In rabbits, B lymphopoiesis takes place in the fetal liver and the neonatal bone marrow. By four months of age, virtually all proB and preB cells are absent in rabbit bone marrow and the only B lineage cells present are diversified, re-circulating B cells. I am testing the possibility that the absence of on-going B lymphopoiesis in adults is due to changes in the bone marrow 'niche' as rabbit's age. This 'niche' is includes stromal cells that produce several types of proteins important in the homing, growth, survival and differentiation of hematopoietic stem cells (HSCs). We have recently found several molecules that are differentially expressed in bone marrow of young and adult rabbits and I am investigating the role of these molecules in B lymphopoiesis.