Most clinical applications of haematopoietic stem/progenitor cells (HSCs) would benefit from their ex vivo expansion to obtain a therapeutically significant amount of cells from the available donor samples. We studied the impact of cellular interactions between umbilical cord blood (UCB) haematopoietic cells and bone marrow (BM)-derived mesenchymal stem cells (MSCs) on the ex vivo expansion and differentiative potential of UCB CD34(+)-enriched cells. UCB cells were cultured: (a) directly in contact with BM MSC-derived stromal layers (contact); (b) separated by a microporous membrane (non-contact); or (c) without stroma (no stroma). Highly dynamic culture events occurred in HSC-MSC co-cultures, involving cell-cell interactions, which preceded HSC expansion. Throughout the time in culture [18 days], total cell expansion was significantly higher in contact (fold increase of 280 + or - 37 at day 18) compared to non-contact (85 + or - 25). No significant cell expansion was observed in stroma-free cultures. CD34(+) cell expansion was also clearly favoured by direct contact with BM MSCs (35 + or - 5- and 7 + or - 3-fold increases at day 18 for contact and non-contact, respectively). Moreover, a higher percentage of CD34(+)CD38(-) cells was consistently maintained during the time in culture under contact (8.1 + or - 1.9% at day 18) compared to non-contact (5.7 + or - 1.6%). Importantly, direct cell interaction with BM MSCs significantly enhanced the expansion of early lymphoid CD7(+) cells, yielding considerably higher (x3-10) progenitor numbers compared to non-contact conditions. These results highlight the importance of dynamic cell-cell interactions between UCB HSCs and BM MSCs, towards the maximization of HSC expansion ex vivo to obtain clinically relevant cell numbers for multiple settings, such as BM transplantation or somatic cell gene therapy.