Morphometric studies of the spleen of the hypoxic mouse.

Adult male mice were exposed for periods of up to 7 days at atmospheric pressures of either 72 or 52 kPa (540 or 390 mm mercury). Changes in body weights, spleen weights and red and white splenic pulp weights were compared with those in normoxic controls. Contrary to expectation, spleen weights and amounts of red pulp were greater (in terms of experimental-control differences) in the animals kept at 72 kPa, suggesting that at 52 kPa loss of overall fitness reduced the adaptive erythropoietic response. However, in view of the fact that increased erythropoiesis has disadvantages as well as advantages, the lesser response at the lower pressure may not be wholly disadvantageous. Coefficients of variation of the spleen variables examined showed a tendency to decrease in the animals at 72 kPa, but rose markedly at 52 kPa. This finding was interpreted as indicating that at 72 kPa spleen and red pulp changes are adaptive, but at 52 kPa they indicate an overall relative failure of adaptive mechanisms, with consequent reduced somatic fitness.