Reproduction, nutrition, developpement最新文献

Comparisons are drawn between the photoperiodically driven breeding cycles in "long-day" birds and "short-day" mammals, emphasizing the importance of photorefractoriness as a key regulator in the timing processes. It is argued that the two types of breeding cycle may not be so radically different as previously thought and, indeed the cycles may be strictly homologous. Evidence in support of this comes from the role of the thyroid glands in seasonality. In starlings and quail, thyroidectomy prevents refractoriness developing and the birds remain in breeding indefinitely under long days. If the processes underlying refractoriness are similar across species then thyroidectomy should greatly alter the ewe's breeding cycle. In two experiments, Welsh Mountain ewes were thyroidectomized in the summer during anoestrus and their subsequent periods of oestrus monitored under various daylengths. There was no effect of thyroidectomy on the time when oestrous cyclicity began in the Autumn but the onset of anoestrus was profoundly disrupted. All the ewes continued to cycle well beyond the end of the normal breeding season and a number have continued throughout the entire period of anoestrus.

Photoperiod cues play an important role in the timing of puberty in the female lamb. Removal and replacement of photoperiod cues by denervation of the pineal gland and timed melatonin infusions, respectively, indicate that the pathway for transmission of photoperiod information develops well before puberty. This is reinforced by manipulation of artificial photoperiods during various periods of development. Such approaches reveal that even in the first few weeks of life, the pattern of melatonin secretion accords with daylength and modulates prolactin secretion. Several months later, after internal, growth-related cues indicate that sufficient body size has been achieved to initiate reproduction, photoperiod history is used as an important predictor of reproductive success, and thus, whether puberty should occur. In the female spring-born lamb, the decrease in daylength in autumn is the critical cue for the initiation of estrous cycles. Experimentally, this may be achieved by surgically disrupting the pathway for transmission of photic cues after appropriate long-day exposure. In the autumn-born lamb and in the slowly growing lamb, sexual maturation may be masked by the transition into seasonal anestrus the following spring. In these young females, a decreasing photoperiod or "removal of long days" (surgical) is not necessary for puberty the following autumn. Sufficient photoperiod history may be acquired in such lambs that they enter puberty as a consequence of becoming refractory to the long days of summer. We hypothesize that the phenomenon of refractoriness reflects the expression of an innate rhythm of reproductive activity and that changes in daylength experienced early in life serve to synchronize this rhythm with the seasonal environment.

Differences in nitrogen and RNA contents were found between liquid-associated and solid-adherent bacteria isolated from the reticulo-rumen of ruminants offered various diets. Consequences on the estimation of the bacterial nitrogen flow to the duodenum are discussed.

We and others have previously shown that in the rat and the sheep gonadectomy increases the translational capacity of mRNAs encoding gonadotropin subunits alpha, LH beta and FSH beta. Injection of estradiol or of testosterone or dihydrotestosterone depresses the translational capacity of the mRNAs. After using estradiol to induce progesterone receptors in the pituitary of castrated animals, it was determined that progesterone does enhance the inhibitory effect of estradiol. We have also observed this inhibitory effect of gonadal steroids in vitro, suggesting that at least part of the steroid action is exerted at the pituitary level. Hybridization assay (Northern blot) using oligodeoxynucleotide probes complementary to a short portion in the cDNA strand of each subunit, showed gonadal steroids to act by decreasing the number of copies of mRNAs encoding LH and FSH subunits. Using anterior pituitary cells in culture, incubated in the presence of labeled methionine, we have confirmed our previous observation that GnRH stimulates the biosynthesis of the polypeptide chains of LH. This effect is not secondary due to LH release. It is not inhibited when incubation is performed in the presence of tunicamycin, an inhibitor of glycosylation. SDS-polyacrylamide gel electrophoresis of specific immunoprecipitates of polypeptides immunologically related to alpha allowed us to identify 3 forms of alpha-polypeptide differing in their apparent Mr:21K (partially glycosylated), 23K (authentic) and 25K (hyperglycosylated). Besides its stimulatory effect on the release and synthesis of LH, GnRH also stimulated the release of the 23K and 25K forms of alpha. In the presence of tunicamycin an additional 16K form of apoprotein-alpha was detected which accumulated within the cells. A cAMP analogue (8-Br-cAMP), intracellular cAMP generators (choleragen, forskolin), as well as an analogue of diacylglycerols (TPA) mimic the stimulatory action of GnRH. However, although no evidence has been obtained at present that either cAMP or diacylglycerols mediate the GnRH effect on the biosynthesis of the polypeptide chains of LH, our data suggest that phosphorylation of intracellular phosphoproteins plays a major role in this process.

Human endometrium synthesizes a number of proteins. Two of the major proteins are the low molecular weight insulin-like growth factor-binding protein (IGF-bp25) and endometrial protein PP14 (placental protein 14). Both proteins have been cloned from human decidual cDNA library and their complete amino acid sequences have been deduced. IGF-bp25 is found in various tissues and body fluids including secretory and decidualized endometrium, amniotic fluid, liver, follicular fluid and luteinized granulosa cells of preovulatory ovarian follicles. Clinical studies have shown that there is no systematic variation in the circulating levels during normal menstrual cycle, whereas in hyperstimulated cycles the levels are higher when there are many preovulatory follicles and immediately after follicle aspiration for in vitro fertilization (IVF). Some women with polycystic ovarian disease have a subnormal level of IGF-bp25 in serum. The other protein, PP14, is synthesized by secretory and decidualized endometrium, and it is also abundant in amniotic fluid. PP14 is mainly released by secretory endometrial glands during the last week of ovulatory cycles. There is a consistent variation in serum PP14 levels during normal menstrual cycle. The levels are lowest at the time of ovulation and rise steeply during the last week of luteal phase and peak at the onset of menstruation. Administration of micronized oral progesterone to normally ovulating infertile women brings about elevation in their serum PP14 levels during late luteal phase. In postmenopausal women cyclical estrogen-progestogen replacement causes elevation of serum PP14 level, but this does not take place in hysterectomized postmenopausal women.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of a dietary alpha-linolenic acid (18:3 n-3) deficiency on lipid fatty acid composition of the liver and serum of lactating rats have been studied during three gestations and over three generations. These females were compared to corresponding females which remained sterile. Two lots of female rats received, respectively, a diet containing lipids either in the form of 1.50 g of sunflower oil per 100 g of diet (deficient diet) or as 1.87 g of soya oil per 100 g of diet (control diet). Both diet contained the same amount of linoleic acid (18:2 n-6), i.e. 940 mg/100 g of diet, but the sunflower diet supplied 43 times less 18:3 n-3 than the soja diet, or 3 mg vs 130 mg/100 g of diet. Results show that successive gestations appeared to be more efficient means of depleting material n-3 PUFA stores than successive generations. The 18:3 n-3 deficient diet caused a considerable decrease in the level of n-3 polyunsaturated fatty acids (n-3 PUFA) in liver and serum lipids, and particularly of 22:6 n-3. This decline was compensated by an increase in the level of n-6 polyunsaturated fatty acids (n-6 PUFA), and particularly by a very high augmentation of 22:5 n-6. The ratio n-6 PUFA/n-3 PUFA in liver phospholipids and in serum lipids was a good index of the adequacy of dietary n-3 PUFA supply. However, the ratio 22:5 n-6/22:6 n-3 was a finer index. This ratio appeared to be a reliable index of dietary n-3 PUFA deficiency when it was higher than 1 in serum lipids of a fasting animal. The proportion of 22:5 n-6 as well as the ratios n-6/n-3 and 22:5 n-6/22:6 n-3, were also increased in the liver phospholipids of lactating females receiving the soya oil diet; this suggested that a supply of 130 mg/100 g of diet, corresponding to a ratio of n-6/n-3 = 7.2, was not sufficient for these rats during pregnancy and lactation. A supply of 200 mg of n-3 PUFA/100 g of diet, corresponding to a ratio of n-6/n-3 = 5, is recommended for these animals.

The initial step in muscle formation is the fusion of undifferentiated myoblasts into multinucleated myotubes which synthesize the specific proteins of the muscle. During this transition a whole series of genes are turned on. A simple explanation for this activation process is that each gene contains a structure which is common to all genes of the family and which is recognized by (a) specific factor(s). Experimental evidence supporting this model is described. Appearance of a new protein may not involve gene activation but be the result of a new mode of splicing as in the case of tropomyosins. Examples have been chosen to illustrate the various strategies used to control gene expression and provide the necessary diversity required during the process of muscle maturation. They include controlled expression of linked genes, coexpression of genes from unrelated phenotypes or alternative splicing of unique genes.