Cell culture is the process by which cells are grown under controlled conditions outside their natural environment. After the cells of interest have been isolated from living tissue, they can subsequently be maintained under controlled conditions; these conditions vary for each cell type, but consist of a suitable vessel with a substrate or medium that supplies the essential nutrients, growth factors and gases, regulates the physio-chemical environment. Most cells require a surface or an artificial substrate whereas others can be grown free floating in culture medium; the lifespan of most cells is genetically determined, but some cell culturing cells have been “transformed” into immortal cells which will reproduce indefinitely if the optimal conditions are provided. In practice, the term "cell culture" now refers to the culturing of cells derived from multicellular eukaryotes animal cells, in contrast with other types of culture that grow cells, such as plant tissue culture, fungal culture, microbiological culture.

The historical development and methods of cell culture are interrelated to those of tissue culture and organ culture. Viral culture is related, with cells as hosts for the viruses; the laboratory technique of maintaining live cell lines separated from their original tissue source became more robust in the middle 20th century. The 19th-century English physiologist Sydney Ringer developed salt solutions containing the chlorides of sodium, potassium and magnesium suitable for maintaining the beating of an isolated animal heart outside the body. In 1885, Wilhelm Roux removed a portion of the medullary plate of an embryonic chicken and maintained it in a warm saline solution for several days, establishing the principle of tissue culture. Ross Granville Harrison, working at Johns Hopkins Medical School and at Yale University, published results of his experiments from 1907 to 1910, establishing the methodology of tissue culture. Cell culture techniques were advanced in the 1940s and 1950s to support research in virology.

Growing viruses in cell cultures allowed preparation of purified viruses for the manufacture of vaccines. The injectable polio vaccine developed by Jonas Salk was one of the first products mass-produced using cell culture techniques; this vaccine was made possible by the cell culture research of John Franklin Enders, Thomas Huckle Weller, Frederick Chapman Robbins, who were awarded a Nobel Prize for their discovery of a method of growing the virus in monkey kidney cell cultures. Cells can be isolated from tissues for ex vivo culture in several ways. Cells can be purified from blood. Cells can be isolated from solid tissues by digesting the extracellular matrix using enzymes such as collagenase, trypsin, or pronase, before agitating the tissue to release the cells into suspension. Alternatively, pieces of tissue can be placed in growth media, the cells that grow out are available for culture; this method is known as explant culture. Cells that are cultured directly from a subject are known as primary cells.

With the exception of some derived from tumors, most primary cell cultures have limited lifespan. An established or immortalized cell line has acquired the ability to proliferate indefinitely either through random mutation or deliberate modification, such as artificial expression of the telomerase gene. Numerous cell lines are well established as representative of particular cell types. For the majority of isolated primary cells, they undergo the process of senescence and stop dividing after a certain number of population doublings while retaining their viability. Cells are maintained at an appropriate temperature and gas mixture in a cell incubator. Culture conditions vary for each cell type, variation of conditions for a particular cell type can result in different phenotypes. Aside from temperature and gas mixture, the most varied factor in culture systems is the cell growth medium. Recipes for growth media can vary in pH, glucose concentration, growth factors, the presence of other nutrients.

The growth factors used to supplement media are derived from the serum of animal blood, such as fetal bovine serum, bovine calf serum, equine serum, porcine serum. One complication of these blood-derived ingredients is the potential for contamination of the culture with viruses or prions in medical biotechnology applications. Current practice is to minimize or eliminate the use of these ingredients wherever possible and use human platelet lysate; this eliminates the worry of cross-species contamination. HPL has emerged as a safe and reliable alternative as a direct replacement for FBS or other animal serum. In addition, chemically defined media can be used to eliminate any serum trace, but this cannot always be accomplished with different cell types. Alternative strategies involve sourcing the animal blood from countries with minimum BSE/TSE risk, such as The United States and New Zealand, using purified nutrient concentrates derived from serum in place of whole animal serum for cell culture.

Plating density plays a critical role for some cell types. For example, a lower plating density makes granulosa cells exhibit estrogen production, while a higher plating density

The Ford GT is a mid-engine two-seater sports car manufactured and marketed by American automobile manufacturer Ford for the 2005 model year in conjunction with the company's 2003 centenary. The second generation Ford GT became available for the 2017 model year; the GT recalls Ford's significant GT40, a consecutive four-time winner of the 24 Hours of Le Mans, including a 1-2-3 finish in 1966. The Ford GT began life as a concept car designed in anticipation of the automaker's centennial year and as part of its drive to showcase and revive its "heritage" names such as Mustang and Thunderbird. At the 2002 North American International Auto Show, Ford unveiled a new GT40 Concept car. Camilo Pardo, the head of Ford's "Living Legends" studio, is credited as the chief designer of the GT and worked under the guidance of J Mays. Carroll Shelby, the original designer of the Shelby GT 500, was brought in by Ford to help develop the GT. While under development, the project was called Petunia; the GT is similar in outward appearance to the original GT40, but is bigger and most 4 in taller than the original's 40 in overall height.

Although the cars are visually related, there is no similarity between the modern GT and the 1960s GT40 that inspired it. After six weeks from the unveiling of the GT40 concept, Ford announced a limited production run of the car. Three pre-production cars were shown to the public in 2003 as part of Ford's centenary celebrations, delivery of the production version called the Ford GT began in the fall of 2004; as the Ford GT was built as part of the company's 100th anniversary celebration, the left headlight cluster was designed to read "100". A British company, Safir Engineering, who built continuation GT40 cars in the 1980s, owned the "GT40" trademark at that time; when production of the continuation cars ended, they sold the excess parts, tooling and trademark to a small Ohio based company called Safir GT40 Spares. This company licensed the use of the "GT40" trademark to Ford for the initial 2002 show car; when Ford decided to put the GT40 concept to production stage, negotiations between the two firms failed as Ford didn't have the resources to pay US$40 million demanded by the owners of the name, thus the production cars are called the GT.

The GT was produced for the 2006 model years. The car began assembly at Mayflower Vehicle Systems in Norwalk and was painted and continued assembly at Saleen Special Vehicles facility in Troy, through contract by Ford; the GT is powered by an engine built at Ford's Romeo Engine Plant in Michigan. Installation of the engine and transmission along with seats and interior finishing was handled in the SVT building at Ford's Wixom, Michigan plant. Of the 4,500 cars planned 100 were to be exported to Europe, starting in late 2005. An additional 200 cars were destined for sale in Canada. Production ended in September 2006 without reaching the planned production target. 550 cars were built in 2004, nearly 1,900 in 2005, just over 1,600 in 2006, for a grand total of 4,038 cars. The final 11 car bodies manufactured by Mayflower Vehicle Systems were disassembled, the frames and body panels were sold as service parts; the Wixom Assembly Plant has stopped production of all models as of May 31, 2007. Sales of the GT continued from cars held in storage and in dealer inventories.

When the Ford GT was first announced, the demand outpaced supply, the cars sold for premium prices. The first private sale of Ford's new mid-engine sports car was completed on August 4, 2004, when former Microsoft executive Jon Shirley took delivery of his Midnight Blue 2005 Ford GT. Shirley earned the right to purchase the first production Ford GT at a charity auction at the Pebble Beach Concours d'Elegance Auction after bidding over US$557,000. A few other early cars sold for as much as a US$100,000 premium over the suggested retail price of US$139,995. Optional equipment available included a McIntosh sound system, racing stripes, painted brake calipers, forged alloy wheels adding US$13,500 to the MSRP; the Ford GT features many technologies unique at its time including a superplastic-formed frame, aluminum body panels, roll-bonded floor panels, a friction stir welded center tunnel, covered by a magnesium center console, a "ship-in-a-bottle" gas tank, a capless fuel filler system, one-piece panels, an aluminum engine cover with a one-piece carbon fiber inner panel.

Brakes are four-piston aluminum Brembo calipers with cross-drilled and vented rotors at all four corners. When the rear canopy is opened, the rear suspension components and engine are visible; the 5.4 L longitudinal rear mounted Modular V8 engine is an all-aluminum alloy engine with an Eaton 2300 Lysholm screw-type supercharger. It features a forged rotating assembly housed in an aluminum block designed for the car. A dry sump oiling system is employed; the DOHC 4 valves per cylinder heads are a revision of the 2000 Ford Mustang SVT Cobra R cylinder heads. The camshafts have unique specifications, with more lift and duration than those found in the Shelby GT500. Power output is 500 lb ⋅ ft of torque at 4,500 rpm. A Ricardo 6-speed manual transmission is fitted featuring a helical limited-slip differential. Car and Driver tested the GT in January 2004 and recorded a 0-60 mph acceleration time of 3.3 seconds. Performance: Top speed: 205 mph 1⁄4 mile: 11.8 seconds 0–62 mp

The mass attenuation coefficient, or mass narrow beam attenuation coefficient of the volume of a material characterizes how it can be penetrated by a beam of light, particles, or other energy or matter. In addition to visible light, mass attenuation coefficients can be defined for other electromagnetic radiation, sound, or any other beam that can be attenuated; the SI unit of mass attenuation coefficient is the square metre per kilogram. Other common units include cm2/g and mL⋅g−1⋅cm−1. Mass extinction coefficient is an old term for this quantity; the mass attenuation coefficient can be thought of as a variant of absorption cross section where the effective area is defined per unit mass instead of per particle. Mass attenuation coefficient is defined as μ ρ m; when using the mass attenuation coefficient, the Beer–Lambert law is written in alternative form as I = I 0 e − λ where λ = ρ m ℓ is the area density known as mass thickness, ℓ is the length, over which the attenuation takes place. When a narrow beam passes through a volume, the beam will lose intensity to two processes: absorption and scattering.

Mass absorption coefficient, mass scattering coefficient are defined as μ a ρ m, μ s ρ m, where μa is the absorption coefficient. In chemistry, mass attenuation coefficients are used for a chemical species dissolved in a solution. In that case, the mass attenuation coefficient is defined by the same equation, except that the "density" is the density of only that one chemical species, the "attenuation" is the attenuation due to only that one chemical species; the actual attenuation coefficient is computed by μ = 1 ρ 1 + 2 ρ 2 + …, where each term in the sum is the mass attenuation coefficient and density of a different component of the solution. This is a convenient concept because the mass attenuation coefficient of a species is independent of its concentration. A related concept is molar absorptivity, they are quantitatively related by × =. Tables of photon mass attenuation coefficients are essential in radiological physics, dosimetry, interferometry and other branches of physics; the photons can be in form of X-rays, gamma rays, bremsstrahlung.

The values of mass attenuation coefficients are dependent upon the absorption and scattering of the incident radiation caused by several different mechanisms such as Rayleigh scattering. The actual values have been examined and are available to the general public through three databases run by National Institute of Standards and Technology: XAAMDI database. If several known chemicals are dissolved in a single solution, the concentrations of each can be calculated using a light absorption analysis. First, the mass attenuation coefficients of each individual solute or solvent, ideally across a broad spectrum of wavelengths, must be measured or looked up. Second, the attenuation coefficient of the actual solution must be measured. Using the formula μ = 1 ρ 1 + 2 ρ 2 + …, the spectrum can be fitted using ρ1, ρ2, … as adjustable parameters, since μ and each μ/ρi are functions of wavelength. If there are N solutes or solvents, this procedure requires at least N measured wavelengths to create a solvable system of simultaneous equations, although using more wavelengths gives more reliable data.

Absorption coefficient Absorption cross section Attenuation length Attenuation Beer–Lambert law Cargo scanning Compton edge Compton scattering Cross section High-energy X-rays Mean free path Molar attenuation coefficient Propagation constant Radiation length Scattering theory Transmittance