We investigated the rearrangement and expression of TCR genes in mouse
fetal thymus organ culture, a system that avoids subsequent entry of
hematopoietic precursor cells, The first observable rearranged TCR gen
e was homogeneous V gamma 2-J gamma 2, detectable as early as fetal da
y 11 (d11) in the thymic primordia, The productive TCR was homogeneous
V gamma 5-J gamma 1, first detectable in d13 thymocytes, followed by
adult-type TCR gamma (V gamma 4 and V gamma 7). Sequence analysis of T
CR revealed five types of V-J junctional sequences, In the very early
stage, a homogeneous V-J junction is generated via a short homology se
quence in the coding region (Type I), while a short homology sequence
in the P-nucleotide rather than the coding region is used in the follo
wing stage (Type II), In the later embryonic stages, diverse V-J junct
ions are generated by well-known mechanisms, such as P-nucleotide (Typ
e III), N-region insertion (Type IV) or trimming of the coding ends (T
ype V), These findings suggest that the generation of homogeneous TCR
gamma (V gamma 2 and V gamma 5) in the early fetal stages is due to th
e intrinsic rearrangement mechanisms and is in stage specific manner.