Fetal bovine serum is the liquid fraction obtained after blood from bovine fetuses clots, used as a complex supplement for some cell cultures. It supplies a mixture of proteins, carrier molecules and growth-supporting components. Because it is a biological mixture, composition and growth effects can vary between lots. A result obtained in serum-containing medium therefore depends on a less chemically defined environment than one using individually specified components. [ref]
Animal Cell Culture Guide by ATCC. Media and supplements: sera and serum variability
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