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Encyclopedia of reproduction

About: The article was published on 1998-01-01 and is currently open access. It has received 702 citations till now.
Citations
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Journal ArticleDOI
TL;DR: Evaluated clinical assays for total and free testosterone showed that laboratory proficiency testing should be based on the ability to measure accurately and precisely samples containing known concentrations of testosterone, not only on agreement with others using the same method.
Abstract: Objective: The objective of the study was to evaluate the current state of clinical assays for total and free testosterone. Participants: The five participants were appointed by the Council of The Endocrine Society and charged with attaining the objective using published data and expert opinion. Evidence: Data were gleaned from published sources via online databases (principally PubMed, Ovid MEDLINE, Google Scholar), the College of American Pathologists, and the clinical and laboratory experiences of the participants. Consensus Process: The statement was an effort of the committee and was reviewed in detail by each member. The Council of The Endocrine Society reviewed a late draft and made specific recommendations. Conclusions: Laboratory proficiency testing should be based on the ability to measure accurately and precisely samples containing known concentrations of testosterone, not only on agreement with others using the same method. When such standardization is in place, normative values for total and ...

1,105 citations

Journal ArticleDOI
TL;DR: In this article, the effects of progestin, androgens, and estrogens on global testicular gene expression patterns (microarray analysis), and molecular mechanisms by which steroids regulate specific candidate genes (identified by subtractive hybridization approaches) during early stages of testis maturation are discussed.

977 citations

Journal ArticleDOI
TL;DR: Variations in photoperiodic response are seen not only between species but also between breeding populations within a species and between individuals within single breeding populations, and among species differences with respect to the importance and specific functions of various melatonin target sites.
Abstract: Photoperiodism is a process whereby organisms are able to use both absolute measures of day length and the direction of day length change as a basis for regulating seasonal changes in physiology an...

728 citations

Journal ArticleDOI
21 Apr 2006-Cell
TL;DR: It is suggested that high circulating FSH causes hypogonadal bone loss and that Osteoclasts and their precursors possess G(i2alpha)-coupled FSHRs that activate MEK/Erk, NF-kappaB, and Akt to result in enhanced osteoclast formation and function.

650 citations

Journal ArticleDOI
TL;DR: Adolescence in rodents may represent a suitable animal model with enough face- and construct-validity, Actually, this model is able to show behavioral features that resemble those found in human adolescents, including vulnerability to the consumption of psychoactive drugs.

563 citations

References
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Book ChapterDOI
TL;DR: The seasonal cycle highlights the seasonal changes in daylight length, rutting behavior, testicular diameter, sexual skin flush, and concentrations of plasma follicle-stimulating hormone, prolactin, and testosterone in a group of rams throughout the year.
Abstract: Publisher Summary This chapter focuses on the seasonal control of fertility. It discusses two aspects in detail—the way in which changes in the secretion of luteinizing hormone-releasing hormone by the hypothalamus controls the activity of the pituitary and testis, and the way changes in day length influence the activity of the hypothalamus. The downstream events evoked by the hypothalamus are relatively easy to understand, whereas the mechanisms involved in the photoperiodic control are very complex and largely unresolved. While seasonal changes in temperature, rainfall, and food availability are the factors of the environment that dictate survival of adults and young—and are thus ultimately responsible for dictating the timing of the birth season—these are not necessarily the factors used as cues by the animals to regulate their reproductive endocrinology. This is because it is necessary to anticipate the timing of birth by dictating the timing of conception, as the duration of gestation in mammals is usually fixed. As accurate timing of conception is important, animals tend to become reliant on cues from the environment, which are the best predictors of the time of year. The seasonal cycle highlights the seasonal changes in daylight length, rutting behavior, testicular diameter, sexual skin flush, and concentrations of plasma follicle-stimulating hormone, prolactin, and testosterone in a group of rams throughout the year.

595 citations

Journal ArticleDOI
TL;DR: Ewes of many breeds can be stimulated to ovulate by the reintroduction of rams and appear to be effected both through a delay in the onset of the LH surge and through a direct action of progesterone on the ovary.

322 citations

Journal ArticleDOI
TL;DR: LH is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre‐pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle.
Abstract: Summary (1) Luteinizing hormone (LH) is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre-pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle. Secretion is probably also pulsatile during the preovulatory surge of LH. (2) The secretion of LH is affected by the ovarian steroids, oestradiol and progesterone, both of which act principally to reduce the frequency of the pulses. During the luteal phase the two steroids act synergistically to exert this effect, and during anoestrus oestradiol acts independently of progesterone. Androstenedione secreted by the ovary apparently has no role in the control of LH secretion. (3) The amplitude of the pulses may also be affected by the steroids but there are conflicting reports on these effects, some showing that amplitude is lowered by the presence of oestrogen and others showing increases in amplitude in the presence of oestrogen and progesterone. (4) The secretion of LH pulses is affected by photoperiod, social environment and nutrition. Under the influence of decreasing day-length, oestradiol alone cannot reduce the frequency of pulses and the ewe experiences oestrous cycles. When day-length is increasing, the hypothalamus becomes more responsive to oestradiol which reduces the frequency of the pulses. (5) A hypothetical pheromone secreted by rams can increase the frequency of the LH pulses in anoestrous ewes and thereby induce ovulation, possibly by inhibiting the negative feedback exerted by oestradiol. (6) The relationships between nutrition and reproduction are poorly understood, but it seems likely that the effects of nutrition are mediated partly through the hypothalamus and its control of the secretion of LH pulses. (7) The pulses of LH secreted by the anterior pituitary gland are evoked by pulses of GnRH secreted by the hypothalamus. The location of the centre controlling the GnRH pulses and the neurotransmitter involved are not known.

231 citations

01 Jan 1995
TL;DR: In this article, it was shown that nutritional signals exert powerful effects on the reproductive system of mature male ruminants, and the responses are partly independent of changes in gonadotrophin secretion.
Abstract: Summary Changes in the nutrition of mature rams and goat bucks lead to profound responses in testicular size and therefore the rate of production of spermatozoa. These effects are largely due to changes in the size of the seminiferous tubules and in the efficiency of spermatogenesis. With the exception of severe undernutrition, the effects on spermatogenic function are not accompanied by similar changes in the endocrine function of the testis, as measured by the production of testosterone or inhibin. In rams, moderate changes in nutrition affect gonadotrophin secretion for only a few weeks, whereas testicular growth is affected for several months. In mature male goats during the non-breeding season, nutrition-induced testicular growth does not seem to be associated with any gonadotrophin response. Such observations have led us to develop the hypothesis that nutrition-driven testicular growth is at least partly independent of changes in gonadotrophin secretion. The energetic components of the diet, rather than the protein content, seem to be responsible for affecting gonadotrophin secretion in the ram. The volatile fatty acids, and not glucose, are the active factors, although intracerebral insulin may also play a role. Where these substrates act and whether they are also involved in the gonadotrophin-independent pathways, requires testing. In conclusion, nutritional signals exert powerful effects on the reproductive system of mature male ruminants, and the responses are partly independent of changes in gonadotrophin secretion. At gonadal level, the gametogenic tissue responds rapidly to changes in plane of nutrition, but the endocrine compartments of the testis are less affected. Variations in the expression of the nutritional responses between sexes, breeds and species probably reflect variations in the role of this environmental factor as a modulator of reproductive function.

93 citations

Journal Article
TL;DR: In this article, it was shown that nutritional signals exert powerful effects on the reproductive system of mature male ruminants, and the responses are partly independent of changes in gonadotrophin secretion.
Abstract: Changes in the nutrition of mature rams and goat bucks lead to profound responses in testicular size and therefore the rate of production of spermatozoa. These effects are largely due to changes in the size of the seminiferous tubules and in the efficiency of spermatogenesis. With the exception of severe undernutrition, the effects on spermatogenic function are not accompanied by similar changes in endocrine function of the testes, as measured by the production of testosterone or inhibin. In rams, moderate changes in nutrition affect gonadotrophin secretion for only a few weeks, whereas testicular growth is affected for several months. In mature male goats during the non-breeding season, nutrition-induced testicular growth does not seem to be associated with a gonadotrophin response. Such observations have led us to develop the hypothesis that nutrition-driven testicular growth is at least partly independent of changes in gonadotrophin secretion. The energetic components of the diet, rather than the protein content, seem to be responsible for affecting gonadotrophin secretion in rams. The volatile fatty acids, and not glucose, are the active factors, although intracerebral insulin may also play a role. Where these substrates act and whether they are also involved in the gonadotrophin-independent pathways requires testing. In conclusion, nutritional signals exert powerful effects on the reproductive system of mature male ruminants, and the responses are partly independent of changes in gonadotrophin secretion. In the gonads, the gametogenic tissue responds rapidly to changes in nutrition, but the endocrine compartments are less affected. Variations in the expression of the nutritional responses among sexes, breeds and species probably reflect variations in the role of this environmental factor as a modulator of reproductive function.

77 citations