Chronological Changes in the Accumulation of Poly(A)

Chronological changes in the accumulation of poly(A)+ RNA in developing cells of Xenopus laevis were traced by in situ hybridization with 3 H-poly(U) as a probe. Almost all the somatic cells acquired 3 H-poly(U) binding sites by 3.5 days after fertilization (stage 44). At this stage, the primordial germ cells (PGCs), which had just moved to the genital ridges from the deep endodermal cell mass showed moderate 3 H-poly(U) binding activity. The silver grains disappeared from the PGCs after stage 47 (6 days after fertilization), while the somatic cells, which constituted the neural tube, genital ridges, kidney and intestine exhibited strong 3 H-poly(U) binding activity. Activity for 3 H-poly(U) binding reappeared in the FGCs after stage 52 (21 days after fertilization) when sexual differentiation of gonads became detectable morphologically. The chronological accumulation and disappearance of 3 H-poly(U) binding activity in the PGCs in contrast to the constant activity in somatic cells is discussed.