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A Stereometric Analysis of Karyokinesis, Cytokinesis and Cell Arrangements during and following Fourth Cleavage Period in the Sea Urchin, Lytechinus variegatus
Author(s) -
Summers Robert G.,
Morrill John B.,
Leith Ardean,
Marko Michael,
Piston David W.,
Stonebraker Alan T.
Publication year - 1993
Publication title -
development, growth and differentiation
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.864
H-Index - 66
eISSN - 1440-169X
pISSN - 0012-1592
DOI - 10.1111/j.1440-169x.1993.00041.x
Subject(s) - cleavage (geology) , cytokinesis , blastomere , mitosis , sea urchin , biology , anatomy , cell division , embryo , microbiology and biotechnology , paleontology , cell , embryogenesis , genetics , fracture (geology)
Fourth cleavage of the sea urchin embryo produces 16 blastomeres that are the starting point for analyses of cell lineages and bilateral symmetry. We used optical sectioning, scanning electron microscopy and analytical 3‐D reconstructions to obtain stereo images of patterns of karyokinesis and cell arrangements between 4th and 6th cleavage. At 4th cleavage, 8 mesomeres result from a variant, oblique cleavage of the animal quartet with the mesomeres arranged in a staggered, offset pattern and not a planar ring. This oblique, non‐radial cleavage pattern and polygonal packing of cells persists in the animal hemisphere throughout the cleavage period. Contrarily, at 4th cleavage, the 4 vegetal quartet nuclei migrate toward the vegetal pole during interphase; mitosis and cytokinesis are latitudinal and subequatorial. The 4 macromeres and 4 micromeres form before the animal quartet divides to produce a 12‐cell stage. Subsequently, macromeres and their derivatives divide synchronously and radially through 8th cleavage according to the Sachs‐Hertwig rule. At 5th cleavage, mesomeres and macromeres divide first; then the micromeres divide latitudinally and unequally to form the small and large micromeres. This temporal sequence produces 28‐and 32‐cell stages. At 6th cleavage, macromere and mesomere descendants divide synchronously before the 4 large micromeres divide parasynchronously to produce 56‐ and 60‐cell stages.