Proximal-distal gradient of pre-HSC activity in the VA. (A) Subdissection scheme of the VA: 1, portion of VA associated with YS; 2, distal VA portion; 3, proximal VA portion; 4, portion of VA associated with midgut. Original magnification ×32. (B) VA subsections were transplanted into irradiated recipients after 4 days of culture (1 e.e. per mouse). Donor engraftment was assessed 14 weeks posttransplantation (n = 2 independent experiments).