The developmental pattern of red and yellow bone marrow was studied in rabbits of different ages from newborn to 6 months of age. At birth no adipose cells were seen in any of the marrow cavities. Most bones were still developing and marrow cavities were relatively limited. Nonetheless a distinct difference in the cellularity of marrow in the trunk versus limb bones was noted, the latter being significantly less cellular. Adipose cells began to develop at 2 weeks of age and proceeded so that the adult pattern of red and yellow marrow was fully established by 4 months. The development of adipose cells occurred in both trunk and limb bones; the magnitude of the process, however, was considerably greater in the limb bones. Adipocyte precursors may be present in the marrow at birth with a differential distribution in the areas of prospective red and yellow marrow. Thus, fatty involution of marrow appears to be a programmed developmental event.
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