Analysis of Biological Development (K. Kalthoff)

Updates to Topic 05: Cleavage


Answers to Questions in Text

Fate of Mammalian Morula Cells (p. 106/107)

  1. Which general behavior of cells is utilized when researchers “feed” miniature plastic beads to them? Answer: Endocytosis
  2. What does the stepwise formation of the inner cell mass according to Fig. 5.13 imply for the likelihood with which a fate map at the morula stage would predict the fate of a given blastomere? Answer: While outside cells may contribute to trophoblast or inner cells mass (ICM), inside cells most likely will form much of theICM.

Role of the Egg Cortex in Positioning the Mitotic Spindle (p. 112/113)

  1. In the light of the experiments illustrated in Figs. 5.22 and 5.23, how would you explain the fact that the cleavage furrows of the first two cleavages in frog eggs always begin to form at the animal pole (see Fig. 5.1)? Answer: In the zygote as well as the two-cell embryo, the cell nuclei are displaced towards to animal pole, and the centrosomes are positioned so that the mitotic spindle axes are perpendicular to the animal-vegetal axis. Hence, the mitotic spindle fibers will interact first with the egg cortex near the animal pole.
  2. In which kind of embryo, illustrated in section 5.2, does the majority of all cleavage furrows normally arise between asters that belong to different spindles? Answer: In the insect embryo, see Figs. 5.18 and 5.19.

 

Comments

See Movies on Cleavage on Movies page.

 

Clarifications and Corrections

p. 115, legend to Fig. 5.25 should begin "Centrosome replication and movements..."

p. 123, paragraph 2, line 2 from below, should read "meroblastic cleavage..."

 

New Review Articles

Paria B.C., Reese J., Das S.K., and Dey S.K. (2002) Deciphering the cross-talk of implantation: Advances and challenges. Science 296: 2185-2188

Schuyler S.C., Pellman D. (2001) Search, capture and signal: games microtubules and centrosomes play. J. Cell Sci. 114: 247-255

Sisson J.C., Rothwell W. and Sullivan W. (1999) Cytokinesis: Lessons from Rappaport and the Drosophila embryo. Cell Biology International 23: 871-876

 

New Research Articles

Lehmann D.A., Patterson B., Johnston L.A., Balzer T., Britton J.S., Saint R. and Edgar B.A. (1999) Cis-regulatory elements of the mitotic regulator, string/Cdc25. Development 126: 1793-1803

In Drosophila embryos past the midblastula trasition, mitosis in most cells is triggered by bursts of transcription of string+, which encodes a phosphatase that activates the mitotic kinase CDK1 (see text Fig. 5.34). It is expected that mitotic divisions are differentially regulated so as to allow cell turnover and regional proliferation, to support morphogenetic movements, and to avoid interference with cell differentiation. In accord with the expected complexity in the control of string+, Lehmann et al. (1999) found that the gene's transcription is controlled by an unusually large regulatory region (more than 30kb) containing a plethora of regulatory elements. They identified regulatory elements specific to subsets of epidermal cells, mesoderm, trachea, nurse cells, and in particular the nervous system.

Korinek W.S., Copeland M.J., Chaudhuri A. and Chant J. (2000) Molecular linkage underlying microtubule orientation toward cortical sites in yeast. Science 287: 2257-2259

Lee L., Tirnauer J.S., Li J., Schuyler S.C., Liu J.Y. and Pellman D. (2000) Positioning of the mitotic spindle by a cortical-microtubule capture mechanism. Science 287: 2260-2262

The asymmetrical cleavage patterns observed in many embryos require the correct positioning and orientation of mitotic spindles. The latter may be facilitated by a search-and-capture mechanism, in which dynamic microtubules are stabilized through interactions with specific sites of submembrane cortex. In yeast, oriented microtubules align the mitotic spindle between the mother cell and its bud. Korinek et al. and Lee et al. found independently that a cortical protein, Kar9, localizes to the bud tip and interacts with Bim1, a phylogeneticlly conserved protein that occurs specifically at the free (plus) ends of microtubules. A similar cortical-microtubule capture mechanism seems to exist in animal cells.


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