TCR gene rearrangement is strictly regulated during mouse ontogeny. Th
e V-(D)-J junctions of alpha beta and gamma delta TCR transcripts expr
essed in the adult thymus are more highly diverse than those in the fe
tal thymus, We previously showed that adult hematopoietic stem cells (
HSC) have a higher capacity to insert N nucleotides into V(gamma)4 TCR
transcripts than fetal HSC and that the level of N nucleotide inserti
on is determined, at least in part, at the level of HSC. To analyze th
is developmental change of HSC at the single cell level, we investigat
ed N nucleotide insertions in three TCR transcripts (V(gamma)4, V(gamm
a)2 and V(beta)8) derived from limiting numbers of fetal liver HSC by
fetal thymic organ culture, Eight day-14 fetal liver HSC clones showed
various levels of N nucleotide insertions in V(gamma)4 transcripts (0
-78%). On the other hand, the level of N insertions was similarly regu
lated in V(gamma)4, V(gamma)2 and V(beta)8 TCR transcripts in a clone-
specific way, These results suggested that the level of N insertion is
programmed at the level of single HSC and that fetal liver contains a
heterogeneous population of HSC in terms of N insertion capacity. Aft
er 3 weeks culture with a stromal cell line, fetal HSC showed higher l
evels of N insertion capacity than before culture. This result and the
presence of HSC with intermediate N insertion capacity support the hy
pothesis that the developmental potential of individual HSC gradually
changes from fetal to adult type in one stem cell lineage.