Age-dependent bone marrow mast cell maturation in specific pathogen-free rats

In the present study we have investigated an age-dependent mast cell increase. The mast cell numbers in bone marrow from (BN x Wi/Fu)F1 rats were determined using Alcian blue staining of cells in solution. From the 6th to the 19th week of age the number of mast cells in the bone marrow increased 6-fold with a marked peak around the 12th week of age. This increase in mast cell numbers was accompanied with increased histamine levels. Immunization increased the numbers further, also when the age-dependent mast cell increase was corrected for. The results should be considered in relation to previous studies revealing increased mast cell numbers in the bone marrow after immunization and also increasing numbers in the lungs of rats immunized with protein antigens or exposed to environmental antigens.