Southern blot analysis of PCR products from stromal cultures initiated by FACS sorted single cells. (A) RNA was extracted from six fetal bone marrow stromal cultures that had been initiated by CD34⁺ or CD34⁻ FACS sorted single cells. (+) cDNA was synthesized by the addition of random hexamer primers and reverse transcriptase. (−) The same reaction conditions lacking reverse transcriptase were used. The presence of specific PCR products was detected by Southern blot using a Fibronectin, SCF, CD34 ³²P-labeled specific probes as indicated. (B) RNA was extracted from 4 unselected stromal cultures of adult bone marrow stromal cells. cDNA was synthesized by the addition of random hexamer primers and reverse transcriptase. The presence of specific PCR products was detected by Southern blot using a FL ³²P-labeled internal probe as indicated. (C) RNA was extracted from 10 stromal cultures initiated by FACS sorted single cells with the CD34⁺ or CD34⁻ phenotype. cDNA was synthesized by the addition of random hexamer primers and reverse transcriptase. The presence of specific PCR products was detected by Southern blot using a TPO specific ³²P-labeled internal probe as indicated.