A method of generating, ex vivo, a matrix of cells that mimics at least one phenotype of a naturally occurring mammalian tissue, the method comprising: introducing stem cells, progenitor cells, mature cells or a mixture of these into a bioreactor, the bioreactor comprising: i) a matrix material configured to receive cells in a first fluid, ii) a first chamber configured to flow the first fluid including the cells tangentially to a surface of the matrix material; iii) a first gas permeable membrane configured to permit a first gas to flow therethrough to the first fluid in the first chamber; iv) a first gassing chamber operably linked to the first chamber, said gassing chamber providing gas that flows through the first gas permeable membrane; v) a second chamber configured to flow the second fluid through a second flow path to the matrix material, wherein the first fluid and said second fluid may be the same or different; vi) a second gas permeable membrane configured to permit a second gas to flow therethrough to the second fluid in the second chamber, wherein the first gas and said second gas are different; and vii) a second gassing chamber operably linked to the second chamber, said gassing chamber providing gas that flows through the second gas permeable membrane; wherein the matrix in the bioreactor is disposed between the first and the second flow paths providing a culture media to the bioreactor for a time and under conditions suitable for the cells to arrange on the matrix material of the bioreactor whereby the cells so arranged mimic at least one phenotype of a naturally occurring mammalian tissue. introducing stem cells, progenitor cells, mature cells or a mixture of these into a bioreactor, the bioreactor comprising: i) a matrix material configured to receive cells in a first fluid, ii) a first chamber configured to flow the first fluid including the cells tangentially to a surface of the matrix material; iii) a first gas permeable membrane configured to permit a first gas to flow therethrough to the first fluid in the first chamber; iv) a first gassing chamber operably linked to the first chamber, said gassing chamber providing gas that flows through the first gas permeable membrane; v) a second chamber configured to flow the second fluid through a second flow path to the matrix material, wherein the first fluid and said second fluid may be the same or different; vi) a second gas permeable membrane configured to permit a second gas to flow therethrough to the second fluid in the second chamber, wherein the first gas and said second gas are different; and vii) a second gassing chamber operably linked to the second chamber, said gassing chamber providing gas that flows through the second gas permeable membrane; i) a matrix material configured to receive cells in a first fluid, ii) a first chamber configured to flow the first fluid including the cells tangentially to a surface of the matrix material; iii) a first gas permeable membrane configured to permit a first gas to flow therethrough to the first fluid in the first chamber; iv) a first gassing chamber operably linked to the first chamber, said gassing chamber providing gas that flows through the first gas permeable membrane; v) a second chamber configured to flow the second fluid through a second flow path to the matrix material, wherein the first fluid and said second fluid may be the same or different; vi) a second gas permeable membrane configured to permit a second gas to flow therethrough to the second fluid in the second chamber, wherein the first gas and said second gas are different; and vii) a second gassing chamber operably linked to the second chamber, said gassing chamber providing gas that flows through the second gas permeable membrane; wherein the matrix in the bioreactor is disposed between the first and the second flow paths providing a culture media to the bioreactor for a time and under conditions suitable for the cells to arrange on the matrix material of the bioreactor whereby the cells so arranged mimic at least one phenotype of a naturally occurring mammalian tissue.