Fig. 3. Potential of day-11 and day-12 embryonic haematopoietic tissues to produce/expand definitive HSC/RUs as assessed by an organ culture approach. (A) Cultured day-11 tissues, (B) cultured day-12 AGM region and YS, (C) cultured day-12 liver. Note that among day-11 tissues (A) only the AGM region was able to expand HSC/RUs and among day-12 tissues (B) only the YS was able to expand HSC/RUs, suggesting two consecutive waves of HSC generation/expansion and liver colonisation, firstly from the AGM and secondly, the next day, from the YS. 12 d.p.c. embryonic liver was not capable of maintaining HSCs: a dramatic decrease in number of HSC/RUs was observed after 3 days in culture (see Discussion). Numbers of HSC/RUs in tissues are estimated using a limiting dilution method and presented as described in Materials and Methods. Numbers of recipient mice (RM) and the dilutions (D) used were as follows. Day 11 tissues (A): (AGM, uncultured) 27RM, 2D; (AGM, 3-5 days in culture) 41RM; 8D; (AGM, 7-9 days in culture) 14RM, 3D; (YS uncultured) 25RM, 3D; (YS 3-5 days in culture) 20RM, 6D; (YS 7-9 days in culture) 15RM,3D; (liver, uncultured) 23RM, 2D; (liver, 3-5 days in culture) 29RM, 9D; (liver, 7-9 days in culture) 14RM, 4D. Day 12 tissues (B): (AGM, uncultured) 25RM, 7D; (AGM, 3-5 days in culture) 31RM, 7D; (AGM, 7-9 days in culture) 14RM, 3D; (uncultured YS) 39RM, 8D; (YS, 3-5 days in culture) 17RM, 4D; (YS, 7-9 days in culture) 11RM, 2D; (liver, uncultured) 37RM, 5D. Day 12 liver (C): (3-5 days in culture) 14RM, 3D; (7-9 days in culture) 24RM; 6D.