The durations and compositions of cell cycles in embryos of the leech, Helobdella triserialis

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Abstract
When tritiated thymidine triphosphate ([3H]TTP) or its immunohistochemically detectable analogue, bromodeoxyuridine triphosphate (BrdUTP), is injected into blastomeres of leech embryos it passes throughout the entire embryo and is rapidly incorporated (within 2 min after injection) into nuclei of cells synthesizing DNA (S phase). In the same embryos a DNA-specific stain can be used to identify cells in mitosis (M phase) or nonreplicative interphase (G1 or G2 phase) on the basis of nuclear or chromosomal morphology. Using this procedure, we have determined the lengths and compositions of the mitotic cell cycles of identifiable cells in early embryos of the leech, Helobdella triserialis, and have analysed how the cell cycles change during the first seven stages of development. The relatively short cell cycles of the early blastomeres comprise not only phases of M and S, but also postreplicative gap (G2) phases. The lengthening of the cell cycles that occurs as development progresses is primarily accomplished by an increase in the length of G2 and secondarily by an increase in the length of S and, in some instances, the addition of a prereplicative gap (G1) phase; M phase remains relatively constant. These data suggest that the durations of the cell cycles of embryonic cells are regulated by a variety of mechanisms.