Bone marrow response to large volume blood collection in the horse

Evaluation of erythropoietic regeneration in horses is difficult unless serial bone marrow aspirates are performed. To investigate the acute and chronic erythropoietic regenerative response of equine bone marrow following acute removal or loss of blood, sequential bone marrow aspirates over 4 weeks were taken from the sternum of five horses from which 20 ml kg-1of blood had been removed. We found that the total number of erythroid cells counted (expressed as a percentage of the total number of erythroid and myeloid cells counted) expanded initially by 13·7 per cent within 3 days after blood removal, the erythroid response peaking by 9 days with a further 13·5 per cent increase. This peak coincided with the lowest M:E ratio. Concomitantly, a shift from proliferative phase cells to maturing phase cells occurred, which appeared to persist beyond 31 days post collection. Thus, we found that the equine bone marrow mounted a regenerative erythropoietic response more slowly than previously determined and, also, regeneration of the erythroid compartment was incomplete 31 days after blood removal of this magnitude.