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We describe the spatial and temporal patterns of cell division in the early Xenopus embryo, concentrating on the period between the midblastula transition and the early tailbud stage. Mitotic cells were identified using an antibody recognising phosphorylated histone H3. At least four observations are of interest. First, axial mesodermal cells, including prospective notochord, stop dividing after involution and may not divide thereafter. Second, cell division is more pronounced in the neural plate than in nonneural ectoderm, and the pattern of cell division becomes further refined as neurogenesis proceeds. Third, cells in the cement gland cease proliferation completely as they begin to accumulate pigment. Finally, the precursors of peripheral sensory organs such as the ear and olfactory placode undergo active cell proliferation when they arise from the sensorial layer of the ectoderm. These observations and others should provide a platform to study the relationship between the regulation of developmental processes and the cell cycle during Xenopus embryogenesis.
FIG. 1. Immunofluorescence microscopy of a Xenopus embryo stained with anti-phosphohistone H3 antibody (aPH3) at stage 9. The embryo was double-stained with the DNA dye DAPI (A and D, blue) and aPH3 (B and E, red) and observed with confocal microscopy. Merged images are shown in (C and F). (A C) Images at low magnification and (D F) at high magnification. Arrows in (A C) indicate a cell in anaphase, white arrowheads a cell in metaphase, and green arrowhead a cell in prophase/prometaphase. In (D F), individual condensed chromosome arms are clearly seen. Scale bar 5 50 mm (A C), or 5 5 mm (D F).
FIG. 2. Whole-mount immunostaining by aPH3 after treatment with nocodazole (NOC) and hydroxyurea (HU). Animal pole views of (A) control, (B) NOC-treated, and (C) HU-treated embryos at stage 9. Dark brown dots are aPH3-positive nuclei. A small population of cells in NOC-treated embryos was not stained by aPH3 antibody (2B, arrows), presumably because these cells had just completed the previous mitosis at the start of nocodazole application and had a cell-cycle period longer than the duration of the treatment (2 h).
FIG. 3. Xenopus cell division pattern at stage 9. (A) Animal pole view of a whole embryo stained with aPH3 after clearing. Brown dots are mitotic nuclei. (B) Midtransverse section of a stained embryo. Mitotic nuclei appear as brown dots and other nuclei are light green.
FIG. 4. Cell divisions detected by aPH3 in gastrula stage embryos. (A, C, and E) Sagittal sections of aPH3-stained embryos at stage 10.5, 11, and 12, respectively. Brown dots are mitotic nuclei and blue dots are nonmitotic nuclei. Arrows indicate dorsal lip. Scale bar is 200 mm.
(B, D, and F) Composite images of 11 serial sagittal sections of a representative embryo at stages 10.5, 11, and 12, respectively. Mitotic nuclei appear as black dots. Dorsal axial mesoderm is highlighted in red; mitotic cells are absent. Note that due to distortion during
sectioning, sections could not be registered with each other precisely.