Showing papers in "Scientific American in 1976"
TL;DR: Cell aggregation occurs during centrifugation washes and can lead to tremendous losses of cells due to mega-clumps, which can be minimized by using round bottom tubes and Polypropylene is the best plastic to use with cells.
Abstract: 1. Cell Preparation: The goal is to prepare a sample containing as many single cells as possible. Doublets or higher order clumps will either be detected or thrown out by the cytometer, or will confuse the instrument and result in sort mistakes (resulting in reduced purity). To avoid this, cells should be filtered through a 30-100 μm nylon mesh, depending on cell size, at each step to continually remove cell aggregates. Cell aggregation occurs during centrifugation washes and can lead to tremendous losses of cells due to mega-clumps. This can be minimized by using use round bottom tubes. Do not use conical or tapered bottom tubes. Use a relatively large diameter tube e.g. 17mm. Polypropylene is the best plastic to use with cells – do not use polystyrene. Centrifuge cells only as fast as necessary to pellet. Do not leave the cells in the pellet for a significant time it is best to be at the centrifuge when it stops spinning and immediately remove tubes, decant supernatant (never suck off fluid using a pipette) and resuspend cells (hint: break pellet up before adding any additional fluid/media).
670 citations
TL;DR: The great uniformity of composition and the nature of alkyl substituents suggest that most of the PAHs in the soil and in the sea-bed have been derived over eons from the fall-out of soot from forest and prairie fires, while the aromatic hydrocarbons of crude oil have been formed over millions of years from organic matter in sediments at 100 to 150 C.
Abstract: A great variety of polycyclic aromatic hydrocarbons (PAHs), including both known and suspected carcinogens, are widespread throughout soils, sea-bottom sediments and petroleum deposits. They are formed whenever organic substances are exposed to high temperatures. The great uniformity of composition and the nature of alkyl substituents suggest that most of the PAHs in the soil and in the sea-bed have been derived over eons from the fall-out of soot from forest and prairie fires. Conversely, the aromatic hydrocarbons of crude oil have been formed over millions of years from organic matter in sediments at 100 to 150 C. This has resulted in a much richer composition of alkylated PAH species than in soil or sea-bed. Oil spills therefore result in a changed local spectrum of PAHs, which may be detected for at least 6 yr after such an event. There is controversy about whether living organisms are able to synthesize PAHs. The ubiquity of PAHs in nature makes a direct experimental approach to this problem very difficult, but there is evidence that certain less volatile categories of PAHs are not present in plants grown in an environment free of such PAH species. This suggests that living organisms do not make PAHs.more » Natural fires and reactions in sediments produced PAHs long before the advent of man. They may be ranked with other natural mutagens such as uv radiation and natural radioactivity.« less
432 citations
432 citations
TL;DR: In this article, hot spots, regions of unusual volcanic activity, are studied and their physical properties are discussed, particularly their role as indicators of the passage of plates or continental drift.
Abstract: Hot spots, regions of unusual volcanic activity, are studied Their physical properties are discussed, particularly their role as indicators of the passage of plates or continental drift Included in the treatment are plate tectonics, hot spots and plumes, island chains, the population of hot spots, the opening of the Atlantic, motion at subduction zones, doming and rifting, and aulacogens (JFP)
177 citations
TL;DR: To ask how can an insect navigate by polarized light, three separate questions are raised: What makes the visual cell of an insect sensitive to polarized light?
Abstract: T he eyes of insects are sensitive to a natural phenomenon that man is blind to: the polarized light of the daytime sky. It is this capacity that under lies the remarkable navigational ability of many insect species. Exactly how can an insect navigate by polarized light? To ask this question is really to raise three separate questions: What makes the visual cell of an insect sensitive to polarized light? How do some minimum number of visual cells coop erate to determine the direction of polariza tion at one point in the sky? How much information from how many points in the sky does the insect need for unambiguous navigation? The light radiated by the sun is unpolar ized, that is, its waves vibrate in all direc tions at right angles to the line of sight. In by RUdiger Wehner
159 citations
155 citations
TL;DR: The evidence supporting the establishment of the zones of a natural fission reactor that occurred about 2 billion years ago is presented in this paper, which was found in an open-pit uranium mine in the southeastern part of the Gabon Republic, near the Equator on the coast of West Africa.
Abstract: The evidence supporting the establishment of the zones of a natural fission reactor that occurred about 2 billion years ago is presented. The reactor evidence was found in an open-pit uranium mine in the southeastern part of the Gabon Republic, near the Equator on the coast of West Africa. The history of the probable sequence of events leading to the reaction is recounted. Reasons for the ceasing of the fission reaction are set forth. The spectrum of fission products in the area is thought to be so distinctive that it serves as an unmistakable sign that a chain reaction has taken place. Geological features of the area are related to conditions before and after the fission took place. (BLM)
TL;DR: The interactions between steroid hormones particularly sex hormones and nerve tissue are discussed and nerve cell circuits which may be established by steroid hormone stimulation during sexual differentiation of the brain become permanent and are not susceptible to further hormonal influence.
Abstract: The interactions between steroid hormones particularly sex hormones and nerve tissue are discussed. Steroid hormones act on the genetic material of the cell nucleus. Estradiol is responsible for the reversible activation of the lordosis response and ovulation in adult female rats. When certain areas of the developing brain are exposed to hormones such as estradiol permanent structural changes in the brain lead to a male pattern of behavior in the adult. if the developing rat male brain is deprived of testosterone female behavior results. These differences are probably due to the state of differentiation of the neurons at the time of hormonal interaction. The nerve cell circuits which may be established by steroid hormone stimulation during sexual differentiation of the brain become permanent and are not susceptible to further hormonal influence. Rather the hormones affect the functional efficacy of the circuits. Further research is needed to understand the chemical properties of the target neurons necessary for sexual behavior response.
TL;DR: Success of the 10 year long world eradication campaign depended on production of heat-stable vaccines and a reusable pronged needle that used little material, and the concept of an infected village was introduced, and house to house searches were instituted.
Abstract: The key events in the eradication of smallpox worldwide are related. Smallpox virus was spread by droplets only from the appearance of the rash until scabs form 4 weeks later. It only infected humans making it a potential disease for eradication. It had been endemic in populous areas largely China and India in ancient times appearing in Europe in the 6th century and in America in 1520. Smallpox vaccination was known as variolation before the modern practice of vaccination with cowpox (Vaccinia) was demonstrated in 1796. Success of the 10 year long world eradication campaign depended on production of heat-stable vaccines and a reusable pronged needle that used little material. The U.S.S.R. suggested the campaign in 1959 but the current campaign began in 1976. The 1st strategy was intensive vaccination with moderate success. Subsequent strategies involved surveillance and containment along with improved reporting methods. The concept of an infected village was introduced and house to house searches were instituted. Victims were put under guard and all villagers were vaccinated. The last case of virulent smallpox occurred in Bangladesh in October 1975 and of mild smallpox in Ethiopia in August 1976. The cost of the entire 10-year global eradication was $83 million for foreign assistance and about $160 million spent by the individual countries. This is small compared to an estimated $2 billion yearly spent to control smallpox. It is ironic that smallpox became an epidemic pestilence upon the growth of populations yet it played a major role in preventing population growth until variolation and vaccination became common.
TL;DR: In this article, the inverse of Malthus' proposition is considered: whether the effective utilization of resources for food production can be made to increase the limits set by human population size.
Abstract: The physical resources of earth fire and water although they are very large are ultimately fixed. However the biological and social resources are far from being pressed to the limit. According to Thomas Malthus the human population will always increase until it reaches the limit set by the food supply. This limit is determined by the physical resources available for agriculture. He recognized that farm production increases with improved technology; however the rates of increase would always be lower than the potential capacity of human beings to multiply. This paper considers the inverse of Malthuss proposition--whether the effective utilization of resources for food production can be made to increase the limits set by human population size. A more important inquiry is whether rates of growth of agricultural production can be made to exceed rates of population growth. It is noted that the amount of cultivated land per person could be increased in every part of the world by the year 2000. However large capital investments would be needed for a major increase in crop yield in favorable regions. One way to accomplish this is to increase scientific knowledge of plant and animal biology and of the environment and to transform scientific advances into practical knowledge farmers can use. In addition many resources other than arable land and water must be utilized to increase world food production.