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Differences were apparent at all levels , with a greater diversity present in tissues of diseased colonies . The research available to date indicates that experimental oral BSE challenge of pigs and poultry with brain material from cattle with BSE does not result in disease and that there is no evidence for residual infectivity present in tissues ; there have been no reports of a naturally occurring TSE in these species either ( 16 ) . The female hormone , estrogen , directs and controls cell growth and differentiation by binding to the estrogen receptor located in tissues throughout the body . When there is not enough oxygen present in tissues , HIF is stabilized and is allowed to activate gene expression .