been static or dynamic. The later have known a recent development but the former have until now largely contributed to the knowledge progress. The complexity of dynamic models is proportional to the number of biological mechanisms taken into account. The simplest models describe, for instance, a degradation or a transit elementary process. The aggregated models take simultaneously into account 2 to 4 processes while the general models aim at describing the digestive behaviour of the ruminal ecosystem by integrating its main functions. These mechanistic models have been studied for about 20 years. They are on the whole satisfactory but they are not sufficiently able to describe some diet differences and some variations in the dynamic properties of feed particles.

The production of luteal oxytocin in ewes, resulting from the intrajugular injection of 200 micrograms of PGF2 alpha, could be determined by the increase in intramammary pressure. This simple indirect method of measuring the activity of the corpus luteum enabled easy detection of renewed post-partum ovulation or the onset of pregnancy. The response was monitored every two days between Days 0 and 25, then every 4 days between Days 25 and 59 in: 9 cyclic ewes (group B); 9 cyclic ewes treated with three daily intramuscular injections of 25 mg of Trilostane, a steroid synthesis inhibitor, between Days 7 and 25 (group A); 11 pregnant ewes (group C). Progesterone levels were determined each day from blood sampled in the jugular vein. Trilostane produced a decrease in plasma progesterone, not a total suppression (fig. 3), but did not significantly modify the intramammary pressure variations resulting from PGF2 alpha injections. These were identical in both cyclic and pregnant ewes during the first 15 days: they increased from D0 to D7 and decreased between D12 and D15 (fig. 4). After D15, the increase in intramammary pressure progressively weakened and became 0 at D17 in the cyclic ewes, whereas in the pregnant animals there was a renewed increase in intramammary pressure until D20; this regressed progressively afterwards and disappeared towards D45. This transitory, renewed activity between D15 and D20 might be an indirect or direct result of the message delivered by the embryo to maintain the corpus luteum. Several hypotheses are discussed with a view to explaining this phenomenon.

In utero, glucose utilization by fetal muscles (heart and hindlimb) displays important interspecies differences. In the fed state, it is 5-fold higher in rat than in rabbit fetal muscles. Maternal fasting induces a decrease in glucose utilization in fetal muscles of the rat but not of the rabbit.

Haemochorial placentation, as it occurs in the human and other primate and rodent species, requires a connection of the placenta with supplying maternal (uteroplacental) arteries. Very little is known of the initial stages but endovascular trophoblast invasion seems to represent an essential element for further elaboration of an adequate uteroplacental circulation. In the human, endovascular trophoblast arrives in myometrial segments of spiral arteries only from about 15 weeks of pregnancy. This is preceded by an interstitial type of cytotrophoblast invasion which seems to be associated with regressive changes in spiral artery walls. It is possible that the latter forms an essential priming factor to allow subsequent endovascular migration. Endovascular trophoblast invasion has been documented in different laboratory animals, including the rat and the golden hamster. Especially in the latter case a sequence of changes in the maternal component, i.e. the maternal cellular elements in the spiral artery walls, precede the arrival of trophoblast. Besides, there is also some evidence of haemodynamical factor(s) influencing trophoblast migration. Because of the importance of this phenomenon in the establishment of an adequate uteroplacental circulation, it is essential to develop further experimental models for studying pathological situations as exist in human pregnancy.