SEARCH · PubMed Health

Results for “Basic Reproduction Number”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24Linked to original sources

PubMed1984

[Role of nonenterotoxigenic Escherichia coli strains (K99-,ST-) in the neonatal pathology of the calf].

Enterotoxigenic strains of Escherichia coli have been extensively studied over the last ten years, especially in the calf. On the other hand, septicemic strains have received much less attention during the same period, in spite of a lack of information about their pathogenicity in the calf and about the attributes of their virulence. Virulence of septicemic E. coli includes invasion from the portal entry into the blood, multiplication in blood and organs, resistance to phagocytosis and lethal action of serum, and production of lesions by toxins. Most information obtained recently concerns the last phases of microbial pathogenicity, i.e. multiplication and production of lesions. Antigens K are considered to be mainly responsible for the resistance to phagocytosis. Antigens O would play the major role, qualitatively and quantitatively, in the resistance to bactericidal effect of serum. The ability to grow in iron-deficient medium would constitute a decisive advantage. This ability is attributed to proteins of the outer membrane, some of them being coded by plasmids. Endotoxin is the basic toxic principle that accounts for the production of lesions in the host. It is generally admitted that its toxic effect is due to the lipid A portion of LPS. The biochemical composition of lipid A being very homogeneous in Enterobacteriaceae, its pathological effect is considered to be directly related to the concentration in serum, i.e. without specific endotoxic activity exerted by some serotypes. Numerous clinical and experimental arguments suggest the occurrence of an enterotoxemic form of colibacillosis in calves. Enterotoxemic colibacillosis would result from the strictly intestinal multiplication of some strains of E. coli and the systemic diffusion of corresponding endotoxins. Such a syndrome has been well-documented in piglets, namely oedema disease. Oedema disease is attributed to a limited number of serotypes. Experimental reproduction has led to the evidence that two toxic principles are needed to fully produce the disease: the endotoxin and a neurotoxin, called EDP. These results, together with observations on humans and other animal species, raise two major questions: 1) what are the circumstances that favour the passage of endotoxins through intestinal epithelium ? 2) could a specific endotoxic activity be attributed to some serotypes of E. coli?

Animals↗

PubMed2004

Association between birth weight and serum lipid concentration in premenopausal Japanese women.

BACKGROUND: The relationships between birth weight and serum lipid concentrations in pre-menopausal Japanese women were not well identified and also diet and serum hormone status in these women would be considered. METHODS: A total of 59 premenopausal Japanese women completed a self-administered questionnaire including basic demographic information, disease histories, and menstrual and reproductive histories. They were asked to obtain information on birth weight recorded in mother-and-baby notebook issued by municipality from their mother. Diet was assessed by daily diet records from day 2 through day 10 of the menstrual cycle. Blood sample was collected on day 11 of the cycle to measure serum lipid and hormone concentrations (total and high-density lipoprotein [HDL] cholesterols, triglyceride, estrone, estradiol, and sex hormone-binding globulin). RESULTS: Birth weight was significantly correlated with HDL cholesterol (r = 0.32, p = 0.03), but not with total cholesterol and triglyceride after controlling for age. Neither estrogen nor sex hormone-binding globulin was significantly correlated with serum lipid concentrations after controlling for age and the number of days prior to the next menses. The correlation between birth weight and HDL cholesterol was not affected after additional adjustment for serum estrogen and intakes of protein, calcium, and iron. CONCLUSION: These data suggest that intrauterine growth may be associated with lipid profile.

Adult↗

PubMed2005

Integrating a bovine rectal palpation simulator into an undergraduate veterinary curriculum.

There are problems associated with teaching bovine rectal palpation to undergraduate veterinary students. The students need opportunities to examine enough cows to develop the required skills, but increasing student numbers and limitations on access to cows have made this more and more difficult to achieve. A virtual reality-based teaching tool, the Bovine Rectal Palpation Simulator, has been developed as a supplement to existing training methods. The student palpates computer generated virtual models of the bovine reproductive tract while interacting with a haptic (touch feedback) device. During training sessions, the instructor follows the student's actions inside the virtual cow on the computer screen and gives instruction. A trial integration of the simulator into the fourth-year bovine reproduction course was undertaken at the University of Glasgow Veterinary School during the 2003/2004 academic year. Students were offered two training sessions, and feedback was gathered using questionnaires. In the first session, all students were taught a range of basic skills using a standardized teaching protocol. The second training session was customized to each student's learning needs and included practice in dealing with a range of on-farm scenarios. Student feedback indicated that the training had been useful for learning various aspects of bovine rectal palpation and provided information that helped in the further development of the simulator as a teaching tool.

Animals↗

PubMedDate not supplied

Controlled ovarian hyperstimulation for the new reproductive technologies.

The aim of controlled ovarian hyperstimulation (COH) is to induce multiple morphologically and functionally adequate follicles with the aim of harvesting multiple fertilizable oocytes. We compared several treatment regimens with different FSH/LH ratios: group I was the basic 2 human menopausal gonadotropins (2hMG) protocol in which 2 ampules of hMG were administered starting on day 3 of the cycle; group II was the basic 2hMG in which 2 ampules of "pure" follicle stimulating hormone (p-FSH) were added on day 3 and 4; group III was the 2 FSH protocol in which 2 ampules of p-FSH were administered beginning on day 3; finally, group IV was the 4FSH: 4 ampules of p-FSH on day 3 and 4 followed by 2 ampules of p-FSH daily. The reference regimen was the 2hMG. Except for the higher rate of immature oocytes in 2FSH and 2hMG protocols, the number of preovulatory oocytes and fertilization rate were similar in all protocols. No differences occurred in the pregnancy outcome. The low dose Lupron (LDL) stimulation was an experimental protocol applied to two groups of women who had previously failed COH. Eight women who had a premature luteinization and 8 women who showed a low response agreed to participate in the protocol. Ten micrograms of the GnRH agonist leuprolide acetate (Lupron) were injected subcutaneously every 6 hours starting on cycle day 2 and menotropin stimulation was begun on day 3-5, according to individual patient response. The LDL protocol was successful in determining a favourable estradiol pattern and fertilizable oocytes.

Drug Administration Schedule↗

PubMed1984

Posttesticular surface modifications and contributions of reproductive tract fluids to the surface polypeptide composition of boar spermatozoa.

Caput and cauda epididymal fluids were found to be exceedingly rich in the numbers and kinds of polypeptides when analyzed by two-dimensional (2-D) gel electrophoresis. Only a few of the major (Coomassie-stained) and minor (silver-stained) epididymal fluid polypeptides were identified on epididymal sperm plasma membranes (PM) and even fewer identified in ejaculated sperm. The 2-D electrophoretic patterns of caput sperm PM differed little from those of cauda sperm PM. Thus, epididymal transit resulted in relatively minor quantitative and qualitative modifications in sperm PM composition. Seminal plasma showed a few major polypeptides from the cauda epididymal fluid, but the major constituents were those polypeptides from the seminal vesicle secretions. Sperm appear to acquire one acidic high molecular weight polypeptide from either the bulbourethral gland or prostate gland, and another major acidic polypeptide of high molecular weight from the seminal vesicle gland. Numerous neutral and basic low molecular weight polypeptides, originating from the seminal vesicles, adhered tightly to sperm. These were major polypeptides and constituted a substantial percentage of the total PM protein. Thus, major contributions to the sperm PM polypeptide profile occurred at ejaculation. This study did not address loosely bound polypeptides but is the first to analyze, in a comprehensive way, the origins of tightly bound sperm polypeptides from a single species.

Animals↗

PubMed1990

Active cross-bridge model. Reproduction of the myocardial force-length-time relation and the left ventricular pressure-length-time relation in systole.

A mathematical model has been derived to describe cardiac contraction. This model assumes (1) a linear end-systolic force-length relation for the intact left ventricular (LV) myocardium with proportionality constant Ec, and (2) that the rate of increase in the number of active cross-bridges is proportional to the number of cross-bridges capable of being activated at that moment with a proportionality constant Ka. Quantitative evaluation of this model was performed using parameter values measured in the normal human LV. The model predictions agreed well with many earlier experimental data concerning the myocardial force-time relation, myocardial force-velocity-length relation, systolic time interval-afterload relation, time-varying elastance concept, several LV end-systolic relations, and LV dP/dtmax-end-diastolic volume relation. Moreover, two important basic properties of contractile proteins, i.e., the maximal force-generating capacity and the rate of activation of cross-bridges, were evaluated in the model from the values of Ec and Ka, respectively. Although no definitive biological proof has yet been provided for the assumptions, this model might mathematically integrate the myocardial and ventricular dynamics over the entire systole and then provides a new method for evaluation of the human LV systolic function from two basic properties of active cross-bridges.

Blood Flow Velocity↗

PubMed2005

Contraception: an international perspective.

Although it is estimated that the population growth rate will decline to a replacement level by 2050, it is also now predicted that the total world population will reach 8.9 billion in that year -- far higher than the 2004 estimate of 6.4 billion. More than 26 billion new couples will need contraceptives in the next half century. Although a steady increase in contraceptive use has been observed in both developed and developing countries, the contraceptive needs of a high percentage of couples have not yet been met and the number of unplanned pregnancies continues to increase. The actual use of contraception differs from region to region. Although no new method has been registered for many years, several new products have been marketed during the last 5. Among these are new implants, medicated intrauterine systems, contraceptive vaginal rings, transdermal patches and several new combined oral contraceptive formulations. New contraceptive methods have been developed to meet the objectives of expanding contraceptive choices for both women and men and answering an unmet need for contraceptives with a long-term action that meet the expectations of consumers. Simplicity, reversibility and effectiveness are the desired features of a male contraceptive, but no new male contraceptive method is yet available. New areas of basic research include studies on genes, proteins and enzymes involved in the reproductive system. The new methods will be targeted to specific interactions within the reproductive system at the level of ovaries and testes, as well as between spermatozoa and ova. This futuristic approach still keeps in mind the need for better access to existing contraceptive methods, as well as the discovery of new contraceptives that are simple to use, safe, reversible and inexpensive. In the future, contraceptives may be combined with other medicinal agents to provide dual protection against both pregnancy and other preventable conditions, such as sexually transmitted infections.

Contraception↗

PubMed2004

Crossover invariant subsets of the search space for evolutionary algorithms.

This paper addresses the relationship between schemata and crossover operators. In Appendix A a general mathematical framework is developed which reveals an interesting correspondence between the families of reproduction transformations and the corresponding collections of invariant subsets of the search space. On the basis of this mathematical apparatus it is proved that the family of masked crossovers is, for all practical purposes, the largest family of transformations whose corresponding collection of invariant subsets is the family of Antonisse's schemata. In the process, a number of other interesting facts are shown. It is proved that the full dynastic span of a given subset of the search space under either one of the traditional families of crossover transformations (one-point crossovers or masked crossovers) is obtained after [log2n] iterations where n is the dimension of the search space. The generalized notion of invariance introduced in the current paper unifies Radcliffe's notions of "respect" and "gene transmission". Besides providing basic tools for the theoretical analysis carried out in the current paper, the general facts established in Appendix A provide a way to extend Radcliffe's notion of "genetic representation function" to compare various evolutionary computation techniques via their representation.

Algorithms↗

PubMed2000

Soluble factors guide gonadotropin-releasing hormone axonal targeting to the median eminence.

Axons of GnRH neurons terminate at the median eminence in the medial basal hypothalamus (MBH) of the brain early in development. Similarly, GnRH neurons in grafts of preoptic area (POA) tissue within the third ventricle of hypogonadal mice preferentially innervate the median eminence. Organotypic cocultures of POA explants with other neural tissues suggest that a soluble substance(s) derived from the MBH may be directing this targeting. To begin to identify diffusable chemoattractants, we used preincubated heparin-coated acrylic beads to present specific solutes to POA explants on collagen- and laminin-coated membranes in insert chambers. GnRH axons grew on the membrane in greater number and with longer axons toward conditioned medium from MBH cultures than on the side away from the beads (P < 0.01). In contrast, GnRH axons showed no preferential outgrowth when incubated with beads soaked in control, defined medium. The attraction of MBH-conditioned medium was not generalizable to all neuroendocrine neurons, as it was not seen for galanin immunoreactive outgrowth from POA explants. There also were more GnRH axons toward conditioned medium from mouse brain microvascular endothelial cells, but no difference in axon length. Basic fibroblast growth factor (bFGF), a component of both endothelial cells and ventricular tanycytes, significantly attracted more and longer GnRH axons. Thus, bFGF may be one of the soluble factors directing GnRH outgrowth to the median eminence. However, as with so many other redundancies in the reproductive system, it is unlikely that it is the only targeting factor, as bFGF knockout mice are reported to be reproductively competent.

Animals↗

PubMed1986

Synchronization of estrus in dairy heifers: a field demonstration.

Use of prostaglandin F2 alpha to synchronize estrus was demonstrated over 3 yr with 2966 dairy heifers in 45 herds. Reproductive status of all heifers was determined before any injections occurred. Anovulatory heifers (8.3%) were not injected or inseminated. Ovulatory heifers received one intramuscular injection and most heifers received two injections of prostaglandin F2 alpha spaced 11 d apart. Observations for estrus and insemination were scheduled and performed by dairy farmers. Insemination of heifers occurred 12 h after detection of estrus or at 80 h after second injection of prostaglandin F2 alpha. Pregnancy rate averaged 50.9% but varied from 0 to 100%. Pregnancy rate after insemination according to estrus (62.8%) was greater than when insemination was at 80 h (39.1%). Low pregnancy rates were associated with limitations in basic management of heifers before or during insemination. Managerial limitations were in timing insemination, range of ages or weights of heifers housed together, number and types of vaccinations, season and diet, frequency of observing heifers for estrus, and number of straws of semen thawed concurrently. Managerial factors that affect fertility are similar whether or not estrus is synchronized. Although agents used to synchronize estrus do not affect fertility directly, managerial limitations on fertility may be accentuated by increased intensity of activity associated with synchronization.

Animals↗

PubMed1996

Focus on women in Cambodia.

Dr. Mam Bunheng, Undersecretary of State for Health, Cambodia, provided an overview of the health situation of women and children in Cambodia during a visit to JOICFP on December 8. He also expressed his country's desire to seek collaboration with JOICFP in the future. Providing background on the maternal and child health (MCH) system in Cambodia, Mam explained that the birth-spacing program is an integral part of the MCH activities and is primarily aimed at ensuring the health of women. Cambodia, which has a population of about 10 million people, presently has a high maternal mortality rate of 500/100,000 deliveries. Noting that the country also has a high total fertility rate of about 5.1, Mam emphasized that the government's birth spacing program is not aimed at controlling the population, but rather to ensure balanced development and population growth with the ultimate goal of guaranteeing the health of women. Cambodia also has a high infant mortality rate of 113/1000 live births. Mam explained that lack of access to services, supplies, and trained personnel hamper efforts to promote birth spacing. To overcome these obstacles, Cambodia needs support to develop human resources and to ensure supplies of basic medical equipment and essential drugs. Specifically, Mam stressed the need to train and retrain health staff to ensure adequate staffing at the community level. At present, the total number is insufficient, he said. While in Japan, Mam visited many health facilities as part of JICA's technical cooperation program and was impressed most with the quality services and facilities of the public health center.

Asia↗

PubMedDate not supplied

Concentration of glutathione and expression of glutathione peroxidases 1 and 4 in fresh sperm provide a forecast of the outcome of cryopreservation of human spermatozoa.

Oxidative stress imbalance potentially leads to damage of the structure of the cell and macromolecules such as plasma membrane components, proteins, and DNA. The plasma membrane of the sperm cell, which has high levels of polyunsaturated fatty acids, renders it particularly sensitive to free radical-mediated attacks. The freezing and subsequent thawing of sperm is a physically stressful process carried out during routine procedures in assisted reproduction techniques, which results in a highly variable and unpredictable reduction in the number of motile sperm cells. Subsequently, oxidative status can positively or negatively affect the motility, viability, and fertilizing capacity of thawed sperm. These effects are counteracted by various oxidative defense enzymes and anti-oxidants such as glutathione peroxidase isoforms GPx1 and GPx4, glutathione reductase (GR), and cellular glutathione (reduced) (GSH). In this way, oxidative status could represent a predictive marker of sperm quality following the freeze-thaw process. This study was based on 56 human sperm samples. We observed direct positive and negative relationships between the postthaw motile sperm recovery rate and GPx1 and GPx4 expression and activity, on the one hand, and GSH concentrations, on the other. No correlation was found between this recovery rate and GR or basic semen parameters. Predictive values clearly demonstrate that, among the molecules analyzed, the most accurate diagnoses result when analyses are conducted for GPx1 and GPx1 messenger RNA expression, GPx1 and GPx4 enzymatic activity, and GSH concentration. In conclusion, a reserve of glutathione, together with GPx expression, is necessary to eliminate free radicals using GSH or a like structural protein and seems to be essential for a good postthaw recovery. These molecules can be employed as indicators of postthaw sperm quality.

Cryopreservation↗

PubMed1993

Optimizing management of natural and artificial matings in swine.

A successful mating involves the coordination of many physiological events including insemination, gamete transport and ovulation. As more basic information about these processes becomes available, it is apparent that there are opportunities for manipulating these physiological events and enhancing the reproductive success of natural and artificial matings. The primary intent of this review is to examine the effectiveness of several of these strategies, as determined by their influence on farrowing rate and litter size. It is evident that the timing, duration and pattern of ovulation in pigs are extremely variable. Thus, increasing the frequency and changing the timing of matings during oestrus has received renewed interest. In general, if a female exhibits oestrus for more than 1 day, then increasing the frequency of matings has a greater influence on reproductive performance than does altering the timing of matings. In addition, litter size is more responsive to increased mating frequencies than is farrowing rate. Increasing the number of matings per oestrus via the use of combinations of natural service and artificial insemination, compared with the use of either alone, appears to enhance reproductive performance. It has been well documented that several aspects of the mating process itself can advance the onset of ovulation and enhance sperm transport and storage in the female reproductive tract. As a result, the use of these stimuli in conjunction with natural and artificial matings provide opportunities for enhancing fecundity. Pre- and post-breeding treatments with vasectomized boars, nonviable semen, oestrogens and seminal plasma have all been reported to increase farrowing rate and litter size. With artificial insemination, additions of prostaglandins, oxytocin, oestrogens and leucocytes to semen have been reported to enhance reproductive performance, albeit with varying degrees of success. Personnel and boar needs are important components in optimizing breeding management. Estimates of these needs for artificial and natural matings are discussed.

Animal Husbandry↗

PubMed1993

Testicular and epididymal development in the brown marsupial mouse, Antechinus stuartii (Dasyuridae, Marsupialia).

Reproductive tissues were collected monthly from male Antechinus stuartii during the first 5 months of post-partum development, a period corresponding to the time between birth and the initial increase in plasma androgen above non-detectable levels. The gonad appeared undifferentiated at day 3 after birth, but the basic structure of the testis (tunica albuginea, sex cords, stroma) was well established at 1 month of age. At this stage the developing sex cords contained a single layer of pre-Sertoli cells which surrounded a central core of gonocytes. Mitotic division of cells within the cords was common. Intertubular fetal Leydig cells, often observed in clumps, and perivascular and peritubular fetal Leydig cells were common and readily identified. By 2 months of age there was an obvious increase in cord diameter and the abundance of pre-Sertoli cells, while a marked reduction in the density of connective tissue cells and fetal Leydig cells was observed in the interstitium. Fetal Leydig cells appeared to persist only in close association with the developing seminiferous cords. Testicular size and the diameter and convolutions of the seminiferous cords increased substantially (two fold increase in cord diameter) by 3 months of age. Gonocytes had begun to migrate toward the basal lamina of the cords, and connective tissue cells and Leydig cells appeared in large numbers throughout the interstitium. By 4 and 5 months of age, gonocytes were commonly seen in contact with the basement membrane, and the cords remained non-patent. Leydig cell number and density increased greatly during these months. The epididymal epithelium remained undifferentiated throughout the first 5 months of development. Epithelial cells characteristically contained a large nucleus which occupied most of the cell, very little cytoplasm and few organelles. The diameter of the epididymal duct was similar throughout for the first 3 months of the study. In months 4 and 5 the diameter of the duct in caput and corpus regions increased, ahead of that of the cauda, possibly in relation to variations in androgen exposure at different regions along the developing duct. Further histological and quantitative studies on the growth and development of Leydig cells within the Dasyuridae are needed for comparison with eutherian mammals, which together with knowledge of the changing levels of fetal androgens may provide a greater understanding of the role of the different populations of Leydig cells in the differentiation of the testis and male reproductive tract.(ABSTRACT TRUNCATED AT 400 WORDS)

Age Factors↗

PubMed2002

Playing for half the deck: the molecular biology of meiosis.

Meiosis reduces the number of chromosomes carried by a diploid organism by half, partitioning precisely one haploid genome into each gamete. The basic events of meiosis reflect three meiosis-specific processes: first, pairing and synapsis of homologous chromosomes; second, high-frequency, precisely controlled, reciprocal crossover; third, the regulation of sister-chromatid cohesion (SCC), such that during anaphase I, SCC is released along the chromosome arms, but not at the centromeres. The failure of any of these processes can result in aneuploidy or a failure of meiotic segregation.

Animals↗

PubMed2003

Genetic mating system and the significance of harem associations in the bat Saccopteryx bilineata.

We analysed the polygynous mating system of the bat Saccopteryx bilineata using behaviour observations and genetic data on 11 microsatellite DNA loci. Basic social units in S. bilineata are harem groups that consist of single males and up to eight females. Colonies comprise several harem groups, and the composition of colonies and harems is often stable over several reproductive seasons. The combination of parentage exclusion and likelihood-based parentage assignment in this study produced detailed parentage information for a large colony of S. bilineata. Reproduction occurred mostly within the colony (17% extra-colony paternity), but social associations in harems within the colony did not represent reproductive units (70% extra-harem paternity). The latter finding was consistent over three reproductive seasons. Spatial association of the roosting sites of males and females could not explain parentage patterns in the colony. Even though intra-harem paternity was less frequent than expected, it contributed significantly to reproduction of harem males. On average, the number of offspring sired by a male with females in his harem territory increased significantly with harem size, which corresponds to the higher energetic investment that is related to the maintenance of large harems. However, extra-harem paternity was not correlated with a male's harem size or intra-harem reproductive success. This suggests that individual preferences of females rather than male traits associated with the ability to defend large harems are most likely to cause the detected differences between social association and genetic mating system.

Animal Population Groups↗

PubMed2004

[Karyotypic analysis from two clones of gynogenetic Pengze crucian carp (Carassius auratus of Pengze)].

Pengze crucian carp (Carassius auratus of pengze), one of the most popular cultural fishes in China, is an endemic bisexual population with natural gynogenetic reproduction mode. Just like Silver crucian carp, investigation on the karyotype of Pengze crucian carp is important to understanding the diversity and the evolution of the unisexual vertebrates. Two distinguishable gynogenetic clones of Pengze crucian carp, named clone H and clone L, respectively,were detected recently. In this study, the chromosomes number and karyotype of the two clones were analyzed using chromosomes of kidney cell-PHA culture in vivo prepared by flame-drying technique. The results show that the chromosome numbers and karyotypes of the two clones are different. Clone H has 156 chromosomes in its karyotype with 42 metacentric, 36 submetacentric, 39 subtelocentric, 33 telocentric, and six supernumerary chromosomes; the karyotype of clone L contains 162 chromosomes with 36 metacentric, 45 submetacentric, 33 subtelocentric, 36 telocentric, and twelve supernumerary chromosomes. The karyotype formula of 150 basic chromosomes of clone H is 3n = 42M +36SM +39ST +33T,NF = 228; and the karyotype formula of 150 basic chromosomes of clone L is 3n = 36M +45SM +33ST +36T, NF = 231. The discovery of the two different gynogenetic clones indicates that the genetic diversity also exists in the Pengze crucian carp, similar to that in the Silver crucian carp.

Animals↗

PubMed1999

What are natural retinoids?

Vitamin A (retinol) is the prototype of all other natural retinoids. It is composed of a nonaromatic ring structure, a polyprenoid side chain and a carbonyl end group. These features make it liable to metabolic interconversion and protein interactions but also cause detergent-like properties and a sensitivity to UV irradiation and oxidation. Natural retinoids are present in all living organisms, either as preformed vitamin A or as carotenoids, and are required for a vast number of biological processes, e.g. vision, cellular growth and differentiation and reproduction. Although retinol is the most omnipotent compound, natural retinoids like all-trans-, 9-cis- and didehydroretinoic acid (RA) are clearly more potent outside the retina and trigger gene expression via binding to nuclear retinoid receptors. Retinaldehyde takes an intermediate position in this respect, with ability to convert to both retinol and RA. Over the years, many natural retinoids have been tried therapeutically against skin disorders with the best effects achieved with retinol, retinaldehyde, 13-cis-RA and all-trans-RA. The latter compound was the prototype when new, synthetic derivatives of vitamin A were sought, hoping for a better therapeutic index and a higher functional specificity. Inevitably, treatment with such drugs will influence the effects of coexisting natural retinoids. An understanding of the basic principles of these interactions may have major impact on patient outcome.

Carotenoids↗