Some physiological and developmental considerations of the temperature‐gradient hypothesis of bone marrow distribution

Abstract In adult humans, active bone marrow is confined to the proximal portion of the skeleton. Huggins and Blocksom (J. Exp. Med., 64 : 253, '36) concluded that a high temperature is needed for hematopoiesis in rats. However, precise thermal regulation of human marrow was not found (Petrakis, J. Appl. Physiol., 4 : 549, '52). Because these experiments made on the rat tail are the basis for a commonly accepted hypothesis attempting to explain marrow distribution in man, it was considered of importance to re‐examine the caudal vertebra model upon which the temperature‐gradient hypothesis is based. The sacral and coccygeal vertebrae were examined in rats, mice and humans with respect to marrow cellularity and temperature. In rats and mice and man it was observed that the transition between hematopoietically‐active and inactive (fatty) vertebral marrow cavities is abrupt, occurring at the level of the first and second caudal and coccygeal vertebrae. All vertebrae distal to this point have fatty marrow. Of significance was the finding that the vertebral and coccygeal temperatures, as measured with a thermister needle, remain unaltered over this area of changing cellular activity. These anatomical and thermal observations of the caudal vertebrae of rats, mice, and humans indicate that the use of the tail as an experimental model does not support the hypothesis that temperature is a primary factor in the physiological maintenance of hematopoiesis in bone marrow. The possible relationship of hematopoietic activity to developmental and other factors peculiar to the caudal vertebra model is under study.